VASCULAR SYSTEM. 



327 



moment into two chambers. Of these the right being nearer to the opening 

 of the venous auricle is charged with venous blood which it delivers into 

 the pulmonary artery, whereas the left contains mixed blood near the sep- 

 tum and arterial blood in the cavum arteriosima. Tho former will pass 

 mainly into the left aortic arch, the latter into the right, from which the 

 carotids and subclavians spring. A proper distribution of blood is thus 

 assiu-ed, the carotids receiving arterial blood only. In the crocodile the 

 ventricle is double and its ca\'ity much clearer of muscular trabeculae 

 than in the other orders. Further the auriculo-ventricular openings are 

 guarded not only by the mesially attached pocket- valve, alone found in 

 other reptiles, but also by a muscular flap of the ventricular wall (like 

 that in the right ventricle of birds). 



The vascular arches which persist in reptiles are the 3rd, 4th, 

 and 5th postoral.* The vessel of the third arch (first branchial 

 of fishes) becomes the carotid : in Lacertilia it usually retains 

 its connection dor- 

 sally with the next 

 arch (Fig. 181) by 

 a vessel which is 

 called a ductus 

 Botalli. In Chelonia 

 (Fig. 179) the fifth 

 or pulmonary arch 

 retains its connec- 

 tion with the pre- 

 ceding (fourth or 

 systemic), so that 

 the ductus Botalli 

 between the 



no. 181. — Diagrams illustrating the metamorphosis of the 

 vascular arches in a lizard A. and snake B (from Balfour). 

 a internal, b external, c common carotid ; d in A, ductus 

 Botalli between third and fourth arches, in B, right aortic 

 trunk ; e in A, right aortic trunk, in B vertebral artery ; 

 / subclavian in A, left aorta in B ; g dorsal aorta ; h in A 

 left aorta, in B pulmonary artery ; i pulmonary artery in 

 A, ductus Botalli in B ; k connection, lost in the adult 

 between pulmonary and systemic arch. 



IS 



fourth and fifth 

 arches. 



The lymphatic system is similar to that of Amphibia. There 

 are some wide lymphatic spaces, and posterior lymph hearts 

 opening into the ischiadic veins are present. Lymphatic glands 

 are absent, except in crocodiles in which there is one in the 

 mesentery. 



Spleen, paired thymus, thyroid, and suprarenal bodies are 



always persent. 



The kidneys are usually lobed structures placed posteriorly. 



* It is maintained by some anatomists that the persisting arches are 

 the third, fourth, and sixth, the fifth early imdergoing atrophy. In this 

 case the pulmonary artery of reptiles will be derived from the vessel of the 

 sixth postoral arch as in Amphibia (see Boas, Morph. Jahrbuch, 13, 1887, 

 p. 115). 



