Leonardo da Vinci - the anatomist (1930) 12

From Embryology
Embryology - 15 Aug 2020    Facebook link Pinterest link Twitter link  Expand to Translate  
Google Translate - select your language from the list shown below (this will open a new external page)

العربية | català | 中文 | 中國傳統的 | français | Deutsche | עִברִית | हिंदी | bahasa Indonesia | italiano | 日本語 | 한국어 | မြန်မာ | Pilipino | Polskie | português | ਪੰਜਾਬੀ ਦੇ | Română | русский | Español | Swahili | Svensk | ไทย | Türkçe | اردو | ייִדיש | Tiếng Việt    These external translations are automated and may not be accurate. (More? About Translations)

A personal message from Dr Mark Hill (May 2020)  
Mark Hill.jpg
I have decided to take early retirement in September 2020. During the many years online I have received wonderful feedback from many readers, researchers and students interested in human embryology. I especially thank my research collaborators and contributors to the site. The good news is Embryology will remain online and I will continue my association with UNSW Australia. I look forward to updating and including the many exciting new discoveries in Embryology!

McMurrich JP. Leonardo da Vinci - the anatomist. (1930) Carnegie institution of Washington, Williams & Wilkins Company, Baltimore.

   Leonardo da Vinci (1930): 1 Introductory | 2 Anatomy from Galen to Leonardo | 3 Possible Literary Sources of Leonardo’s Anatomical Knowledge | 4 Anatomical Illustration before Leonardo | 5 Fortunes and Friends | 6 Leonardo’s Manuscripts, their Reproduction and his Projected Book | 7 Leonardo’s Anatomical Methods | 8 General Anatomy and Physiology | 9 Leonardo’s Canon of Proportions | 10 The Skeleton | 11 The Muscles | 12 The Heart | 13 The Blood-vessels | 14 The Organs of Digestion | 15 The Organs of Respiration | 16 The Excretory and Reproductive Organs | 17 The Nervous System | 18 The Sense Organs | 19 Embryology | 20 Comparative Anatomy | 21 Botany | 22 Conclusion | References | Glossary of Terms | List of Illustrations
Historic Disclaimer - information about historic embryology pages 
Mark Hill.jpg
Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Leonardo da Vinci - The Anatomist

Chapter XII The Heart

The Arabistic idea as to the structure of the heart prevalent in Leonardo’s day was to the effect that it was composed of three ventricles, two larger ones and one which is in the middle, as it were, and serves as —

“a receptacle for nutriment in which is produced a strong condensation similar to the substance of the heart itself and as a storehouse for the spiritus generated in it by the subtle blood. And between the two are passages like openings.”

Such are the words of Avicenna as translated from the Latin version of Gerard of Cremona. This median ventricle is a myth, probably having its origin in an attempt by Avicenna to harmonize the descriptions of Aristotle and Galen. The former recognized three ventricles, two of which undoubtedly correspond to what we now term the right and left ventricles, while the third was probably the left atrium, the right atrium being regarded merely as a portion of the vena cava, this vessel, therefore, opening directly into the right ventricle. Galen on the other hand admitted only two ventricles, and the atria were regarded as of minor importance, practically as portions of the vena cava and pulmonary veins respectively, but the septum between the ventricles was supposed to be perforated by minute pores through which some of the blood in the right ventricle could pass into the left.

Avicenna followed Galen in giving the atria a subordinate importance, speaking of them as ear-like, sinewy, rugose attachments of the ventricles, but felt bound to recognize Aristotle’s authority in the matter of the ventricles and consequently magnified Galen’s septal pores into a third median cavity. Thus it was, apparently, that the misconception of the third ventricle became established in mediaeval anatomy. Mondino follows Avicenna fairly closely, disregarding the atria completely, but stating that the third or median ventricle is not a single cavity but many, thus making it undoubtedly equivalent to Galen’s pores. In one of the editions of Mondino, that published at Strassburg in 1513, the editor, Iohannes Adelphus, adds as a commentary to the description of the heart a diagrammatic representation of the organ, in which he endeavors to picture Avicenna’s idea (fig. 43). The atria are omitted and the third ventricle is obtrusively shown. Openings a, b, c and d in the walls of the major ventricle represent respectively those for the aorta, pulmonary vein, i.e., the left atrium, pulmonary artery and vena cava, i.e. the right atrium, while groups of small circles represent the valves guarding the adjacent openings.

In its finer structure Galen considered the heart to be composed of fibers arranged longitudinally, transversely and obliquely, but these fibers were not muscular, though they greatly resemble muscle fibers.


•r ittribcnie cttxtm a potman* ab car.tfufua (cffrofa fat perfect* daubunC/fcne tunic* vnica/qt nature pap follici tacftb**oquobper ipfum tranfif.

€ qp0*necbdid/p*rctoeoaficifitrabffcot fantfuin? ab cpatc/'t m£bat ab otatncbta.ClaubifboracjpuIOonie'i apcnf bo:a btlafat5ie.i£tu6 fcofliola apcriunf ab cjrtra ab (nrtis/? imperfect* clatiburuur.

flDene arterialie que potrat fanguin? ab pulmanettt a cotbe: art male f c$. 0 uap tu ntcap jjpter accc ffu$ ei*ab membp pritiui matua.iStq: poztat fanguinc coIcricO rat be fubtilc:eiue bof^ota apertunf cb inrue ab tjet ra/? cku bunfccducrfo pcrfrctc.pcr tjocojificiu coxtma (t erpdht £014 pf?ri«ot 6 /'i ntb»I rennet bora bilatatSie. #op buop Arterierenatie rencarterialie/prranu b$ (Balie.pifjrba rnliratepticulanu ir.ca.^tbe uiuamctie m?b:o?.r(f.c»

tbt<Ixrm f a© lunrp^.cap.prtmo. Fig. 43. Diagram of structure of heart in Ioannes Adelphus’ edition of Mondino’s Anathomia (Strassburg, 1513). After C. Singer, Fasciculo di mcdicina, vol. 1,

fig. 59, 1925.

He regarded the muscles as being organs of voluntary motion, but the contraction of the heart was involuntary; furthermore, it was rhythmical and continuous, and its tissue differed from muscle in its firmness, in the threefold arrangement of its fibers, in its color and in its taste. It was therefore not muscle, but a tissue sui generis. Avicenna adopted these views in their entirety, claiming that those were wrong who regarded the heart as muscular. His object was an abstract of Galen’s teaching and consequently he omits the arguments upon which this conclusion rests, but he calls attention to the firm, almost cartilaginous, tissue at the base of the ventricles. Mondino omits all reference to the minuter structure of the heart.

Physiologically the mediaeval conception of the heart was essentially that of Galen, whose ideas may be thus summarized. The heart was endowed with a pulsative faculty which acted through the fibers of which it was composed, the longitudinal fibers by their contraction broadening the organ and so producing the condition of diastole, during which blood was attracted to the ventricles, while the transverse or circular fibers elongated it and produced systole, expelling the contents. The oblique fibers were retentive in function, producing the condition of peristole, during the occurrence of which the heart was utilizing the material that had been attracted. The blood, concocted in the liver, was carried to the right ventricle by the vena cava and also to the various organs of the body, supplying them with nourishment, the pulmonary artery serving to carry nourishing blood to the lungs and also certain fuliginous vapors, engendered in the heart by the innate heat, which had its center in that organ. Some of the blood contained in the right ventricle passed through the pores of the ventricular septum and in the left ventricle mingled with the pneuma, which, manufactured from the air inspired by the lungs, is conveyed to the left ventricle by the pulmonary veins. The blood of the left ventricle with its contained pneuma or spiritus is thence distributed to the body by the aorta, carrying to the various organs the energy necessary for the performance of their functions. There could be no question of a circulation of the blood with such a theory. So far as its function was concerned the venous blood was quite distinct from the arterial, and while the two might intermingle through the pores of the ventricular septum or through anastomoses, which Galen supposed occurred in the organs, the blood in both veins and arteries merely underwent an ebb and flow, a flux and reflux from and to the right and left ventricles.

These were the views that Leonardo had to guide his observations and our task now is to ascertain how far he "was governed by them or how far on the basis of his own observations he was able to modify or discard them. The heart was evidently one of the organs whose mechanism especially interested him and his admiration for its efficiency led him to write beneath one of his illustrations of it — a very imperfect illustration at that — the words “Wonderful instrument invented by the Supreme Master” (AnB, 12). Many pages are devoted to illustrations of its structure, indeed the majority of those that have been assembled in QII are concerned either with its structure or action, and the text on several pages is arranged in paragraphs with headings, as if intended for incorporation in the prospective book. Many of his observations were undoubtedly made upon the hearts of animals, probably those of bullocks for the most part, though an observation on a pig’s heart is also recorded (QI, G). His figures show the form of the heart, both isolated and in place, and the internal structure of the ventricles is represented by dissections and both longitudinal and transverse sections.

Special methods, suggested as possible aids in studying the movements of the heart and blood, have already been mentioned in Chapter VII (p. 90, el seq.).

The location of the heart is stated to be midway between the brain and the testicles (QIII, lOv). Usually it is represented in an oblique position, inclining downward and to the left; only in a few cases in which it is shown (fig. 44) isolated is it given a vertical position (QII, 1, 2, 3v for example). It is suspended from the middle of its base by the two blood-vessels, the inferior vena cava and aorta (QII, 17), and throws itself toward the left side and is then suspended by the left vessel (QII, 15); its obliquity is explained on the Galenic principle of the arrangement of the organs so that there will be a proper adjustment of weight on either side of the middle line. The liver lies chiefly upon the right side and its weight is counterbalanced by the heart and spleen on the left; part of the heart is, indeed, to the right of the midline, but this is counterbalanced by the greater weight of the stomach being to the left (QII, 15). In another passage (QII, 17v) the obliquity is directly ascribed to the greater weight of the right ventricle, this ventricle containing more blood than the left and on the verso of the same folio it is argued that the greater thickness of the wall of the left ventricle is not a provision for counterbalancing the greater weight of the right, since such counterbalancing is unnecessary in four-footed animals and in man when lying down. But, Leonardo enquires, how is it with the bat who sleeps suspended upside down?

The position of the apex of the heart is supposed to be somewhat higher in the cadaver than it is in the living body, since, at death the heart enlarges and shortens, because its transverse muscles relax and the longitudinal ones contract (QI, 4v). Leonardo does not take into consideration the weight of the liver in the recumbent cadaver pressing upon the diaphragm, now devoid of tone, and so forcing the thoracic organs to a slightly higher level. The conclusion, however, that the heart is a little shorter and therefore a little higher in the cadaver is borne out by certain interesting observations made on pigs (see p. 91).

It is noteworthy that neither in his figures nor his text does Leonardo take definite cognizance of the atria of the heart. He figures in some cases (QII, 3v, 4) the auricles (fig. 44) and speaks of them in various passages as orecchi or addilamenti del core , this latter term recalling



Fig. 44. Two figures of the heart. (QII, 3v.)


Fig. 45. Dissection of the heart showing papillary muscles and a moderator band. (QII, 14.)


their Avicennian description; but of the more important atria never a word. As a consequence, in endeavoring to interpret his idea of the structure and action of the heart one is confronted with the alternatives, did his orecchi include the atria as well as the auricular appendages, or were the atria for him, as for Galen, merely the basal portions of the venae cavse and pulmonary veins? Some of his illustrations incline one toward the latter view, especially those on QV, 1 ; QIV, 7 (fig. 45) and QIII, lOv, in which the venae cavae seem to unite to open directly into the right ventricle, while the left pulmonary veins open directly into the left ventricle, those from the right lung uniting with the venae cavae. Furthermore in speaking of the passage of blood from the liver, the source of the blood, to the heart, he describes it as passing directly to the right ventricle; indeed, with Leonardo’s views as to the movement of the blood, he could hardly describe it otherwise. The figures mentioned above probably belong to the earlier period of his anatomical studies and, as such, should not be taken as representing accurately his final views; it is noteworthy that in these figures not even the auricles are shown.

But in looking through the folios one finds evidence pointing strongly to the first alternative. His theory of the movement of the blood calls for somewhat capacious atrial cavities, since they must be able to accomodate a considerable portion of the blood expelled from the ventricles during their contraction. As soon as he had conceived this idea he began to represent the auricles as circular or spherical cavities, situated upon the base of the ventricles and communicating with them by the atrioventricular openings as in QI, 3, 3v, and QII, 17, while on QII, 14 (fig. 45) he shows the two vense cavse opening into the auricle. Unfortunately the text on the page which shows this last figure, the nearest approach to accuracy in the representation of the atrium, contains no reference to the figure and nothing anatomical; it is a eulogy of mathematics and a tirade against human folly. The auricles having thus gained some importance, Leonardo proceeds to exalt them still higher, speaking of them as ventricles, maintaining that the heart has four ventricles, two upper ones which are the auricles of the heart and two lower which are called the right and left ventricles (QII, 17v). ] In another passage essentially the same statement is made and supported by arguments against supposed objections, the lower ventricles being now described as being in the substance of the heart, while the upper ones are outside it (QI, 3). It seems clear from these passages and figures that Leonardo’s auricles or upper ventricles included both the auricles and atria as now recognized, and this identification is confirmed by QII, 11, on which is recorded an observation of a patent foramen ovale, placing the right orecchio in communication with the left, and a memorandum is made to see whether it is the same in the orecchi of other hearts. The accompanying figure shows that Leonardo had observed the atrial septum and therefore the atria, but under the influence of Avicenna these were regarded as of less importance than the auricles, w T hose irregular inner surfaces are indicated in all the representations given by Leonardo of his upper ventricles. As will be seen later, these rough and irregular surfaces were essential to his ideas of what took place in the heart.


1 In passing it may be noted that Holl’s (1913) suggestion that Leonardo was the first to apply the term ventricles ( ventricholi ) to the lower chambers of the heart can not be accepted.


The upper ventricles or orecchi, using Leonardo’s term for them to avoid confusion, possess dilatable and contractile walls (QII, 3, 3v) composed of muscle and fleshy membrane (QI, 3). All the ventricles are muscular, but in the extrinsic 2 ones, i.e. the orecchi, the muscles of the walls form a continuous coating (QII, 3v) in which, it is supposed there are both transverse and oblique muscles (QI, 4), although no fibers are visible. This absence of fibers permits the walls to readily extend lengthwise (QI, 3). The interior of the orecchi (QI, 4v) is described as being cellular (QI, 4; QIV, 13v), the cells being evidently what are elsewhere termed the cavernous depressions between the muscles that contract the orecchi (QI, 4v), that is to say between the mm. pectinati. These are not described in detail, but they are said to be of the same nature as the mm. papillares of the ventricles (QII, 3v).

Of the two ventricles, i.e. Leonardo’s lower or intrinsic ventricles, the right is said to be larger (QI, 3) and heavier than the left (QII, 17v), its greater weight being due to the greater amount of blood contained in it (QI, 3v; QII, 17). Its length, however, is only three-quarters that of the entire heart (QII, 3v), a statement which may be interpreted to mean that Leonardo recognized the fact that the apex of the heart is formed entirely by the left ventricle. He notes the greater thickness of the wall of the left ventricle (QII, 11, 17) and represents it in figures of transverse sections through the ventricles (QII, 4v). Judging from a passage on AnB, 33, he regarded this greater thickness as a provision whereby the greater heat of the blood in the left ventricle might be resisted; he disputes the possibility of its being intended as a counterpoise for the greater weight of the right ventricle (see p. 152). It is stated that the right ventricle at its base has only one-quarter the thickness it has at the apex (QII, 3v).

The muscle fibers of the ventricular walls are arranged in the longitudinal, transverse and oblique directions, the longitudinal, in accordance with ancient tradition, serving the attractive faculty of the ventricles by causing their dilation, while the transverse and oblique contract the ventricles and therefore subserve the expulsive faculty. The columnse carnese on the inner surfaces of the ventricles are indi s This term is borrowed from Mondino who describes the auricles as extrinsic and the ventricles as intrinsic parts of the heart.


THE HEART


155


cated in a figure on AnB, 12, and the papillary muscles, together with the chordae tendineae that are attached to them, are frequently figured and mentioned (fig. 47). The papillary muscles are grouped with the mm. pectinati as musscholi intrinsici to distinguish them from the musculature of the walls, musscholi estrinsici (QII, 3v) 1 * 3 and usually but a single papillary muscle is shown in the right ventricle, the smaller ones being disregarded, and two in the left ventricle (QII, 14, 23, 10), but sometimes five or six are represented in the right ventricle (QII, 12) and three or four in the left (QIV, 14). They are described as being very hard, almost like cartilage (QII, 12), and in one note they are described as "the first cause of the motion of the heart” (AnB, 12) and in another it is stated that by their great density they prevent the complete closure of the ventricles during contraction and so afford space in the left ventricle for the blood that passes to it from the right ventricle during systole (QII, 4v). But Leonardo’s chief interest in the papillary muscles was in connection with their action on the atrioventricular valves through the chordae tendineae. These he notes are attached to the under surfaces of the valves (QII, 3, 8v). The effect had by the papillary muscles upon the valves is, however, stated somewhat obscurely and at first reading his statements seem to imply that he supposed the chordae to push the valves together, an idea absolutely contradictory to his axiom that it is the function of muscles to pull not to push (AnB, 29). Comparison of several passages dealing with the matter serves to explain Leonardo’s idea without violation of his axiom. When a papillary muscle contracts it shortens, drawing its ends toward its middle (QII, 23) ; 4 consequently, when contracted, the action of the papillary muscles would be to draw the valves down into the ventricular cavities and so open the orifice which they guard. But the valves close during the systole of the ventricles and therefore the papillary muscles can not be contracted at that period of the heart’s cycle, but must alternate in their contraction with that of the ventricular walls. When the ventricles contract the papillary muscles relax and elongate, allowing the impetus of the blood to bring the valve cusps into contact with one another, and when the ventricles dilate the papillary muscles contract and the atrio-ventricular orifices are opened (QII, 17, 17v, 23). A pretty theory when the available evidence is taken into consideration, but, unfortunately for it, the present-day evidence indicates a simultaneous contraction of the papillary muscles and the muscles of the ventricular walls.

1 Holl (1913) is in error when he interprets these terms as applying to the musculature

of the ventricles and atria respectively. True, Leonardo speaks of these chambers as ventricholo intrinsico and v. estrinsico, but in applying these terms to the muscles he

clearly means by intrinsici the papillary and pectinate muscles, as distinguished from those of the heart walls.

4 It was perhaps this idea that led Leonardo to represent a papillary muscle with chordae at either extremity as on AnB, 12.


156


LEONARDO DA VINCI — THE ANATOMIST


To complete his theory as to the movement of the blood, Galen found it necessary to assume direct communication between the right and left sides of the heart, and consequently taught that the septum of the ventricles was perforated by minute pores through which the blood might pass. Coming down to mediaeval times with all the authority of Galen behind it, this idea was blindly accepted and even Leonardo succumbed to it, letting theory master observation. Again and again he speaks of these pores (QI 3, 4, 4v; QII, 12, 17v; QIV, 11) and in two passages (QII, 4v, 10) bestows upon the septum the term "sieve ( cholatorio ) of the heart.” And not only so, but on QII, 3, he represents a portion of the septum, showing its irregular surface and, on the cut edge of it, pores that do not exist! But while in this instance Leonardo allowed tradition to master observation, the latter led him to the discovery of the moderator bands now recognized as important structures in the distribution to the ventricular musculature of the contractile stimulus. On QII, 11, 17v; and QIV, 13 (fig. 45), he shows the band of the right ventricle, probably that of a bullock’s heart, extending across the cavity of the ventricle from the septum to the outer wall at about onequarter the distance from the base of the heart to its apex (QIV, 13). In other figures a greater number of bands are shown schematically (QIV, 7; and QIII, lOv), as many as three in the right and five in the left ventricle (fig. 46). Leonardo speaks of the band as the catena (QIV, 13) 5 and suggests that the action of the bands is to prevent over-dilation of the ventricles, whereby they would attract to themselves too much blood and so become overheated (QII, 4v).

Leonardo gives two representations (QII, 10; and QIV, 14) of the base of the heart with the atria and great vessels removed, both of them decidedly diagrammatic (fig. 47). The aortic and pulmonary orifices are shown, both guarded by their semilunar valves, and in the figure on QII, 10, the atrio- ventricular openings are also shown, with the valves that close them. On QIV, 14, however, while the atrio-ventricular orifices are shown, their valves are not clearly represented. It is evident that Leonardo recognized all four orifices of the base, the right atrio-ventricular and that for the pulmonary artery (arteria venalis) opening into or from the right ventricle and the left atrio-ventricular and the aorta (arteria aorta) opening into or from the left ventricle. Some discussion has been aroused as to the correctness of Leonardo’s identification of these orifices, or rather as to the interpretation of a statement on QII, 2v, in which it is said that —

“The right ventricle has two orihees, one for the vena aorta. . . The other orifice is for the arteria venalis and leads from the heart to the lungs.”

Roll (1912) has suggested that the bands be called the Leonardo da Vinci muscle trabecula; in honor of their discoverer.


WX.TI-' <**'"*'■•* I"'."’ ’

rHtrti *


\**i\t t" *"*

a * y«) '*


A I »#•* «*v

g«TJ*^

) *TH


j »^>V, *f

iK-- r r*


Fig. 46. The thoracic and abdominal viscera, the heart dissected and showing several moderator bands in each ventricle. (QIV, 7.)


V »

. CO > Ah


<1


v^v'/l

j *«, ^7^ '/If o

i '» .. J -**?-> ■ . i r- i H 'f '/ rr

'""SZV "T"T~ T

... w -.w i-r.o<r i




.«/.r


./ *


I . W/T >.


0 )


X /



2/.



lift- 47. Sketches of base of heart and of papillary muscles and chorda? tendinea? of

left ventricle. (QIV, 14.)


THE HEART


157


The Editors of the Quaderni interpreted ‘‘vena aorta” to be a lapsus calami for vena arterialis (pulmonary artery), an interpretation which made the passage imply that both the pulmonary artery and the pulmonary vein (arteria venalis) opened into or from the right ventricle. This was pointed out by Boruttau (1913), who maintained that by “vena aorta” Leonardo really meant the aorta and that he was speaking of the left ventricle and not the right. The Editors of the Quaderni suggest that since there is a gap in the passage, as if there had been some interruption in the writing of it, it is possible that Leonardo started to speak of the right ventricle, but on resuming his writing after the interruption forgot this and went on to speak of the left ventricle. This suggestion is ingenious, but Boruttau’s explanation seems more probable, since Leonardo in more than one other passage confuses right and left, an error, as Boruttau points out, to which his habit of looking-glass writing may have made him especially prone. As to the difficulty in the way of identifying the arteria venalis with the pulmonary veins, Leonardo’s disregard of the atria may be recalled, the left atrium being regarded as part of the pulmonary veins.

The valves that guard the orifices of the heart had long been known and their use determined, and they forcibly attracted Leonardo’s attention, many folios containing memoranda in pen and pencil concerning their form and structure and especially their action. He terms them usscioli or pannichuli, sometimes porte, although it is not always possible to determine whether by this last term he meant the valves themselves or the openings they guarded. It is curious that in considering the atrio-ventricular valves he almost entirely neglects the mitrals and, on the other hand, it is the aortic semilunars upon which he concentrates his attention. The mitral valves are shown closing the left atrio-ventricular orifice in two figures (fig. 47) on QII, 10, and probably in another on QII, 11, but no mention is made of them in the text. In the figures the intermediate cusps are almost as large as the main ones, a condition that may be partly explained by the valves being seen from above, closed, and partly on the supposition that the drawings were made from a bullock’s heart.

Of the tricuspid valve the account is much fuller. It is shown closed (fig. 48), from above and below (QII, 8v), and the same page shows its three cusps spread out flat with their papillary muscles and chordse. These papillary muscles are shown alternating with the cusps and sending chordse tendinese to both the adjacent cusps; this does not agree with the arrangement in the human heart, where there is usually one large principal muscle and one or two smaller, and again it may be supposed that the preparation was made from an animal heart. With regard to the structure of the valves, Leonardo makes statements (QII, 3, 8v) which are difficult to understand. He says that each papillary muscle is divided into two muscles which are continuously


15S


LEONARDO DA VINCI — THE ANATOMIST


in contact, and then each of these divided into its branches, composed of chorde nervose covered by very delicate flesh ( sottilissima chartiositd), and finally becomes converted into a pannichulo nervoso, also covered by flesh. It would seem that by his sottilissima charnositd Leonardo meant the endocardium. Then he goes on to say —

“But just as the muscles whence they arise cover each other, so too do the chorde and pannichuli. But their movements in drawing the chorde and extending the pannichuli are not equal, since the superior membranes ( pan nichuli) have more movement than the inferior, because in extending they cover in great part the inferior chordae, before that they form their pannichulo; and this the inventor has done, for the cause shown in the figure above, which shows that this author does nothing superfluous or defective.”

All this is very difficult, nor does the figure referred to help in understanding it, but it apparently leads to the conclusion (QII, 8v) that the valves of the right ventricle are not entirely double, since this would necessitate their having their thickness where this would be unnecessary, and where it was necessary only a double thickness. “In this case Nature would fail in its law.”

The semilunar valves are shown in position in the pulmonary aorta on QII, 3v (fig. 44), and in both arteries on QII, 10 (fig. 47); they are shown separately, both open and shut, on QII, 4; from both above and below on QII 9v; and from above and open on QIV, 12. This last figure is interesting because it shows the walls of the aorta bowed out opposite each valve to form what are now termed the sinuses of Valsalva, to which Leonardo assigned great importance in connection with the mechanism of the closure of the aortic opening. In discussing this he frequently speaks of the hemicycles (QII, 13v; QI, 11, llv, 12v) or semiventricles (QIV, llv), meaning thereby the hemispheres formed by the individual valves together with their respective sinuses. Holl (1913) identifies the hemicycles with the valves alone, but the identification given above is to be preferred, since in one passage (QIV, 11) Leonardo states that the tissue of the wall of the heart where the three semilunar cusps are attached swells when the left ventricle dilates, and in another passage he locates this swelling tissue in the base of the hemicycles.

“Very powerful is the heart in its dilation, and it would be apt to draw the valves backward, were it not that the base of the hemicycles dilates itself like the flesh of the tongue.” (QII, 13v.)

So confident is he of the swelling of this tissue and its ability to prevent regurgitation of blood from the aorta, that he questions whether Nature might not have done without the valves, and relied solely on this swelling (QIV, 11), forgetting for the time his law that Nature never makes anything superfluous.


\J ** •


¥

■C'

n


if ir :

^,i j > **


it


.A


\ . . . 4$ u 4 rH ' 1 w

j .‘.vA’ &jr; •"■■/ 3 a, y

i-. -y * /I


(ri


\ ■

■ •*


V •*-< v , »‘ ’ .

> '. • d?.<


’ . %v


^|li . - :• « J?

.>4

>- /i »

<* ; <* {» t

■t* *■ j-?

«« . •***



I - *

■*» f

• <*

( , »*.« 

V


\ H


•/ 1


/


% ?


\


■ ‘ Vi . .. '**f

' v .I*-* *


i- V • » A

- t> , Sr /

-'V^/ v < * c .


&


<• ■/>


■ ■-./■ A, > » / w


f 5 ! •



1 '*



"V

S i

tf$w : r®*-in

«?'. •'Tf fp* ; . -> /* 4 '.'•- .

f^p:r



•> V



C j


Fig. 48. Tricuspid valve from above and from below, showing attachments of chordae tendinese. (QH, 8v.)







THE HEART


159


On QIV, 14v, in connection with a figure representing the base of the heart with the atria and great vessels removed, it is stated that the base has a somewhat triangular outline, and that the angles between the cusps of the aortic semilunars are opposite the angles of the base, and the cusps themselves are opposite the surfaces of the heart. Leonardo discusses the question why there should be three semilunars instead of four (QIV, 12) and endeavors to show mathematically that in vessels of a given circumference three triangular cusps will have greater strength than four quadrate ones. The embryological explanation, deriving the semilunars from a four-cusped valve in the aortic trunk, the longitudinal division of the trunk to give rise to the basal portions of the aorta and pulmonary artery dividing each of the lateral cusps into two and so giving three cusps to each of the arteries, was not discovered until long after Leonardo’s time.

So much for Leonardo’s knowledge of the gross structure of the heart; a statement as to his ideas concerning its minuter structure is now called for. On QII, 23 he enumerates the various elements or, as we should call them, tissues that enter into the composition of the heart, illustrating the occurrence of each in a diagram. The tissues mentioned are bone, cartilage, membrane ( paniculo ), nerve (i.e. tendon), muscle and nerve; each of these may be considered in turn, and first as to bone. He represents it on QII, 10, as an elongated plate in the vicinity of the left atrio- ventricular orifice. It is well known that normally no bone occurs in the human heart, but an os cordis does occur in many of the larger mammals, the ox, for example, in the fibrous tissue ( trigonum fibrosum ) at the base of the heart between the two atrio-ventricular orifices. Leonardo’s mention of it is, accordingly, further evidence that he based his account of the heart largely upon the study of bullocks’ hearts. It is evident from his diagram that by cartilage he meant the dense fibrous rings that surround the openings at the base of the heart, by paniculo, the membrane forming the valves; and by nervi, the chordae tendineae.

It has been pointed out that Galen regarded the main substance of the heart as something different from muscle, as something sui generis, and that this view was accepted by Avicenna. Leonardo discards this idea completely and consistently regards the heart as a muscular organ, its fibers in action obeying the same laws as other muscles.

“The heart in itself” he says “is not the beginning of life, but it is a vessel made of dense muscle, vivified and nourished by the artery and vein, as are the other muscles.” (AnB, 33.)

In another passage it is stated that the heart is the strongest of all muscles (G, IV). This was an important step, a long stride, indeed, toward the correct understanding of the physiology of the heart and it naturally led to an inquiry into the nerve supply of the heart, since if it


100


LEONARDO DA VINCI — THE ANATOMIST


were a muscle it should, like other muscles, receive its stimulus from a nerve. On AnB, 33v Leonardo states that he had traced the left vagus nerve to the pericardium and was inclined to believe it to be the nerve of the heart, and elsewhere (QIV, 7) he makes a memorandum that this matter was to be pursued further, in words that are worthy of quotation. He says —

“Do not abandon the reversive (vagus) nerves until the heart and see if these nerves give movement to the heart or if the heart moves of itself; and if the movement comes from the reversive nerves that have their origin in the brain, then it will be clear to you that the soul ( anima ) has its seat in the ventricles of the brain and the vital spirits have origin in the left ventricle of the heart. And if the movement of the heart springs from itself, then you will say that the seat of the soul is in the heart and similarly that of the vital spirits. Consequently attend well to these reversive nerves and also to the other nerves, since the movement of all the muscles arises from these nerves, which, with their ramifications, penetrate the muscles.”

The probability that Leonardo had observed the endocardium, so far at least as it covered the papillary muscles, the chordae and the atrio-ventricular valves, has already been noted (p. 158).

Of the pericardium, Leonardo gives no representation and in the text it is merely mentioned, without description. It is termed the capsule of the heart (QI, 3v), the case that invests the heart (AnB, 17, 33v). On QI, 3v it is said to contain air, which it receives from the lung, and on AnB, 17, it is stated that the diaphragm —

“pressing upon the pericardium forces up the small amount of fluid ( omore ) that is at the bottom of it and so continuously, by so bathing it, moistens the heated heart and prevents it from being dried up by its so great movement.

In the endeavor to ascertain Leonardo’s ideas as to the movement of the blood through the heart, one meets with many difficulties. A complete statement of the movement is not given; certain portions of it are treated in exlenso with much repetition, others are barely mentioned and others entirely neglected, so that there is no continuity and one must piece together the complete story as best one may. It is clear, however, that Leonardo’s concept was essentially that which had been handed down through the centuries from Galen. He elaborates Galen’s theory in some particulars, but a statement of the views of the older author gives a basis for understanding those of the later one.

Galen held that the veins had their origin in the liver. The chyle passed from the stomach and intestines to the concave surface of the liver by way of the portal vein, and in the liver was concocted into blood, certain impurities being removed from it. It then passed from the convex surface, the gibbosity, of the liver by way of what is now termed the hepatic vein, but was regarded by Galen as the beginning of the great vein ( vena magna or vena cava). This quickly divided into two trunks one of which descended to supply the abdominal viscera,


THE HEART


161


forming what we now term the vena cava inferior, while the other ascended through the thorax to the neck and head, giving off a branch to the heart as it passed. This cardiac branch was what is now recognized as the right atrium of the heart and the remainder of the branch is the vena cava superior. Only a small portion of the blood formed in the liver passed to the heart, the greater portion of it being distributed directly to the various organs for their nutrition. Of the blood which reached the right ventricle a portion passed by way of the arterial vein (pulmonary artery) to the lungs for their nutrition, carrying with it certain fuliginous vapors formed in the heart, to be expelled by the lungs. Another portion passed through minute pores in the septum of the heart into the left ventricle, where it met with vital spirits concocted in the lungs from the air and carried to the heart by the venous artery (the pulmonary vein), whose basal portion was the left atrium, whence it is always spoken of in the singular. Under the influence of the innate heat resident in the left ventricle, the blood that had passed through the septal pores and was subtilized by its passage, became imbued by the vital spirits, and, by the contraction of the ventricle, was forced out by way of the aorta to give energy to the various organs.

Such, in brief, was Galen’s conception of the movements of the blood, and two points in his theory may be emphasized in view of what is to follow, namely, (1) the idea that the venous and arterial blood had entirely independent functions, the one supplying nourishment and the other energy to the organs, and (2) the existence of innate heat in the left ventricle, heat implanted in it from the beginning, not developed in it, and of such intensity as to require the cooling effect of respiration to keep it in subjection.

In exposing Leonardo’s views one may start with the right ventricle in diastole and filled with blood. When it contracts the contained blood may escape in any one of three directions. A portion of it, before the tricuspid valve can close, will be forced into the right auricle (■ orecchi ), forcibly dilating it (QII, 4v, 8v); after closure of the valves a portion passes through the minute septal pores into the left ventricle (QI, 3v, 4, 4v; QII, llv, 13v), and a third portion passes to the lung by way of the vena arterialis (pulmonary artery) (QII, 17v). The blood sent to the auricle can not be very great, for it is only that which does not come in contact with the cusps of the valves (QII, 8v), that is to say, that which lies between the cusps at the beginning of the contraction of the ventricles, for with further contraction the blood is forced against the under surfaces of the cusps and the atrio-ventricular orifice is completely closed; indeed, the more complete the contraction, the more perfectly does the valve close the orifice (QI, 3; QII, 11, 17v). The blood thus sent to the auricle dilates it, but it is in the auricle only “on deposit” (QI, 3, 4, 17v) and is returned to the ventricle when this


102


LEONARDO DA VINCI — THE ANATOMIST


passes again into diastole. In its dilation the auricle is entirely passive, but it is actively contractile and forces the blood it has received back into the dilating ventricle (QI, 3, 4), and since the dilation of the ventricle is also an active process, effected by the contraction of its longitudinal muscles, a vacuum is created in it (QII, 11), which will tend to attract blood from the nearest available source, namely the right auricle. Thus there are three forces acting in the return of blood to the ventricle, the recoil of the dilated auricle (QII, 3), the muscular contraction of the auricular walls (QI 4) and the vacuum formed in the ventricle (QII, 11), the result being that the blood returns with great impetus, and what with the friction created by its own currents and that produced by contact with the irregular surfaces of the auricle and ventricle it becomes greatly heated and subtilizes (QI, 3, 4, 4v; QII, 3v, 11; AnB, 12).

The production of this heat is, then, the purpose of the flux and reflux of the blood between the auricle and ventricle. The heat of the body was supposed to have its source in the heart, but while Galen and the ancients attributed it to the mysterious innate heat ( color innatus) implanted in the left ventricle of the heart, Leonardo endeavored to explain it on purely mechanical principles, depriving it of its mysterious character and transferring its source from the heart to the blood. The premise on which he based his explanation, the flux and reflux, was erroneous, but the theory is of interest as a characteristic attempt to explain natural phenomena upon natural rather than supernatural principles. It was the following out of the method initiated centuries before by Hippocrates, and practically neglected from Galen’s day to Leonardo’s; it was an ingenious attempt to solve a problem whose solution was made possible only by the discovery of oxygen some two and a half centuries later.

After the escape of the blood to the auricle the tricuspid valve closes effectively and the continued contraction of the ventricle forces a second portion of blood through the pores of the ventricular septum. This portion has been subtilized by the flux and reflux — in QI, 3v, its subtilization is referred to its passage through the pores, as Galen taught — and, after its passage to the left ventricle, what blood remains to the right is more viscid —

“and to some extent is transformed into minute threads like the vermis of the middle ventricle of the brain. And these threads multiply like thick and short tow and eventually entwine themselves among the chord® of the valves that close the right ventricle, so that in aged animals the opening (porta) can not be well closed and a great part of the blood, which ought to penetrate the narrow porosities of the middle wall of the left ventricle for the production of the spirits mentioned above, escapes through the imperfectly closed opening into the superior ventricle and consequently aged persons lack spirits completely and often die while speaking.” (QI, 3v.)


THE HEART


163


Leonardo had evidently observed the formation of fibrin filaments in the ventricle.

A third portion of the blood of the right ventricle passes out by way of the pulmonary artery, probably for the nutrition of the lungs. This is plainly indicated on QII, 17v, but Leonardo’s expressed ideas as to the passage of blood to and from the lungs are decidedly obscure and to some extent contradictory. The heating of the blood by the flux and reflux could, he believed, be so intense as to suffocate the heart and deprive it of life (QI, 3), and not only does it convert some of the blood into spirits, but it might even convert it into elementary fire, were it not for the succor afforded by the lungs (QII, 11). 6 The cool air which these attract to themselves cools and refreshes the blood of the heart. But as to the exact way in which this is brought about Leonardo seems to be in doubt. On AnB, 12, showing one of the ventricles opened, the wall of the issuing artery is also cut away, revealing an opening into the artery by which the blood is said to be refreshed by air from the lungs. Since the ventricle is termed the "receptacle of the spirits” it is presumably the left ventricle and the artery is the aorta, so that the opening is probably that of one of the coronary arteries, erroneously identified. The figure is very crude and probably belongs to an early period, and this same remark may be made regarding a passage on QIV, 7, which states that the heart can not dilate wdthout drawing to itself air from the lungs, which it immediately returns with an impetuous motion, independently of the pulmonary rhythm. So again with the indistinct statement that there are two passages between the heart and the lung, one in the concavity of the heart (whatever that may be) which carries air to that organ and another which returns it to the lungs (QIII, 10v).

On QII, 1 and 11 Leonardo takes a position more in harmony with modern ideas, in that he asserts that no air passes from the heart to the lung, but the arteries, which are in continuous contact with the branches of the trachea scattered through the lung, take up the freshness of the air that enters the lung (QII, 11), and he goes on to describe (QII, 1) how the minutest ramifications of the trachea are accompanied by ramifications of the veins, with which they are in continuous contact.

“It is not here that the air enclosed in the trachea expires through its delicate branches and penetrates through the tips of the smallest branches of the veins.”

Thus suggesting that there are no anastomoses between the branches of the bronchi and those of the blood-vessels. Then he adds—

6 The increased temperature of fever is explained by the greater frequency of the heart beat as shown by the pulse, since the faster the heart beats the greater will be the friction that produces the heat (AnB, 12).


164


LEONARDO DA VINCI — THE ANATOMIST


“But concerning this I shall not entirely affirm my statement until I have seen the anatomy which I have in hand.”

There is nothing to show what the anatomy revealed. Apparently he believed that blood could flow either from or to the heart in both the pulmonary artery and vein, for he states that the blood regurgitated through the atrio-ventricular opening, evidently the left one, gives due nourishment to the veins of the lung and, after being refreshed in the lung, returns to freshen the blood it had previously left in the ventricle (QII, 4v). This idea of the blood passing in either direction in a vessel comes down from Galen and will be met with again in connection with the portal vein.

Returning to the right ventricle it is evident that, since a portion of its blood passes through the septal pores and another portion through the pulmonary artery, the amount of blood contained in the right auricle will not be sufficient to fill it at the reflux (QI, 3v, 4). But its dilation is an active process and a vacuum will therefor be formed which will attract blood from whatever source it may.

The most available source is the blood contained in the veinules of the gibbosity of the liver and it is thence that the ventricle regains the blood it has lost (QI, 4, 4v; QII, 17v), replenishing itself with new blood freshly concocted in the liver. If, when the ventricle is dilating, the lung is also expanding, the latter will press upon that portion of the vena cava between it and the spinal column and a portion of the blood will be forced into the right ventricle, in which case so much the less is drawn from the gibbosity of the liver (QI, 4).

That there is a flux and reflux of blood between the left auricle and ventricle may be inferred, indeed it is distinctly implied (QII, 4v), but the blood regurgitated through the mitral valves before their closure is described as passing by the pulmonary veins to the lungs, whence it returns to refresh the blood remaining in the ventricle (QII, 4v). In dealing with the left heart, however, Leonardo concentrates his attention on the passage of blood into the aorta and its effects on the semilunar valves. These, like the atrio-ventricular, allow a certain amount of regurgitation into the ventricles before they eventually close (QII, 4v), the blood which is regurgitated being that which occupied the axis of the column of blood in the aorta (QII, 11). This travels faster than the more peripheral portions of the column and passes freely between the valves, while the peripheral portions come into contact with them and force them together. It is not the more axial portion of the column that closes the valves, since it would be more apt to crumple them than force them together, but the peripheral portions, descending into the hemicycles (sec p. 158), are thrown into vortices, which force the valves downward and inward and so bring them into contact (QII,


Fig. 49. Studies of vortices in pockets of semilunar valves, (()I\ , 11.)

Fig. 50. Figures illustrating the comparison of the heart and blood-vessels with a sprouting nut with its plumule and radicle.

In the figure to the right the azygos vein is well shown. (AnB, 11.)


THE HEART


165


12). The assumption of these vortices appealed strongly to Leonardo’s interest in hydrodynamics and several pages of the Quaderni have a greater or less number of sketches (fig. 49) illustrating them (QII, 12, 13v; QIV, 11, llv, 12); furthermore it was in connection with their study that he purposed to employ the glass model, cast in a plaster of Paris mold (see p. 90).

Since new blood is supposed to be formed more or less continuously in the liver, there must be a corresponding destruction of it. Leonardo recognizes this in his statement that the blood continually dies and is remade (AnB, lOv), but statements as to how and where it is destroyed are not very satisfying. It has been seen that a certain amount of blood is converted into vital spirits and this amount is restored to the heart from the veins of the liver. Leonardo attempts (QII, 17v) an estimate of the amount so restored. He estimates the number of dilatations of the heart as 2,000 per hour, a number decidedly below the average, and remarks ‘‘There is a great weight.” On the margin of the page there is a sum in multiplication 24X12 = 300 (evidently the prodduct is a round number) and below this is written “7 ounces per hour.” This may be the result of his estimate, but how it is reached is not clear.

It does not seem probable that Leonardo had studied the beat of the heart in the living organ when he made the statement found on QI, 3, as to the succession of the contractions of its different parts. He there holds that the two ventricles alternate in their contraction and dilation, and that the two auricles do the same, so that when the right ventricle dilates the left auricle contracts, and when the left ventricle dilates the right auricle contracts. This is evidently pure speculation uncontrolled by observation, and yet a little consideration will give a clue to the reasoning that led him to such an erroneous idea. Its foundation was the Galenic assumption that blood passes through pores in the ventricular septum from the right to the left ventricle, an assumption that played an important part in delaying the discovery of the circulation of the blood and had established itself so firmly that even Vesalius at first accepted it and only later began to doubt. Leonardo, as has been seen, accepted it. The passage at best was a difficult one, the pores being so small as to be invisible, and if the two ventricles were to contract simultaneously it w r ould have been impossible; the left ventricle must be relaxed to receive the blood which the right was endeavoring to force into it. Hence the supposed alternation in the contraction of the ventricles. But for the flux and reflux each auricle must alternate in contraction with the ventricle of its side, and so the statement. It was perfectly logical, granting the correctness of the supposed facts upon which it was based, but unfortunately it was unchecked by observation or experiment.


16G


LEONARDO DA VINCI — THE ANATOMIST


However, on another page, QII, 4v, Leonardo does correct this statement, giving a more accurate one, which, however is not very clearly expressed. Pie says, speaking of the ventricles:

“Their dilatation and contraction arc made at the same time by the flux of the blood; and the reflux of the blood is made at the same time, succeeding the first, by the reflux from the upper ventricles (i.e., the atria) placed above the base of the heart.”

Apparently he means by this that the two atria contract synchronously and so do the two ventricles, their contraction, however, alternating with that of the auricles.

Leonardo recognized that the pulse was a result of contraction of the heart; the contraction of the ventricles, the beat of the apex against the chest wall, the beating of the pulse and the entrance of blood into the aorta are simultaneous (QIV, 11), The apex beat is said to occur when the heart “contracts with impetuous motion and drives the blood out of itself into the passage of the vein that is intended for it” (QII, 11), and, further, it is stated that the time that elapses between two beats of the pulse is half a musical tempo, during which the heart closes twice and opens once. In each musical or harmonic tempo the heart makes, accordingly, three movements and since there are 10S0 tempi in an hour, the heart moves in each hour 3540 times. Leonardo seems to have erred here in his calculations, since if from pulse beat to pulse beat there is an interval of half a tempo, in an entire tempo there would be three pulse beats, that is to say three contractions and three dilations, six motions in all. Consequently the number of movements of the heart per hour should be 6480, which would mean 3240 pulse beats per hour, a number considerably below what may be taken as the average normal number. In extenuation it mnst be remembered that Leonardo had no accurate means of counting the pulse; there were no time-pieces in his day and more than sixty years were to elapse after his death before Galileo invented his pulsilogia, the first instrument designed for the accurate measurement of a physiological phenomenon. There is a possibility that by “movements” Leonardo may have meant contractions, but this is negatived by the concluding words of the sentence, and no matter how one attempts to reconcile the discordant statement, it is clear that an error has been made, for the result could be 3540 only if the number of musical tempi per hour was 1180.

Attention is called to the fact that the beat of the heart as determined by the pulse is discontinuous, a distinct interval occurringbetween each two beats. This interval allows time for the liver and the meseraic veins to form and restore to the “gibbous part” of the heart that


THE HEART


167


amount of blood that the left 7 ventricle had taken from it (QI, 4v), and it further brings it about that the nourishment of all parts of the body must also be discontinuous (QI, 7).

That the beat of the heart was automatic seems to have been accepted at one time. Thus it is said of the heart “This beats of itself and never stops, except eternally,” and on the same page it is pointed out that none of the inferior organs of the body can be stopped by the will, unless it be the lungs. The heart acts automatically and so does the stomach and the other intestines joined to it; and it is similar with the liver, the gall-bladder, the spleen, the testicles, the kidneys and the bladder (AnB, 13). But in another passage, probably written later, after he had observed the possible relation of the vagus nerve to the heart, Leonardo is not so certain as to the automaticity of the heart and makes a memorandum to inquire whether the stimulus to contract does not come to the heart through a nerve as in the case with other muscles (QIV, 7). This passage, which is full of interest, has already been quoted (p. 160).


7 Leonardo has “right” here, but it is evidently a slip.


Historic Disclaimer - information about historic embryology pages 
Mark Hill.jpg
Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)
   Leonardo da Vinci (1930): 1 Introductory | 2 Anatomy from Galen to Leonardo | 3 Possible Literary Sources of Leonardo’s Anatomical Knowledge | 4 Anatomical Illustration before Leonardo | 5 Fortunes and Friends | 6 Leonardo’s Manuscripts, their Reproduction and his Projected Book | 7 Leonardo’s Anatomical Methods | 8 General Anatomy and Physiology | 9 Leonardo’s Canon of Proportions | 10 The Skeleton | 11 The Muscles | 12 The Heart | 13 The Blood-vessels | 14 The Organs of Digestion | 15 The Organs of Respiration | 16 The Excretory and Reproductive Organs | 17 The Nervous System | 18 The Sense Organs | 19 Embryology | 20 Comparative Anatomy | 21 Botany | 22 Conclusion | References | Glossary of Terms | List of Illustrations


Reference: McMurrich JP. Leonardo da Vinci - the anatomist. (1930) Carnegie institution of Washington, Williams & Wilkins Company, Baltimore.


Cite this page: Hill, M.A. (2020, August 15) Embryology Leonardo da Vinci - the anatomist (1930) 12. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Leonardo_da_Vinci_-_the_anatomist_(1930)_12

What Links Here?
© Dr Mark Hill 2020, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G