586 



COMPARATIVE ANATOMY. 



• 



arterial circulus eephalicus at the base of the skull. A special optic 

 artery is given off from the vessels of the rudimentary gill, with 

 which a direct branch of the first branchial vein (Selachii), or a 

 branch of the same vessel which surrounds the body of the liyoid 

 (Teleostei), is connected. There are numerous modifications in the 

 mode of origin and arrangement of the different vessels, the most 

 important of which are found in the carotid and optic arteries. 



This portion of the vascular system is arranged in a similar 

 manner in the Amphibia. In the Perennibranchiata the cephalic 



arteries arise from the anterior 

 portion of the roots of the aorta; 

 in the Caducibranchiata, from 

 the first permanent aortic arch, 

 or, they are continuations of the 

 anterior arch itself (Pig. 331, c). 

 In this case, an artery which 

 goes to the tongue (?) represents 

 an external carotid. After this 

 vessel is given off, there is a 

 swelling (c) on the carotid trunk 

 in the Frogs and in the Sala- 

 manders ; this is the so-called 

 carotid gland. The lumen of 

 the vessel is here traversed by a 

 network of bands, which break 

 it up into a number of narrower 

 passages, just as if a capillary 

 network had been intercalated 

 in the course of an artery. The 

 carotid gland appears to be de- 

 rived from an arrangement of 

 this kind, the branchial vascular 

 network not having been com- 

 pletely atrophied. The next pair 

 form the aortic arches (ad as), 

 which converge backwards, and 

 finally unite into an unpaired aortic trunk (a). Each aortic arch 

 gives off a subclavian (sd ss). Just before they unite, a large 

 visceral artery (rn) is given off from the left aorta. The pulmonary 

 artery represents a last aortic arch. Before it goes to the lungs (p) 

 it gives off a large cutaneous branch (cut), which ramifies on the 

 back and neck as far as the posterior region of the head, and affords 

 a distinct proof of the respiratory function of the integument. _ 



In the earliest stages of the Amniota we meet with many similar 

 arrangements of the arterial system. The internal carotid, which 

 supplies the brain and the eye (Fig. 332, A B, c), is seen to be a 

 forward prolongation of the roots of the aorta on either side. The 

 external carotid is a branch of the third primitive aortic arch. If 

 this arch loses its connection with the fourth, the two carotids are 



Fig. 331. Arterial system of the Frog. 

 ha Bulbns arteriosus. c Carotid. 

 C Carotid gland. I Lingual _ artery. 

 ad Eight, as Left aorta, a Aortic trunk. 

 m Visceral artery. sd Eight, ss Left 

 subclavian, oes Oesophageal branches. 

 p Pulmonary artery, cut Its cutaneous 

 branches, occ Posterior cephalic branch. 



