What is the origin of the phrase “it warms the cockles of my heart”?

Solution 1:

          Mary, Mary, quite contrary,
                    How does your garden grow?
          With silver bells, and cockle shells,
                    And pretty maids all in a row.


For the word itself, the OED gives an etymology that vectors through French, as in the fancy dish called Coquilles Saint-Jacques /kɔ.kij sɛ̃.ʒak/, back to the Latin word for conch:

Etymology: Middle English cokille, < French coquille (Old French also cokille) shell, = Italian cocchiglia cockle-shell < Latin type *cocchilia, *cocquilia, by-form of conchylia, plural of conchylium (conquilium in a Gloss.), < Greek κογχύλιον small kind of mussel or cockle, diminutive of κογχύλη = κόγχη (whence Latin concha and by-form *cocca) mussel or (perhaps) cockle. With the English shifting of the stress, cokille has become cockle, like gentille, gentle, etc.

For the idiom, it presents this:

  1. cockles of the heart: used in connection with to rejoice, delight, etc.; also (in modern use) to warm the cockles of one’s heart.

    For derivation cf. quot. 1669. Others have sought its origin in Latin corculum dim. of cor heart. (Latham conjectured ‘the most probable explanation lies (1) in the likeness of a heart to a cockleshell; the base of the former being compared to the hinge of the latter; (2) in the zoological name for the cockle being Cardium, from the Greek καρδία = heart’.)

The putative Latin origin in the 1669 citation is given only as:

  • 1669 R. Lower Tractatus de Corde 25 — Fibræ quidem..spirali suo ambitu helicem sive cochleam satis apte referunt.

The first English citation follows just two years later:

  • 1671 J. Eachard Some Observ. Answer to Grounds Contempt of Clergy 22 — This Contrivance of his did inwardly..rejoyce the Cockles of his heart.

It has been suggested in comments that the heart-shaped shell of the cockle may have reinforced the phrase. Here from Wikipedia is an illustration of the effect:

cockle shell

The English are not the only ones whose hearts have been historically warmed by cockles. Consider Botticelli’s famous painting, The Birth of Venus, again from Wikipedia showing Venus on a half-shell of a giant scallop:

birth of venus

On the Iberian Peninsula, the word for a scallop is a vieira in all three of Galician, Portuguese, and Spanish, a term derived from Medieval Latin conchula veneria meaning Venus’s conch/shell — which brings us back to the cockles of one’s heart.

Solution 2:

Richard Lower (1631 – 17 January 1691) was an English physician who heavily influenced the development of medical science. He is most remembered for his works on transfusion and the function of the cardiopulmonary system.

Wikipedia


Having previously experimented on transfusion between animals, Lower worked with Sir Edmund King to transfuse sheep's blood into a man and the procedure was carried out by Lower and King before the Royal Society on 23 November 1667.

Two years later, Lower produced his Treatise on the Heart, in 1669.

Lower Treatise

Word Histories


Thus, in 1669, in the wake of huge interest in the possibility of routine blood transfusion, there appears in print the expression 'cochlea' in reference to the structure of the human heart.

Then, in 1671, we have the first known reference to 'the cockles of the heart' in John Eachard's The Grounds & Occasions of the Contempt of the Clergy where (on the first line of page 27) he writes

... as much rejoyce the Cockles of his heart, as he phanſies, that what I writ ſometimes did much tickle my spleen ...

Eachard's use of the word 'spleen' is clearly anatomical and therefore it is clear that so is his use of the word (note the capital) 'Cockles'.

John Eachard was made a DD by royal mandate and was twice (1679 and 1695) Vice-Chancellor of Cambridge University so would have been an educated man and presumably well informed of current medical advances and terminology.


From the above, I would suggest it is an anatomical reference at a time when blood transfusion was about to become a routine medical procedure and the organ which pumps the blood was a focus of contemporary interest.


Thereafter, the spread of the expression 'warming the cockles' may well be a matter of mixed metaphor due to

  • the fact that cockles is not a precise term and covers a variety of bivalves all of which squirt water, thus there is the suggestion of the pumping of blood by the chambers of the heart

  • the process of cooking cockles during which they adopt the appearance, not of lean meat, but of offal, thus presenting an image of warming organs.

So the imagery in the consciousness of the general public is of the cooking item and the edible food - this is what is thought of as 'cockles'.

When one feels the warm glow of contentment, therefore, one is reminded of the sight of a simmering pan of warmed items which once squirted liquid.


So I venture to suggest that 'warm cockles' is not a reference to anything cultured or romantic but rather has more earthy origins - namely blood transfusions, anatomy, an attack on the clergy, a squirting seashell and a bubbling pot on the stove.


Details of Morecambe Bay Cockling Disaster 2004 (21 drowned)

Morecambe Bay Cocklers (Date Uncertain : circa 1905)

Morecambe Bay Cocklers (Date Uncertain)

Solution 3:

Dictionary discussions of 'warm the cockles'

Dictionaries of word and phrase origins published in the past century seem split on the derivation of "warm the cockles of [one's] heart." Here are six entries for the phrase from various sources.

From Christine Ammer, The American Heritage Dictionary of Idioms, second edition (2013):

warm the cockles of one's heart Gratify one, make one feel good, as in It warms the cockles of my heart to see them getting along so well. This expression uses a corruption of the Latin name for the heart's ventricles, cochleae cordis. {Second half of 1600s}

From John Ayto, Oxford Dictionary of English Idioms, third edition (2009):

warm the cockles of someone's heart give someone a a comforting feeling of pleasure or contentment. This phrase perhaps arose as a result of the resemblance between a heart and a cockleshell.

From Robert Hendrickson, The Facts on File Encyclopedia of Word and Phrase Origins, fourth edition (2008):

warm the cockles of one's heart. The most popular explanation for the cockles here says that late-17th-century anatomists noticed the resemblance of the shape of cockleshells, the valves of a scallop-like mollusk, to the ventricles of the heart and referred to the latter as cockles. Whether this is the case or not, cockles isn't use much anymore except in the expression to warm the cockles of one's heart, to please someone immensely, to evoke a flow of pleasure or a feeling of affection." Behind the expression is the old poetical belief that the heart is the seat of affection.

From Longman Dictionary of English Idioms (1979):

warm the cockles of someone's heart coll[oquial] to produce a warm feeling of health and contentment in a person, esp. by means of alcohol: drink this! It'll warm the cockles of your heart on a cold night like this || 'it warms the cockles of my heart when I hear the old war songs,' said the old soldier ... {Perhaps from a comparison between the shape of the heart and a cockleshell

From William Morris & Mary Morris, Dictionary of Word and Phrase Origins (1962):

cockles of the heart You have often heard the phrase warm the cockles of one's heart, but these cockles have nothing to do with the cockles and mussels Sweet Molly Malone used to sell.

The cockles of the old ballad are what the dictionaries call "edible bivalve mollusks"—shellfish, to you and me. In appearance they are unlike our scallops, having a somewhat heart-shaped, ribbed shell.

The cockles of your heart, on the other hand are the ventricles and thus, by extension, the innermost depths of one's heart or emotions. The word comes from the Latin phrase cochleae cordis, meaning "ventricles of the heart," while the shellfish cockle comes from the Latin conchylium, meaning conch shell.

From Ernest Weekley, An Etymological Dictionary of Modern English (1921):

cockle2. Shell. F. coquille, from VL. coccylium, L. conchylium, G. κογχύλιον, from κόγχη, whence L. concha, VL. cocca, origin of F. coque, shell (of egg). ... The cockles of the heart are explained (1669) as for the related cochlea (q.v.), winding cavity. Hot cockles, a game in which a blindfolded person has to guess who slaps him, occurs in in Sidney's Arcadia. It is app[arently] adapted from F. jeu de la main chaude, but cockles is unexplained.

Weekley's entry for cochlea is as follows:

cochlea. Cavity of ear. L., snail, G. κοχλίας. From shape.

So it appears that your choice of derivations comes down to a choice of mollusks: gastropod or pelecypod.

Supplementing these authorities are two earlier references. First, J. Dixon makes the cochleae cordis argument in an issue of Notes and Queries (July 9, 1887), and A.F. Chamberlain argues for the cockle weed as the original figure involved in "cockles of the heart" in an issue of American Notes and Queries (May 4, 1889).


Meanings of 'cockle' in early dictionaries

John Kersey, Dictionarium Anglo-Britannicum: Or, A General English Dictionary (1708) offers these definitions for cockle:

Cockle, a kind of Shell-fish ; also a Weed otherwise call'd Corn-rose.

To Cockle, to pucker, wrinkle, or shrink, as some Cloth does.

Cockle-stairs, winding-Stairs.

The mystery of why winding stairs would be called cockle-stairs is perhaps answered by Kersey's definition for cochlea on the same page:

Cochlea, (L.) the Cockle, a Shell-fish; the Sea-snail, or Periwinkle : Also a Screw, one of the Six Mechanick Powers, or Principles : In Anatomy, the Hollow of the inner part of the Ear.

Here we see a degree of confusion in the use of cochlea to refer to both a bivalve (the cockle and a snail (the periwinkle). If the confusion extended in the opposite direction, it would be easy to explain cockle-stair as having originated as cochlea-stair after the periwinkle's winding shell. Edward Phillips, The New World of English Words, or a Generall Dictionary (1658) includes the shell-fish and weed definitions of cockle, but renders the architectural term for winding stairs as cocle-stairs. Elisha Coles, An English Dictionary: Explaining the Difficult Terms... (1692) includes an entry for cockleary, meaning "pertaining to winding stairs."

Thomas Dyche & William Pardon, A New General English Dictionary (1735) reports that the verb sense of cockle is "To shrivel, gather or shrink up, to pucker like an ill-sown seam, &c."—suggesting a similarity to a cockleshell's ribbing. Samuel Johnson, A Dictionary of the English Language (1755) confirms this:

To Cockle. v. a. {from cockle.} To contract into wrinkles like the shell of a cockle.

But Johnson is less sure of the meaning of the adjective cockled, which he finds in Love's Labour's Lost:

Cockled. adj. {from cockle.} Shelled; or perhaps cochleate, turbinated. [Cited occurrence:] Love's feeling is more soft and sensible,/ Than are the tender horns of cockled snails. Shakespeare.

John Kersey, A New English Dictionary, fourth edition (1739) omits the entries for cochlea and cockle-stairs but adds an entry for hot-cockles:

Hot-Cockles, a kind of Sport.

As we saw in Weekley's Etymological Dictionary, the sport consists of having blindfolded subjects try to guess who just slapped them.


Early instances of 'the cockles of [one's] heart'

As tchrist notes in his answer, one very early instance of the phrase appears in John Eachard, "Some Observations upon the Answer to an Enquiry into the Grounds & Occasions of the Contempt of the Clergy" (1671/1685 [fifth edition]):

But of all Strategems that he [a critic who published an answer to Eachard's original publication, The Grounds and Occasions of the Contempt of the Clergy and Religion Enquired Into] makes use of, to shew how vain and successless all my endeavours were likely to be; that certainly argues the most of close and thick thinking, which he lucks upon (p. 12.) Nay, says he, I will venture farther a little to make it appear (and indeed if there were ever Venture made, this was one) that Ignorance and Poverty are not the only grounds of Contempt; for some Clergy-men are as much slighted for their great Learning, as others are for their Ignorance. Now, although he says in his Preface, that he would not much boast of convincing the world, how much I was mistaken in what I undertook; yet, I am confident of it, that this Contrivance of his did inwardly as much rejoyce the Cockles of his heart as he phansies, that what I writ did sometimes much tickle my Spleen. But wherein, I pray, Sir, are they slighted?

But earlier than the earliest identified instance of "the cockles of [one's] heart" is an instance of "the cockles of [one's] head." From Guillaume Du Bartas, "The Decay in Josuah Sylvester's translation of Works of Du Bartas (1608/1641):

Jezebel:/ The Queen had inkling: instantly she sped/ To curl the Cockles of her new-bought head:/ The Saphyr, Onyx, Garnet, Diamond,/In various forms cut by a curious hand,/ Hang nimbly dancing in her hair, as spangles:

Here the "Cockles" appear to be ringlets, perhaps (in this case) on a wig. William Whitney, The Century Dictionary (1899) partially confirms that view, citing Du Bartas's verse in support of its definition 4 of cockle: "A ringlet or crimp." But a ringlet arguably suggests a snail, and a crimp arguably suggests the ridges of a bivalve shell, so we haven't really clarified which creature "cockles of hair" invokes.

Another early instance of "the cockles of [someone's] heart" again uses rejoyce. From Thomas Brown, "Dialogue II" (ca. 1688–1690), from A Collection of All the Dialogues Written by Mr. Thomas Brown (1704):

Crites. Why truly, Mr. Bays, I must needs own that 'tis writ with more Caution than generally such kind of Lives are, and as for the Language and the Ornaments of the Stile, I have nothing to except to them, for, without any more Ceremony, they are extremely fine, both in the Original and in the Translation.

Bays. Nay, now you Rejoyce the Cockles of my Heart, honest Mr. Crites; oh I love dearly to have my pieces commended, and all that, by a Person of Understanding: I'gad I do, Mr. Crites, 'tis the greatest refreshment in the World. Prithee, dear Rogue, let me hugg thee to pieces. Come, I'll give thee a Dish of Tea for this———

The same Thomas Brown (who died in 1704) is responsible for one of the earliest Google Books instances where cockles of the heart are heated—in fact, kindled. From "The Virgin's Answer to Mrs. Behn," in Letters from the Dead to the Living (1702/1707):

First, were I to give my self Liberty (as whether I do or no, is no matter to any Body) I would always bestow my Favours upon those above me, and those beneath me, and never be concern'd with any Man upon an equal footing ; and these are my Reasons: Suppose the vicious Eyes of a great Man are fix'd upon me, and my charms should kindle a Love-Passion in the Cockles of his Heart ; he Writes, Chatters, Swears and Prays, according to Custom in such Cases, I still defend the Premises, by a flat verbal denial ; but at the fame instant, encourage him in my Looks, and am always free to oblige him with my Company, till by this sort of usage, I make him sensible down-right Courtship will never prevail; and that the Citadel he besieges, is not to be surrender'd without bribing the Governess: ...

Nevertheless, Francis Grose, Classical Dictionary of the Vulgar Tongue, second edition (1788) cites the "rejoice" version of the expression:

COCKLES. To cry cockles ; to be hanged : perhaps from the noise made whilst strangling. Cant.—This will rejoice the cockles of one's heart ; a saying in praise of wine, ale, or spiritous liquor.

And the use of rejoice appears at least as late as George James, Pequinillo, volume 1 (1852):

Her [Kit's] services were not forgotten by Ludlow and his wife; and, as they went out, after bidding Kit Markus a hearty farewell, the former slipped a crown piece into her hand, which rejoiced the cockles of her heart, more than most things ever did in life ; for it was the first she ever had, and she knew the bitter taste of poverty.

Google Books also turns up instances of "cheer the cockles of his heart" (1816), "comfort the cockles of his heart" (1821), earlier than the earliest instance I could find in Google Books or Elephind for "warm the cockles of [one's] heart." The earliest seven such instances are as follows.

From Richard Penn Smith, The Triumph at Plattsburg (1830), in Representative American Plays (1916):

ANDRE [MACKLEGRAITH]. Corporal Peabody, it warms the cockles o' my heart to see your good natured face at this present speaking, though you ken weel enow, that the time ha' been, and that na lang syne, when I would ha' preferred your room to your company any day in the week, and ha' been the gainer by it.

From Frederick Marryat, Jacob Faithful, serialized in The Metropolitan (February 1834):

"There now, master, there's a glass o' grog for you that would float a marlingspike. See if that don't warm the cockles of your old heart."

"Aye," added Tom, " and set all your muscles as taught as weather backstays."

From W.H. Ainsworth, Jack Sheppard, serialized in Bentley's Miscellany (February 1839):

"Give me the brandy, and I'll tell you," replied Wood.

"Here, wife—hostess—fetch me that bottle from the second shelf in the corner cupboard.—There, Mr. Wood," cried David, pouring out a glass of the spirit, and offering it to the carpenter, "that'll warm the cockles of your heart. Don't be afraid, man,—off with it. It's right Nantz. I keep it for my own drinking," he added in a lower tone.

From "English Extracts: The Stomachs of London," reprinted from Blackwood's Magazine in the Sidney [New South Wales] Morning Herald (December 27, 1842):

Truly, ravenous reader, it is a goodly stomach that same Smithfield; like our own, empty as a gallipot the greater part of the week, but filled even to repletion upon market days. In our case, you will understand market day to be that when some hospitable Christian, pitying our forlorn condition, delights our ears, warming the cockles of our heart with a provoke; when, he assured, we eat and drink indictively, like an author at his publisher's!

From "A Windfall for the 'Young 'Un'" from the New York Spirit of the Times, reprinted in the Ottawa [Illinois] Free Trader (December 11, 1846):

To my honored Senior (whom I set down in my category as my legitimate "dad,") I would refer you for father particulars. He is tenacious of the character of his progeny and loves me ; I would commend him to you, for it will warm the cockles of his old heart to learn that the "Young 'Un" is in luck.

From "Lord John Russell's Bowl of Bishop," in Punch, or the London Charivari (January 1848):

At this usually inclement season of the year, the cheerful mug of egg-flip and the comforting tumbler of hot-spiced elder cordial are in great request, as the means of raising low spirits and warming the cockles of the chilled heart. Perhaps, however, both egg-hot and elder: wine must yield in their elevating and invigorating properties to a good Bishop.

From "Local Intelligence," in Bell's Life in Sydney [New South Wales] and Sporting Reviewer (March 17, 1849):

"IF YOU DOUBT WHAT I SAY TAKE A BUMPER AND TRY."—The words of this song occurred to us last evening, when we paid a visit to our friend Samuells, the worthy host of the Liverpool Arms, at the corner of Pitt and King-streets, upon which occasion we wound up with a glass of the creamy, which warmed the cockles of our heart. Not content with administering this comfort, he insisted upon cooking us a chop, and washing it down with a first-rate glass of Byas's best, not forgetting a drop of the real pale Cognac to prevent any rebellious qualms in the lower regions.

And from [Anonymous], Harley Beckford, volume 3 (1849):

"Ay, brandy's the stuff for heroes, French, Dutch, or English! Fill up, and I'll mate you!" cried John. The glasses were filled, rung together at the rims, and emptied at the word "Fire." "Hah!" said he, smacking his lips with extreme relish, and looked lovingly at the remainder."It warms the cockles of one's heart."


Conclusions

English-language writers have mentioned "the cockles of [one's] heart" since at least 1671, when John Eachard, in the course of a screed directed at an opponent in a controversy over religious doctrine, asserted that a "contrivance" that his opponent had devised "did inwardly ... rejoyce the Cockles of his heart"—an experience that Eachard suggests is similar to the physiological effect that his own writing has on him, namely, that it "did sometimes much tickle my Spleen." From this I infer that Eachard may have conceived of cockles not as a metaphorical allusion to cockleshells or to the cockle weed/corn-rose/darnell/field campion, but as a description of a winding passageway—that is, a cochlea.

Today, we associate the word cochlea exclusively with the inner ear, and John Kersey's entry for the term in his 1708 Dictionarium Anglo-Britannicum does cite that as the anatomical sense of the word. But cochlea also might refer to either the cockleshell (a bivalve) or the periwinkle (a sea snail)—the one notable for its radiating ridged surface and somewhat heartlike shape, and the other for its turbinate form and winding interior. Moreover, dictionary entries (over the course of 200 years) for cockle-stairs or cocle-stairs (meaning winding stairs) strongly suggest that cochlea made the jump to cockle in at least one area of English.

In Google Books searches, the earliest verbs associated with "the cockles of [one's] heart" are rejoice (1671), cheer (1816), and comfort (1821), with warm first appearing in 1830. By then, the term "cockles of [one's] heart" had fallen into disfavor among cultivated English speakers, as Francis Grose points out in his Classical Dictionary of the Vulgar Tongue (1785), where he describes "rejoice the cockles of one's heart" as part of a cant phrase in praise of alcoholic drink. Of the first eight instances of "warm the cockles of [one's] heart" that I found (all from the period 1830–1849), five clearly involve drinking some type of alcoholic beverage, so from an early date the warming of the cockles is often physiological and not merely metaphorical.

Also intriguing is this excerpt from Marin La Voye, Eugénie, the Young Laundress of the Bastille (1851):

No sooner had you been unlucky enough to utter a word before her [Madame Bonamie], that tended in the least to establish a positive distinction of rank, in what we term the population of a country, whereby you fixed, as it were, the proper place of every individual, according to his birth, fortune, or talent, the offended laundress felt the cockles of her heart, good at all other times, begin to close, and a strange optical delusion instantly fell before her eyes, through which she fancied you looked like a a suddenly declared foe.

Is La Voye describing ventricle fibers compressing or internal passageways of the heart narrowing or both?

A firm and convincing conclusion about the original identity of "the cockles of the heart" is possible at this remove from the term's origin may be beyond our reach. One particularly tantalizing early instance of cockles is from the 1608 translation of Du Bartas referring to Jezebel's effort "To curl the cockles of her new bought head," where the cockles may be ringlets or crimps. Similarly ambiguous is Shakespeare's reference to "cockled snails" in [Love's Labour's Lost], where cockled may mean "turbinated" or simply "shelled."

In any case, it seems to me that interpreting "the cockles of the heart" as being suggested by the similarity between the shape of the bivalve cockle and a human heart (or the Valentine's Day representation of a human heart has a serious drawback—namely, that the expression is plural and subordinate to the heart ("the cockles of the heart"), not singular and equivalent to it ("the cockle that is one's heart," say).

To my mind, the most promising candidates for original meaning of "the cockles of [one's] heart" are (1) the ventricle fibers known to early anatomists as cochleae cordis, or (2) the hidden recesses of the heart, conceived of as chambers connected by a winding passageway. The crucial (and I think unanswerable) question is whether the first conception of "the cockles of the heart" was of a collection of ridges or striations (cochlea as "shell-fish," in Kersey's definition) on the outside of the heart or as a collection of hidden chambers or recesses joined by a winding passageway (cochlea as "sea-snail," in Kersey's definition) in the heart's mysterious interior.

Solution 4:

The answer to your questions,

  1. What is the origin of the phrase “it warms the cockles of my heart”?
  2. Why does a human heart have a ‘shell’?

depends upon the meaning of 'cockles'. Question 2, for example, seems to assume that 'cockles', in the phrase "it warms the cockles of my heart", means 'shells'. That is not necessarily so when speaking of the early uses of 'cockles'.

The earliest uses of 'cockle' (plural 'cockles'), "probably from Old English times" (OED), referred to a plant, Agrostemma githago (flower and seedpod containing seeds shown):

*Agrostemma githago* Cockle seedpod

The plant, now nearly extinct in the UK due to modern farming methods, was a common poisonous weed in corn and wheat fields in the UK, and its seeds (also known as cockle) contaminated those crops.

Although appearing much earlier than other meanings of 'cockle', and finding frequent allegorical and metaphorical employment in 15th-17th century sermons and other religious contexts, for obvious reasons this sense of 'cockle' is unlikely to be the meaning of 'cockle' in the phrase "it warms the cockles of my heart".

At the core of that extended phrase is another phrase, "the cockles of my heart". The earliest use of the core phrase that I could find was this from Eachard's Observations Upon an Answer [to the Grounds and Occasions of the Contempt of the Clergy and Religion] (1671; link is to the eleventh, 1705, corrected edition):

Now, although he says in his Preface, that he would not much boast of convincing the World, how much I was mistaken in what I undertook; yet I am confident of it, that this Contrivance of his, did inwardly as much rejoyce the Cockles of his Heart, as he phansies, that what I write did sometimes much tickle my Spleen.

Eachard's rejoinder to the critic of his Grounds and Occasions presents itself as jocose; it may, however, no less on that account be an ad hominem attack. Simply put, Eachard's response to "R. L.", in its mention of 'cockles', may suggest that "R. L." is given to drunken venery.

The second oldest meaning of 'cockle', which is perhaps the literal meaning behind figurative use in "cockles of my heart", refers to a type of mollusc, or simply the shell of that mollusc. The mollusc commonly called the 'cockle' is

heart-shaped [with] brittle shells [with] distinct radiating ridges.

"cockle." The Columbia Encyclopedia, 6th ed. Encyclopedia.com. 19 Jan. 2018 http://www.encyclopedia.com.

OED, following Dixon (see below), suggests the phrase "cockles of my heart" may derive from anatomist R. Lower's description, in Latin, of the ventricular fibers of the heart. This description was first published in 1669, two years before Eachard's use:

Fibrae...spirali suo ambitu Helicem, sive cochleam, satis apte referunt; ut Tab. 2. in Fig. 3.....

Lower's "Fig. 3", and a cockle shell:

Fig. 3 from Lower Cockle shell (Eulamellibranchia)

The resemblance of the spiraling ventricular fibers in Lower's figure to the "distinct radiating ridges" of the cockleshell is remarkable. That resemblance alone, however, does not explain how Lower's description (or the accompanying figure) provided the origin of the phrase used by Eachard.

For that connection, the usual explanation is something like the one advanced by J. Dixon in Notes and Queries, 7th series, volume 4, July 1887. Dixon's account (the first to connect Lower's anatomy with the phrase), though, raises its own set of questions:

"The cockles of the heart." — Mr. Smythe Palmer, in his 'Folk-Etymology,' s.v. "Cockle," says:
  "Cockles, in the curious phrase 'the cockles of the heart,' has never been explained. It occurs in Eachard's 'Observations,' 1671, 'This contrivance of his did inwardly rejoice the cockles of his heart' (Wright)."
  The phrase is never heard except as used jocosely. If one offers an old friend a glass of good wine one may say, "There! that will warm the cockles of your heart"; but the words could never be used seriously, either in conversation or in writing. Hotten's 'Slang Dictionary' (1864) calls it "a vulgar phrase," and so no doubt it would be if seriously used.
  In an anatomical work on the heart I have met with a passage which seems to give some hint as to the origin of the expression. Lower, one of the most eminent anatomists of the seventeenth century, in his 'Tractatus de Corde,' 1669, p. 25, speaking of the muscular fibres of the ventricles, says:
"Fibrae quidem rectis hisce exterioribus in dextro ventriculo proxime subjectae oblique destrorsum ascendentes in basin cordis terminantur, et spirali suo ambitu helicem sive cochleam satis apte referunt."
  The ventricles of the heart might, therefore, be called cochlea cordis, and this would easily be turned into "cockles of the heart." What we want is some quotation from a grave writer that will bridge over the gap between the cochlea cordis of the anatomist and the phrase "cockles of the heart" used jocosely, as, for instance, by Hood:
    To cure Mamma another dose brought home
    Of Cockles; — not the cockles of her heart.

Dixon's observation concerning the invariably jocose early use of the phrase agrees with the evidence, not only from early uses in general, but from early lexicographers. For example, a general use in Thomas Brown's Letters from the Dead (1730):

Suppose the vitious eyes of a great man are fix'd upon me, and my charms should kindle a love-passion in the cockles of his heart; he writes, chatters, swears and prays, according to the custom in such cases, I still defend the premises, by a flat verbal denial; but at the same instant incourage him in my looks, and am always free to oblige him with my company; till by this sort of usage I make him sensible downright courtship will never prevail; and that the cittadel he besieges is not to be surrendered without bribing the governess....

And the 1788 edition of Francis Grose's A Classical Dictionary of the Vulgar Tongue includes this definition of the phrase as used in cant:

Cockles. ... Cant. — This will rejoice the cockles of one's heart; a saying in praise of wine, ale, or spirituous liquor.

Both Brown's "kindle a love-passion in the cockles" and Grose's connection of rejoicing "the cockles" with alcohol suggest that the sense of the phrase may refer not so much to the cockleshell as to the cavity within the cockle, a cavity which may be filled with a "love-passion", or alcohol, or the warmth inspired by alcohol.

Indeed, another sense of 'cockle', first attested by OED from 1688 but undoubtedly in use before that, is the "fire-chamber or furnace of a hop or malt kiln", which perhaps derives from "16th cent. Dutch kākel, kaeckel, kāchel" (OED).

Yet another early sense of 'cockle', first attested from the 16th century in OED, is an "uneven place, pucker, or bulge on what ought to be a flat surface". This meaning appears to be expressed in two later dictionary entries for "cockles" and "cockles of the heart":

Cockles (popular), more a vulgarism than slang. Literally the wrinkles.

In Bermondsey not long ago there lived a little dame;
She was the cockles of my heart, and Nancy was her name.

A Dictionary of Slang, Jargon and Cant, 1889-90, Albert Barrere and Charles Godfrey Leland.

Cockles, subs. (venery). — The labia minora.

Slang and its Analogues Past and Present, 1890-1904, John Stephen Farmer and William Ernst Henley.

Farmer and Henley's Slang and its Analogs also includes an entry for the phrase "cockles of the heart" which refers to it as a "jocose vulgarism", then cites for its origin the observations of Dixon in Notes and Queries, 1887 (see foregoing).

These then are the three likely accounts competing to explain the origin of the phrase "cockles of my heart":

  1. It arose from the resemblance between the heart's ventricular fibers and the radiating ridges on the shell of a variety of mollusc commonly known as the 'cockle'. As an origin, that resemblance between the heart's fibers and the ridges on the cockleshell may have been reinforced by the heart shape of the mollusc.

  2. Use of the word 'cockle' to describe "a wrinkle or pucker" may have contributed to the influences described in the account relying on the resemblance between the heart's fibers and the ridges on the shell of a cockle, or may independently explain the origin of the phrase. Considering the invariably vulgar, jocose use of the phrase, the "wrinkle or pucker" sense of 'cockle' seems more likely to be an independent than contributory origin of the phrase.

  3. Either alone or in combination with the influences described in the first two accounts of the phrase origin given above, the sense of 'cockle' (possibly) deriving from Dutch kākel, kaeckel, kāchel, that is, "fire-chamber or furnace of a hop or malt kiln", may have sponsored use in the phrase "cockles of the heart".

Of the three accounts, the first has received the most attention and so is most convincing. It, however, still suffers from Dixon's complaint that it lacks "some quotation from a grave writer that will bridge over the gap between the cochlea cordis of the anatomist and the phrase 'cockles of the heart' used jocosely". That quotation from a "grave writer" remains to be discovered.