44 



Anatomy of the Rabbit. 



the distribution of the blood to the lungs for purposes of aeration. It is 

 established by the right ventricle, the pulmonary artery and its paired 

 branches, and by the capillaries of the lungs. The blood is delivered to 

 the left atrium through several pulmonary veins. A similar division of 

 the circulatory organs occurs as a homoplastic modification in birds, 

 which, it will be observed, are also warm-blooded vertebrates. 



Many of the peculiar features of the mammalian circulation which at 

 first sight do not appear to be general, but are so in reality, depend on the 

 circumstance that the complete partition of the organs is a final stage of 

 a general progressive development, observable in air-breathing verte- 

 brates, in which the lungs and their vascular connections become perfected 

 for pulmonary respiration. On the other hand, the vascular system as it 

 appears in the embryo,, more especially its aortic portion, is arranged 

 according to the type of branchial respiration as found in fishes. In this 



condition the blood is sent forward from 

 the heart through a ventral aorta. The 

 latter is connected with a series of paired 

 branchial aortic arches, traversing the 

 rudimentary gill structures, and thus 

 passing upward around the sides of the 

 primitive pharynx. The dorsal aorta is 

 formed by the junction of the branchial 

 aortic arches, and passes backward as a 

 main distributing vessel on the ventral side 

 of the axial support. The heart itself is 

 formed primarily on two-chambered plan, 

 similar to that in fishes, where all the 

 blood is received by single atrium and is 

 delivered forward to the gills by a single 

 ventricle. 



The definitive condition of the chief 

 arterial vessels is arrived at by an extensive 

 modification of the branchial plan. As 

 indicated in the accompanying diagram 

 (Fig. 24) the arch condition is retained by 

 the aorta and by the pulmonary artery, 

 and it is interesting to note also that the 

 primary connections of these vessels, repre- 

 sented in the embryo by an open canal, the ductus arteriosus 

 (Botalli), is indicated in the adult condition by a short fibrous 

 cord, the arterial ligament. The adult aortic arch represents only the 

 left one of a pair, and since that of the right is only represented 

 imperfectly by the innominate artery and the base of the subclavian of 

 that side, a condition of asymmetry results, which is mainly expressed 

 by the sinistral position of the arch with reference to the oesophagus 

 (Plate VII). By comparison with the embryonic plan, it is seen that the 

 primitive features of the heart and the arterial vessels include the 

 ventral position of the heart itself, the equivalence of the two atria and 

 of the two ventricles — these structures being partitioned internally but 



Fig. 24. Plan of the branchial aortic 

 arches. The adulfmamroalian vessels 

 are indicated in black (systemic) or 

 shaded (pulmonary). 1-6, primary 

 arches; ao., aorta; a. p., pulmonary 

 artery; c.e., external carotid; c.i., in- 

 ternal carotid; d.a., ductus arteriosus 

 (Botalli); i., innominate artery; s.d., 

 right subclavian; s.s.. left subclavian. 

 (From Weber, after Boas.) 



