THE CIECULATION OF THE PROG. 185 



trunk enlarges at its extremity, close to the angle of the 

 mandible, into a spongy organ, the carotid gland, from 

 which the carotid artery, 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 

 incomplete longitudinal septum into two passages ; and, 

 at its extremity, divides into two tiimks, each of which 

 is partitioned internally into three passages. The middle, 

 or systemic, passage passes directly into a trxmk, which 

 unites with its f eUow beneath the spinal column into the 

 dorsal aorta. The anterior, or carotid, passage ends, as in 

 Salamandra, in a carotid gland and ductus Botalli ; carotid, 

 hyoidean, and oral branches being given off from the former. 

 The hindermost, or lyuhno-cutaneous, passage ends in the 

 pulmonary and the ciitaneous 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 aoi'ta, and are the pernmnent aortic 

 arches. The right aortic arch is wider than the left, espe- 

 cially towards their junction ; as the left gives off, just before 

 this point, a large coeliaco-mesenteric artery to the ab- 

 dominal viscera. Each aortic arch gives off the subclavian 

 and vertebral arteries of its side. Only venous blood 

 passes into the pvdmonary 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 me- 

 chanical aiTangements by which this is broiight about 

 have been beaxitif uUy analysed by Brlicke, 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, com- 

 mimicates with the ventricular opening of the aortic bulb ; 

 secondly, that the aortic bulb is imperfectly divided by a 

 longitudinal septum, the upper left edge of which is at- 

 tached, 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 septiim ends in a chamber, in 



