THE HEART AND ITS VESSELS. 287 



of the latter having a semilunar transverse section, and its walls 

 being much thinner than those of the other (Fig. 230B, Vd, Vg). 



In both Mammals and Birds the blood from the head and 

 body passes by means of the precavals and postcaval into the right 

 atrium, as does also that from the walls of the heart through the 

 coronary vein: the right atrium is separated from the right 

 ventricle by means of a well-developed valve. In Birds, the latter 

 (Fig. 230A, a, b, c, t) is very large and entirely muscular, while in 

 most Mammals it consists of three membranous lappets (tricuspid 

 valve), to which are attached tendinous cords (chordae t en- 

 din ese), arising from muscular processes (musculi papillares) 

 of the walls of the heart. 



In both Birds and Mammals the left atrio-ventricular aperture 

 is provided with a valve consisting of two membranous folds, called 

 the bicuspid or mitral valve: three semilunar pocket-like 

 valves are also present at the origins of both pulmonary artery 

 and aorta (Fig. 230A, *, *, *). 



As regards the origin of the great vessels, Birds are distinguished 

 from Mammals by the fact that in them the right (fourth) arterial 

 arch persists, while in Mammals the left remains as the aortic 

 arch ; the corresponding arch of the other side in both cases gives 

 rise to part of the subclavian artery. Thus in both Birds 

 and Mammals there is only a single aortic arch. As in 

 Amphibians, the posterior arterial arch gives rise in both cases 

 to the pulmonary artery (comp. Fig. 221, C, D). 



Amongst the more important points in the development of the 

 heart may be mentioned the fact that at first the two atria freely 

 communicate with one another by means of the foramen ovale. 

 through which the blood from the postcaval passes into the left 

 ventricle. 



1 



FIG. 231. FIVE DIFFERENT MODES OF ORIGIN OF THE GREAT VESSELS FROM 

 THE ARCH OF THE AORTA IN MAMMALS. 



Ao, aortic arch ; tb, bracMocephalic trunk ; c, carotids ; s, subclavian s. 



Great variations are seen in the mode of origin of the carotids 

 and subclavians from the arch of the aorta in Mammals. Thus 

 there may be a brachio cephalic trunk on either side (Fig. 

 231, A), or an unpaired common brachiocephalic, from which 

 the carotid and subclavian of one or both sides arise (B, C, E), or, 

 finally, a common" trunk of origin for the carotids, the subclavians 

 arising independently on either side of it (D). 



