THE VASCULAR SYSTEM 333 



heart goes to the lungs and only to the lungs. In reptiles and 

 birds, the carotids arise from the right systemic arch. 



The two channels of the truncus in the Theropsida are the 

 pulmonary and the single systemic aorta. These forms 

 include the mammals, and the Theromorph reptiles, although 

 the latter (fossils) are obviously only known from their 

 skeleton. The heart is four-chambered, and the ventricle is 

 completely divided into two, so that all the venous blood from 

 the right auricle goes into the pulmonary arch, and all the 

 arterial blood from the left auricle into the systemic aorta, and 

 there is no mixture. It so happens that the aortic arch of the 

 right side does not persist, and only the left one remains, but 

 it is of the utmost importance to realise that the reason why 

 there is a single systemic arch in the bird is totally different 

 from that which is responsible for the single arch in the 

 mammal. The structure of the heart in the amniotes shows 

 that the reptiles contain two main lines of evolution (besides 

 other less important lines), the one culminating in the birds 

 and the other in the mammals. The sinus venosus disappears 

 in the highest forms, birds and mammals, and is represented 

 by the so-called sino-auricular node. This structure is of 

 great functional importance, for it acts as the pace-maker to 

 the heart. It is here that the contraction originates, which 

 contraction then becomes taken up by the other parts of the 

 heart, and constitutes its " beat." In birds and mammals the 

 superior and inferior caval veins open direct into the right 

 auricle. The sino-auricular valves give rise in the mammals 

 to the Eustachian and Thebesian valves. 



The Arteries. — In the fish typically, each of the visceral 

 arches has an afferent branchial artery leading from the ventral 

 aorta to the gills, and an efferent branchial artery connecting 

 the gills to the lateral dorsal aorta. The vessels in the mandi- 

 bular arch become reduced. The general arrangement of 

 these vessels is necessitated by the presence of the visceral 

 clefts, which make it impossible for the vessels to reach the 

 dorsal side of the gut from the ventral side except by passing 

 in the visceral arches between the clefts. Since gill-slits or 

 pouches are present in the embryos of all chordates, the same 



