HEART OF BIRDS. 



187 



it does not project freely in front of the great vessels arising from 

 the ventricles, but is tightly tied down to them by the reflected 

 layer of the pericardium. The auriculo-ventricular orifice is an 

 oblique slit, fig. 92, k ; a bristle is passed through it in fig. 90. 

 The manner in Avhich regurgitation by this orifice is prevented is 

 one of the chief peculiarities in the heart of Birds. The right 

 ventricle, fig. 91, /«, is a narrow triangular cavity, applied as it 

 were to the right and anterior 

 side of the left ventricle, but not 

 extending to the apex of the 

 heart. The parietes are of pretty 

 uniform thickness, except at the 

 septum ventriculorum, and are 

 weaker in comparison to those of 

 the left ventricle than in Mam- 

 mals. Short fleshy columns ex- 

 tend from the septum to the 

 free wall of the ventricle at the 

 angle of union of these two parts, 

 leaving deep cells between them ; 

 a strong column, fig. 92, yn, 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 * columnae 

 carneaB ' in the right ventricle ; 

 there being none of a pyramidal form projecting into the cavity, 

 nor any ' chordie tendinea3.' The principal valve which guards 

 the auricular aperture is a strong muscular fold, figs. 90, 91, 

 92, /, nearly as thick as the wall of the ventricle itself, extend- 

 ing from the fleshy column above mentioned obliquely down- 

 ward and backward to the angle formed between the septum 

 and the free wall of the ventricle at the lower and posterior 

 part of the cavity. The convex edge of this muscular valve 

 is turned toward the convex projection made by the septum, 

 and must be forcibly applied to this part during the systole of 

 the ventricles ; so that, while all reflux into the auricle is pre- 

 vented, additional impulse is given to the flow of blood through 

 the pulmonary artery ; the muscidar parietes of the ventricle 

 being thus complete at every part except at the orifice of the 

 artery. This valve is strongest in the Diving Birds, weakest in 



Ventricles of the heart of a Swan. 



