VEINS Do; 



are now known together as the anterior vena cava, those of the 

 right and left sides having separate openings into the auricles. 

 The common and external jugulars represent the anterior parts 

 of the anterior cardinals. In many mammals, such as the rabbit, 

 there is a large ventral anastomosis between the two common 

 jugulars, and in some, including man, the portion on the left 

 posterior to this disappears, so that there is no left anterior 

 vena cava and all the blood enters the auricle on the right side. 



The abdominal veins are of less importance. In the dogfish 

 two small vessels run forward in the abdominal wall and enter 

 the sinus venosus ; their homologues in the anterior region 

 are probably the misnamed jugular sinuses which run from the 

 head back to the heart and are seen in the dissection of the afferent 

 branchials. Actinopterygians are very similar, but in lung 

 fishes there is only a single median vein. In Amphibia and Reptilia 

 not only have the abdominals fused to form a median vessel (the 

 anterior abdominal of the frog) but the anterior portion of this 

 opens not directly into the heart but into the hepatic portal. 

 There is often, as in the frog, connection between abdominal 

 and renal portal veins, so that all the blood from the posterior 

 parts of the body has a choice of routes. In birds and mammals 

 the abdominal veins are present only in the embryo, where they 

 carry oxygen from the allantois and (in mammals) food from the 

 placenta. In the adult they atrophy, and blood from the hind 

 limbs and tail goes direct into the posterior vena cava. 



In lung fishes the pulmonary veins enter the left side of the 

 auricle ; in the tetrapods they enter the left auricle, and are much 

 the same in all classes. 



NERVOUS SYSTEM 



The nervous system of vertebrates, like that of all the higher 

 Metazoa, is divisible into a central part, where impulses are 

 received, relayed, co-ordinated and perhaps originated, and a 

 peripheral part which puts the central nervous system into 

 communication with the rest of the body and so with the 

 outside world. The central nervous system is further divided 

 into the spinal cord, and the brain which develops from 

 the anterior part of this. The spinal cord is much the same m 

 all vertebrates ; an inner mass of grey matter surrounds a small 

 canal, and is itself surrounded by white matter (p. 5jj)- l^e 



