160 THE ANATOMY OF YERTEBRATED ANIMALS. 



and it tliea unites with the second and third to form the root 

 of the dorsal aorta. The basal moiety of the first trunk en- 

 larges at its extremity, close to the angle of the mandible, into 

 a spongy organ, the carotid gland, from which the carotid ar- 

 tery, and that for the supply of the hyoidean and oral regions, 

 are given off. 



In the adult Frog, the aortic bulb is separated by an in- 

 complete longitudinal septum into two passages ; and, at its 

 extremity, divides into two trunks, each of which is parti- 

 tioned internally into three passages. The middle, or systemic, 

 passage passes directly into a trunk, which unites with its fel- 

 low beneath the spinal column into the dorsal aorta. The an- 

 terior, or carotid, passage ends, as in Salamajidra, in a ca- 

 rotid gland and ductus JBotalli / carotid, hyoidean, and oral 

 branches being given ofi" from the former. The hindermost, 

 ox put mo-cutaneous, passage ends in the pulmonary and the 

 cutaneous arteries, the anastomoses of these with the roots of 

 the dorsal aorta being obliterated. The middle pair of aortic 

 trunks thus exclusively constitute the origins of the dorsal 

 aorta, and are the permanent aortic arches. The right aortic 

 arch is wider than the left, especially toward their junction ; 

 as the left gives off, just before this point, a large coeliaco- 

 mesenteric artery to the abdominal viscera. Each aortic arch 

 gives off the subclavian and vertebral arteries of its side. 

 Only venous blood passes into the pulmonary arteries of a 

 Frog ; while mixed blood enters the aortic arches, and is of a 

 brighter arterial hue at the end, than at the beginning, of the 

 systole. The blood in the carotid passages is always bright. 

 The mechanical arrangements by which this is brought about 

 have been beautifully analyzed by Briicke, who shows — first, 

 that the spongy interior of the ventricle contains, in its base, 

 a transversely-elongated cavity, into which the auricles open, 

 and which, by its right extremity, communicates with the ven- 

 tricular opening of the aortic bulb ; secondly, that the aortic 

 bulb is imperfectly divided by a longitudinal septum, the 

 upper left edge of which is attached, while its lower right 

 edge is free ; thirdly, that, of the two passages into which the 

 aortic bulb is thus divided, the one on the right side of the 

 septum ends in a chamber, in which the carotid and systemic 

 passages commence, while that on the left side similarly leads 

 to the entrance to the pulmo-cutaneous passages ; fourthly, 

 that the carotid gland, in which the carotid passage ends, pre- 

 sents a mechanical obstacle to the flow of the blood through 

 it ; fifthly, that there is a valvular fold open toward the hearty 



