VASCULAR SYSTEM 305 



of two atria or auricles, and two ventricles, and the conus arteriosus 

 and sinus venosus become eventually more or less incorporated in 

 the ventricles and right auricle respectively. Thus a right (venous) 

 and a left (arterial) half can be distinguished ; and a new vessel, 

 the pulmona7'y artery, arising irom. the l&st arterial arch, becomes 

 connected with the right ventricle : this conveys venous blood to the 

 lungs, while special vessels (pulmonary veins) return the oxygenated 

 blood from the lungs to the left auricle, from which it passes into the 

 left ventricle and so into the general circulation of the body. 



The branchial vessels never become functional as such, in any 

 period of development either in Sauropsida or Mammalia ; but those 

 which persist give rise, as already mentioned, to important vascular 

 trunks of the head, neck (carotids), anterior extremities (sub- 

 clavians), and lungs (pulmonary arteries), and also to the roots of 

 the aorta, one or both of which may remain (comp. Fig. 243). 



The primitive number of arterial arches is six, the first two of 

 which (belonging to the mandibular and hyoid arches respectively) 

 almost always disappear early : in caducibranchiate Amphibia 

 (including Anura) and in Amniota, the fifth arch also disappears. 

 The third gives rise to the carotid arch ; the fourth of both sides 

 (Amphibia, Reptilia), or of one side (Aves, Mammalia), to the aortic 

 or systemic arch, and the sixth to the pulmonary arch (Fig. 243). 



From the Dipnoi onwards, the posterior cardinals become more 

 or less completely replaced functionally by a large unpaired vein, 

 the postcaval or posterior vena cava, v/hich opens independently into 

 the right auricle. 



The Heart, together with the Origins of the Main 



Vessels. 



Fishes (including Cyclostomes). — The heart in Fishes is 

 situated in the anterior part of the body-cavity, close behind the 

 head. It is formed on the same general plan as that described on 

 p. 300, consisting of a ventricle with a truncus arteriosus or 

 merely a bulbus (Cyclostorai, Teleostei), and an atrium or 

 auricle, the latter receiving its blood from a sinus venosus, and 

 being laterally expanded to form the appendices auricidcc (Figs. 245 

 and 246). 



In correspondence with the work which each portion has to 

 perform, the walls of the atrium are thin, while those of the ventricle 

 are much stronger, its muscles giving rise in the interior to a network 

 and usually to a series of large trabeculse : this holds good through- 

 out the Craniata. 



Between the sinus venosus and atrium, and also between 

 ventricle and atrium, membranous valves are present ; there are 

 primarily two atrioventricular valves, but they njay be further sub- 

 divided. Numerous valves, arranged in rows, are present in the 



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