258 PROFESSOR MARSHALL AND EDWARD J. BLES. 



no object in giving with regard to this stage any details concerning 

 the general anatomy, or the condition of the heart, or the vessels of 

 the anterior part of the head. We therefore pass at once to the 

 consideration of the vessels of the four branchial arches. 



In each arch there is now only a single vessel, or aortic arch, as it 

 is commonly termed. This aortic arch is shown by stages inter- 

 mediate between the present and the last described one, to be 

 formed by enlargement of the direct communication between the 

 afferent and efferent vessels, followed by gradual shrinking and 

 atrophy of the part of the afferent vessel above the aperture of 

 communication. Each aortic arch thus consists of the proximal or 

 cardiac portion of the afferent branchial vessel together with the 

 whole length of the efferent branchial vessel of the arch. 



Of the four aortic arches, that of the first branchial arch becomes 

 the carotid arch of the adult. It is interrupted opposite the base 

 of the lingual artery by the carotid gland, which is the persistent 

 plexiform communication between the former afferent and efferent 

 vessels, from which the lingual artery still arises by two roots. The 

 portion of the dorsal aorta between the points of opening of the first 

 and second aortic arches remains an open tubular vessel for some 

 time, and may even retain its lumen in the adult. More usually, 

 however, the cavity becomes obliterated, and the walls of the vessel 

 merely form a pigmented band connecting the dorsal ends of the 

 first and second, i.e., the carotid and systemic arches, together. 

 After the obliteration of this part of the aorta, the blood passing 

 along the carotid arch is distributed exclusively to the head. 



The second aortic arch, in the second branchial arch, becomes the 

 systemic arch of the adult, and requires no further description. 



The third aortic arch, in the third branchial arch, disappears 

 altogether. In young frogs of the first year it loses its connection 

 with the aorta, and then gradually shortens up, the distal part 

 becoming a solid cellular cord, and the proximal or cardiac part 

 retaining for a time its lumen. Before the end of the first year this 

 vessel has entirely disappeared. 



The fourth aortic arch, in the fourth branchial arch, becomes the 

 pulmo-cutaneous arch of the adult. It retains its communication 

 with the aorta for some time after the dorsal part of the third arch 

 has atrophied ; but before the end of the first year the part of the 



