AVES. 



331 



gurgitation by this orifice is prevented is one second muscular valve, which, though small, is 

 of the chief peculiarities in the heart of Birds. 



Fig. 169. 



n 

 m \ 



\ 



Fig. 168. 



Ventricles of the heart of a Swan. 



The right ventricle (k,Jig. 1 68) is a narrow trian- 

 gular cavity, applied as it were to the right and 

 anterior side of the left ventricle, but not ex- 

 tending to the apex of the heart. The parietes 

 are smooth, and, except at the septum ventri- 

 culorum, they are of pretty uniform thickness, 

 but weaker in comparison to those of the left 

 ventricle than in Mammalia. A number of 

 short fleshy columns extend from the septum 

 to the free parietes of the ventricle at the angle 

 of union of these two parts, leaving deep cells be- 

 tween them ; a strong fleshy column (m,fig. 169) 

 also extends from the right side of the base of 

 the pulmonary artery to the upper extremity of 

 the auriculo-ventricular valve ; but these are 

 the only columns earner in the right ventricle ; 

 there being none of a pyramidal form pro- 

 jecting into the cavity, nor any chordte ten- 

 dinex. The principal valve which guards the 

 auricular aperture is a strong muscular fold 

 (I, fig. 167, 168, 169), nearly as thick as the 

 walls of the ventricle itself, extending from the 

 fleshy column above mentioned obliquely down- 

 wards and backwards to the angle formed be- 

 tween the septum cordis and the wall of the 

 ventricle at the lower and posterior part of the 

 cavity. The free rounded edge of this muscular 

 valve is turned towards the convex projection 

 made by the septum, and must be forcibly ap- 

 plied to this part during the systole of the 

 ventricles, so that, while all reflux into the 

 auricle is prevented, additional impulse is given 

 to the flow of blood through the pulmonary 

 artery; the muscular parietes of the ventricle 

 being thus complete at every part except at the 

 orifice of the artery. 



The small muscular column (in, fig. 169) at 

 the upper part of the auricular orifice is analo- 

 gous in its position to the single valve which 

 guards the corresponding orifice in Reptiles ; 

 in which class the Crocodiles alone present a 



r 



Section of the ventricles, Pelecan. 



analogous in its position, and evidently a rudi- 

 mental form of the large muscular valve in 

 Birds. 



The right ventricle is remarkable for the 

 smoothness and evenness of its inner surface. 

 The pulmonary artery is provided at its origin 

 with three semicircular valves (,./?. 169). It 

 divides, as usual, into two branches (n, n,jig. 

 168), one for each lung ; the right branch passes 

 under the arch of the aorta. 



The aerated blood is returned from the lungs 

 by two veins which open into the back part 

 of the left auricle; a strong semilunar ridge, 

 which is hardly sufficiently produced to be 

 called a valve, divides the cavity of the auricle 

 in which the veins terminate from the mus- 

 cular part or appendix. The fleshy columns 

 are very numerous and complicated in this part 

 of the auricle, which is closely tied down to 

 the ventricle by the serous layer of the pericar- 

 dium and dense cellular tissue. 



The left ventricle (o,fig. 168, 169) is an elon- 

 gated conical cavity, the parietes of which are 

 three times as thick as those of the right ventricle, 

 andexhibit strongfleshy columns extending from 

 the apex towards the base; two of the largest 

 of these columns present in the Emeu a short 

 convex eminence towards the auriculo-ventri- 

 cular orifice (r,_/?g.l69), and give off short thick 

 tendons to the margin and ventricular surface 

 of two membranous folds, (p, q,fg. 168, 169) 

 which correspond to the mitral valve in Mam- 

 malia. Of these valves, the one next the aorta (y) 

 corresponds to the single valve which guards 

 the auricular opening in the heart of Reptiles, 

 and is most developed in Birds; the oppo- 

 site valve is of much less size. In many Birds 

 the chordae tendineae pass from the valves at 

 once to the parietes of the ventricle, and are 

 not attached to columnar carneae. The surface 

 of the ventricle formed by the septum is smooth 

 from the orifice of the aorta down to the apex 

 of the heart. The aorta is provided, as in Mam- 

 malia, with three semicircular valves. In Rep- 

 tiles, even in the Crocodile, the great arteries 

 arising from the ventricles are each provided 

 with two valves only. 



We have observed that in general the valves 

 at the base of the pulmonary artery were 

 thicker and stronger than those at the origin 

 of the aorta, and our lamented and talented 

 friend Mr. Home Clift discovered some years 



