590 



HEART. 



which are apparently of the same number and 

 thickness as in that portion of the artery im- 

 mediately above, form a distinct curved edge 

 (Jig- 267, ej, as they pass from the extremity 

 of one festoon to the other. As we trace the 

 middle coat of the artery downwards into the 

 concavities formed by each festoon, we find 

 that below this curved edge they become stri- 

 kingly thinner and continue to diminish in 

 thickness and in length, (since they can only 

 stretch between the projecting extremities,) 

 until we arrive at the bottom of the concavity. 

 These three thin portions of the middle coat 

 must then be placed behind the semilunar 

 valves, and correspond to the sinuses of Val- 

 salva.* The thinness of the middle coat at the 

 sinuses of Valsalva will render this portion of 

 the artery more dilatable, and predispose it 

 to rupture when its coats are diseased.f The 

 tendinous zones are distensible, but to a con- 

 siderably less extent than the middle coat of 

 the arteries. I am not aware that this account 

 of the manner in which the middle coat of the 

 arteries is attached to the tendinous rings has 

 been previously given. I suspect, however, 

 that Dr. Duncan must have been perfectly 

 aware of it from some parts of his manuscript. 

 The differences between these tendinous fes- 

 toons and the yellow elastic coat of the arteries, 

 and the manner of tlieir attachment, can easily 

 be made out in the human heart ; they are, 

 however, more apparent in the larger animals, 

 as the horse and ox. The different characters 

 of the two tissues are obvious at the first 

 glance after boiling, even in the human heart. 

 Muscular tissue.\ The greater part of the 



* So striking is the difference between the middle 

 coat as it fills up the concavity of these festoons, 

 and where it stretches between the projecting ex- 

 tremities in the hedgehog, that at first sight it ap- 

 pears to be deficient at that part. 



t According to Valsalva aneurisms are frequently 

 found in this situation : "jAtque hie aortae sinus 

 maximus ille est, in quo saepe aneurysmata circa 

 praecordia contingunt, ut propria obseivatione 

 edoctus sum." Valsalvae Opera. Epist. Anat. 

 cd. Morgagni, torn. i. p. 131. 1740. This greater 

 tendency to aneurismatic dilatation must depend 

 upon two circumstances. The increased calibre of 

 the artery at this part will increase the pressure 

 upon its walls from the well-known hydrostatic law, 

 that " in a quantity of fluid submitted to compres- 

 sion, the whole mass is equally affected, and simi- 

 larly in all directions," and the diminished thick- 

 ness of the middle coat will materially favour this 

 distending force. 



J While I was engaged in examining the arrange- 

 ment of the muscular fibres of the heart, Dr. Alison 

 had the kindness to procure for me the manuscript 

 of the late Dr. A. Duncan, jun. on this subject. It 

 was well known not only in this country but on the 

 continent that Dr. Duncan had for a very long pe- 

 riod attended very particularly to this question, and 

 was in the habit of demonstrating the parts he had 

 ascertained to his pupils. Unfortunately his inten- 

 tions of publishing on the subject were never car- 

 ried into execution, and his papers referring to it 

 were left in so confused a state that it is exceed- 

 ingly difficult and in most parts impossible to make 

 out the description. I have availed myself of those 

 parts that are legible in the following pages, and 

 these I have scrupulously acknowledged. J)r. Dun- 

 can's dissections of the heart were taken entirely 

 from the ox and sheep. 



heart is composed of muscular fibres arranged 

 in a very intricate manner. These fibres are 

 connected together by cellular tissue,* which, 

 however, exists in much smaller quantity in 

 the heart than in the other muscles of the body. 

 These fibres are attached generally by both 

 extremities to the tendinous rings situated 

 around the orifices of the heart; the fibres of 

 the auricles pass upwards to form the auricles, 

 and those of the ventricles downwards to form 

 the ventricles, so that these tendinous rings 

 must form the fixed points towards which all 

 the contractions of the heart take place. None 

 of the muscular fibres of the auricles are con- 

 tinuous at any part with those of the ventricles, 

 and we will find that while some of them are 

 confined to a single auricle, others belong to 

 both. In the same manner a great part of 

 the fibres of the ventricles are common to 

 both, and are interwoven together, while others 

 again belong exclusively to a single ventricle, 

 or, as Winslowf expressed it, the heart is com- 

 posed of two muscles enveloped in a third. 

 The intimate arrangement of these muscular 

 fibres, particularly those of the ventricles, is 

 exceedingly complex, as the contraction of the 

 organ is not in one particular direction only, 

 but in all directions, and has long been con- 

 sidered as a kind of Gordian knot in anatomy. 

 Vesalius, Albinus, and Haller j confessed their 

 inability to trace them, and more lately De 

 Blainville assures us, from his own experi- 

 ence, that we can only arrive at very general 

 conclusions (des chases tris-g'tnirales) on this 

 subject. By adopting the method of long- 

 continued boiling of the organ before com- 

 mencing to attempt to trace the course and ar- 

 rangement of its fibres, we will find that after a 

 few trials several of the most important points 

 connected with the distributicn of these can be 

 ascertained, and by perseverance they can be 

 unravelled to a great extent. By long boiling 

 the muscular fibres are rendered hard and firm, 

 while the tendinous and cellular tissues are 

 softened or dissolved, and the fat melted. Dr. 

 Duncan, who employed this method to a great 

 extent, states that the essential circumstance is 

 to continue the boiling long enough, and that 

 he has never been able to carry it too far. I 

 have found from eighteen to twenty hours gene- 

 rally sufficient for this purpose. Some have 

 recommended that the heart should be pre- 

 viously put for a short time into a strong solu- 

 tion of salt, and Vaust advises that it should 

 be boiled in a solution of nitre, for the purpose 

 of rendering the fibres firmer. The boiling is 

 infinitely superior to the maceration in vinegar. 

 By stopping the boiling before the tendinous 

 rings are rendered too soft, we can easily see 

 their form and their connexions to the muscular 

 fibres. 



The general connexion and distribution of 



* [This however is denied by other observers, 

 and from very recent and careful examinations. 

 See the succeeding article by Mr. Searle. ED.] 



t Memoires de 1'Academie Royale des Sciences, 

 1711, p. 197. 



J El. Phys. torn. i. p. 351. 



{ Cours de Physiologic, &c. torn. ii. p. 359. 



