184 VERTEBRATES. 



divides into two branches, each of which is made up of 

 three aortic arches, which, curving dorsally, go a little 

 way together, bound up in a mass of fatty and connective 

 tissue- Carefully dissect away this tissue, and disclose 

 the three arches of one side. The anterior (carotid} arch 

 first leaves its fellows, and at once divides into two 

 branches which go to the head. The posterior (pulmo- 

 cutaneous) arch sends branches to the lung and to the 

 skin. The larger middle aortic arch curves dorsally and 

 posteriorly. Lift the stomach and adjacent organs a 

 little away from the dorsal wall, and see this middle arch 

 extending backward, and uniting with its fellow of the 

 opposite side on the median line. Observe that the single 

 dorsal aorta thus formed gives off at once a very large 

 branch to the organs of the body cavity, itself extends 

 posteriorly along the dorsal wall, and finally divides into 

 the two iliac arteries, which go to the hind legs. 



Trace now the three arches of one side back to their 

 origin. Cut each off at the point of its separation from 

 its fellows, and pass into it a bristle. Then with fine 

 scissors lay each one open, cutting backward toward the 

 ventricle. Compare the aortic arches of the frog with 

 those of the catfish. 



That the hindmost aortic arch conveys blood to the lung 

 for aeration has already been noticed. From the lung a 

 pulmonary vein conveys the aerated blood back to the 

 left auricle of the heart, whence it passes into the 

 ventricle, to be mixed with the venous current from 

 the right auricle, 



V. Respiratory Organs, Cut the vein and artery con- 

 necting each lung with the vascular system, arid trace the 

 lungs forward to their union in the trachea. Dissect out 

 the trachea, and trace it forward to the mouth. Observe 

 that it passes through a notch between two posterior horns 

 of a broad, flat cartilage in the floor of the mouth, the 



