36o 



COMPARATIVE ANATOMY 



more, it is customary to assume six pairs as the primitive number of 

 aortic arches in gnathostomes. Of these the first two pairs belonging to 

 the mandibular and hyoid arches partly lose their respiratory function 

 and, consequently, the associated aortic arches. In fishes the persistent 

 aortic arches are broken up into a capillary net-work in the gills. In 

 urodeles the last four pairs of aortic arches persist, the blood from the 



Artery 



r Endothelium. 

 Intima i Inner elastic 

 1 membrane. 



Media ■ 



Muscle fibers. 



. Conn, tissue. 

 Outer elastic 

 membrane. 



Externa \ Conn, tissue s 



For orientation. 



Vasa 

 k vasorum 



Fat cells ' 







^yfe?^ 



^^^^ 



Fig. 300. — A section through a human ulnar artery and vein, showing the wall of 

 the artery on the left and of the vein on the right. The upper part of the figure {a-d) is 

 from a section of the same vessels stained with resorcin-fuchsin, an elastic tissue stain. 



a. Circular, and b, radial elastic fibers of the media of the artery; c, external elastic 

 membrane; d, elastic fibers in the media of the vein; e, circular, and g, longitudinal 

 muscle fibers of the media; /, endothelium. XS50. (From Bremer's "Text Book of 

 Histology. ") 



ventral aorta largely short-circuiting the gills. In anura gills are lost 

 and blood passes directly through the aortic arches to the dorsal aorta. 

 In the anura the fifth pair of aortic arches degenerate. That portion, the 

 ductus arteriosus, which connects the pulmonary branches of the sixth 

 aortic arch with the dorsal aorta also atrophies. The third pair of arches 

 persist as the roots of the carotid arteries. In reptiles as in anura three 

 pairs of aortic arches persist, the 3rd, 4th, and 6th in part as pulmonary. 

 In reptiles the common carotid arteries connect with the right and not 

 with the left systemic or fourth aortic arch. In mammals portions of 



