78 VERTEBRATE ZOOLOGY 



veins, which come from the right and left lungs respectively. Each 

 of these veins is a prominent vessel which lies on the inner side of 

 the lung. Find each, and trace it forward to the pulmonary. 



Exercise 15. Draw a diagram of these veins, so far as observed. 



The Arteries. Two systems of arteries are present : by one of 

 these systems blood is carried from the heart to the gills and by 

 the other from the gills to the tissues. The pulmonary arteries be- 

 long to the last-named system. 



Running forward as a continuation of the truncus arteriosus 

 is the ventral aorta, a large artery which lies in the median plane 

 between the gills. At its forward end this vessel breaks into two 

 pairs of branches. The foremost pair are the first pair of afferent 

 branchial arteries, which carry blood to the first pair of gills. The 

 hinder and larger pair soon divide and give rise to two pairs of 

 arteries, the second and third pairs of afferent branchial arteries, 

 which go to the second and third pairs of gills. Find these arteries 

 and study their relation to the gill arches. 



The blood, after having passed through the gills, is collected 

 in three pairs of efferent arteries, which carry it away from the 

 gills. Find them and note their relation to the gill arches and to 

 the afferent arteries. The foremost efferent artery on each side 

 gives off two arteries which pass straight forward to the head, 

 the larger being the internal carotid and the smaller the external 

 carotid. The second and third efferent arteries unite and form a 

 single vessel, which gives off the large pulmonary artery and is 

 connected with the first efferent artery by a small branch ; it then 

 becomes the aortal arch and, meeting its fellow of the opposite 

 side, forms the dorsal aorta. The pulmonary artery passes straight 

 back along the dorsal side of the lung. 



The dorsal aorta runs back just beneath the spinal column to 

 the hinder end of the animal; in the tail it becomes the caudal 

 artery. At regular intervals, corresponding to the vertebrse, a pair 

 of small spinal arteries spring from the dorsal side of the aorta 

 and supply the body muscles. 



A short distance back of its point of origin the aorta sends off 

 a pair of subclavian arteries, which go to the forelegs. Each sub- 



