188 



ANATOMY OF VERTEBRATES. 



92 



Section of the ventricles. Pelican. 



the Struthious, and especially in the Apteryx, in which it is par- 

 tially membranous and has its margin tied by a few tendons to a 

 fleshy process from the fixed ventricular wall. 



The small muscular column, fig. 92, m, at the upper part of 

 the auricular orifice is analogous in its position to the single valve 

 which guards the corresponding orifice in Keptiles; the Cro- 

 codiles alone present a second muscular valve (vol. i. p. 510, fig. 

 339, r) homologous Avith the larger valve in Birds. 



The right ventricle is remarkable for the smoothness and even- 

 ness of its inner surface. The 

 pulmonary artery is pro\dded at 

 its origin with three semilunar 

 valves, fig. 92, n. It divides, as 

 usual, into two branches, fig. 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 semi- 

 lunar ridge, which is hardly suf- 

 ficiently produced to be called a valve, divides the cavity of the 

 auricle in which the veins terminate from the muscular part or 

 appendix. The fleshy columns are very numerous and compli- 

 cated in this part of the auricle, which is closely tied down to the 

 ventricle by the serous layer of the pericardium and dense cellular 

 tissue. 



The left ventricle, figs. 91, 92, o, is an elongated conical cavity, 

 the parietes of which are three times as thick as those of the 

 right ventricle, and exhibit strong fleshy 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 auri- 

 culo- ventricular orifice, fig. 92, r, and give ofl" short thick tendons 

 to the margin and ventricular surface of two membranous folds, 

 figs. 91, 92, j9, 5^, which correspond to the 'mitral valve' in Mam- 

 mals. Of these valves, the one next the aorta, q, corres2:)onds to 

 the single valve which guards the auricular opening in the heart of 

 Reptiles, and is most developed in Birds ; the opposite valve is of 

 much less size. In many Birds the chords tendinete pass from the 

 valves at once to the parietes of the ventricle, and are not at- 

 tached to columnas carneas. The surface of the ventricle formed 

 by the septum is smooth from the orifice of the aorta down to the 



