320 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



enlarges and becomes the path of the main blood stream and a part of the arch 

 of the adult (fig. 341). Of the four arches (3, 4, 5, and 6 of the primitive 

 scheme) the fifiji is last in tl^e adults of all except a few urodeles and caecilians. 

 The fjiurth connects with the^doigal^a^rta and the ^th vnih the pulmo nary 

 arteries. These last, which have a Botall's duct on one or both sides in some 

 urodeles, are noticeable for the large cutaneous arteries — anterior and posterior 

 — which arise from them in the anura, and which play an important part in 

 respiration. Connected with the carotid arteries are the carotid glands (p 

 264). In the larval stage each consists of a network of blood-vessels — a rete 

 mirabile — between the afferent branchial and the carotid arteries, but in the 

 adult this degenerates into a small muscular organ containing sympathetic cells 

 at the junction of external and internal carotids. The caudal artery and vein 



entirely disappear in the adult anura. 

 The postcava is always well devel- 

 oped and the epigastric (lateral abdomi- 

 nal) veins unite to form an anterior 

 abdominal vein (fig. 332) so that the 

 blood from the hind limbs may return 

 to the heart through either the anterior 

 abdominal and the hepatic portal 

 system or the renal portal system and 

 the postcava. 



In the lungless salamanders the 

 heart and blood-vessels show correlated 

 modifications. Those of the heart were 

 mentioned above. The pulmonary 



arteries and veins fail to develop, but 

 Fig. 341. — Diagram of the aortic ^^ ^ *. ■ j iU n 



arches in amphibiaT Arterial blood cross the cutaneous arteries and the smaller 

 lined, venous black. The gill circulation vessels supplying the pharyngeal region 

 omitted, its course indicated by arrows; ^^e greatly enlarged, respiration being 

 the permanent circulation after the absorp- ~. , , , , . , ^, 



tion of gills shown, eg, carotid gland; da, effected by the skin and the mucous 

 dorsal aorta; d, ductus Botalli; pa, pulmo- membrane of the throat, 

 nary artery; va, ventral aorta; 3-6, aortic ^j^^ ^^^^^^ ^^ ^.^e anuran heart may 



be outlined here. The two atria, filled 

 with blood, contract at the same time, forcing arterial and venous blood into 

 the ventricle, but these are kept from mixing by the muscular trabeculae already 

 mentioned. At the systole of the ventricle, the venous blood, which lies nearest 

 the bulbus, is first forced forward. This takes the most direct course through 

 the wide and shorter pulmonary arteries which are practically empty at the 

 time. The next portion of the blood — mixed arterial and venous, follows the 

 next easiest course through the aortic arches, while the last to leave the ventricle, 

 consisting of pure arterial blood, can only go to the carotids, where the resist- 

 ance is greater on account of the small size of the vessels and the obstacles pre- 

 sented by the carotid glands. 



REPTILES. — The circulation in the reptiles shows several advances over 

 that in the amphibia. The diyigioiLof the heart is_carried into the. Yfintricle 

 and the^giijus y£.nosu&Js not visible from the exterior, though valves at the en- 



