786 COMPARATIVE ANATOMY 



into che right auricle, while the pulmonary veins which carry blood from 

 the lungs to the heart enter the left auricle. As this blood which has 

 returned from the lungs is now oxygenated and ready for distribution 

 to the general system, it is the left side of the heart which becomes the 

 arterial side. 



In the higher forms the ventricle is also divided by a septum. The 

 valves on the right side which separate the auricle from the ventricle 

 are called the tricuspid valves, while those on the left side separating 

 the left auricle and left ventricle are known as the mitral or bicuspid 

 valves. 



In the crocodiles there is an opening between the two sides of the 

 aortic trunk known as the foramen Pannizae (Fig. 447, B. pan), so that 

 there is really some mixture of arterial and venous blood in these ani- 

 mals. 



The separation into four compartments is complete in birds and 

 mammals (Figs. 445, 447, C), so that the blood must pass through the 

 heart twice. Once through the venous, and once through the arterial 

 half, in order to make a complete circuit of the body. 



The heart is formed directly behind the mandibular artery which is 

 the first aortic arch (Fig. 309), so that as other vessels grow, it is forced 

 back further and further until it lies ventrad and caudad to the pharynx, 

 while in the adult higher forms of mammals it is carried back as a result 

 of this unequal growth even into the thorax. (The extreme of migration 

 being seen in the giraffe and the long-necked birds.) 



THE ARTERIES 

 AORTA AND AORTIC ARCHES 



The ventral aorta is that large artery running headwarcl from the 

 heart. It extends to the mandibular artery, which is another name for 

 the first aortic arch (Fig. 309). The mandibular arteries, like other 

 arches, pass dorsad (one on each side of the pharynx) until they meet 

 and form a pair of dorsal longitudinal tubes called the radices aortae. 

 Between the first aortic arch (mandibular artery) and the heart there 

 arise some six or more pairs of arches similar to those forming the man- 

 dibular artery. The number of such arches depends upon the number of 

 gill-clefts the animal has, for these arches develop in the septum be- 

 tween the gill-clefts. (The number of arches is greater in the myxinoids 

 where the number of clefts varies ; seven or eight in the notidanid sharks ; 

 and, as recent investigations tend to show, probably six in the embryos 

 of all other vertebrates.) 



As the embryo continues to grow, the number of these arches, which 

 remain or degenerate, seems to be influenced to a considerable extent 

 by the various changes of the respiratory system the particular animal 

 in question may develop. When gills develop, each aortic arch divides 



