HEAET AND AETEEIES OF VEETEBEATA. 



583 



the arterial and venous trunks. The two kinds of blood are not, 

 however, able to commingle to any great extent. 



The vascular apparatus of Birds is very closely similar to that of 

 Reptiles, and especially of the Cro- 

 codilini. But there is this considerable 

 advance, viz. that the arterial and venous 

 blood are completely separated in the 

 heart, as well as in the great arterial 

 trunks. The auricles of the heart appear 

 to be smaller owing to the smaller size 

 of their anterior (ventral) portion. The 

 musculature of the wall of the ventricle 

 is greatly thickened, especially on the 

 left side. The right ventricle is folded 

 around the greater part of the left one. 

 The atrio -ventricular valve of the right 

 ventricle is formed by a muscular ridge 

 (muscular valve) which projects from the 

 waU of the ventricle, and surrounds the 

 ostium peripherally; the second or mem- 

 branous valve, which is found in the 

 Crocodilini, is but seldom represented, 

 even in rudiment. At the left ostium 

 there is a valve which is connected with 

 the wall of the ventricle by tendinous 



fibres. The arterial arches are reduced in much the same way as 

 in the Reptiha, especially the Ophidii and Crocodilini. But the left 

 aortic arch is not permanently developed. There is, therefore, 

 merely a single right aortic arch, which arises from the left ventricle. 

 Two arterial trunks, art. brachiocephalicae, are given off with it; 

 these are divided into a common carotid, and a subclavian. In 

 some Raptores there is a permanent remnant of a left aortic arch, 

 in the form of a ligamentous chord ; this indicates the course taken 

 by the vessel. The pulmonary artery, which is derived from the 

 last of the primitive aortic arches, is consequently the only arterial 

 trunk which is given off from the right ventricle. 



Fig. 330. Heart and arterial 



trunks of a Chelonian 



(Ohelydra). The letters as in 



tte previous figure. 



Fkitsch, G., Zur vergl. Anatomie der Amphibienherzen. 

 Physiolog, 1869. 



Arch. f. Anat. u. 



§ 435. 



The heart of the Mammalia resembles that of the Birds in 

 having its two halves completely separated ; but there is a great 

 difference in its structure, and in the arrangement of the great 

 vessels. It is in rudiment only that there is any close similarity 

 between either the heart or the general arterial system, which, as in 

 Birds, is derived from a number of paii-ed arches. During the 

 embryonic stage the two auricles communicate ; in the Marsupialia 

 this is effected by a narrow slit, and in the placental Mammals by a 



