XV THE CIRCULATORY SYSTEM 269 



vessels which carry blood away from the heart, have thicker 

 walls than the veins, and after death are usually almost 

 devoid of blood. The ultimate ramifications of the arteries 

 lead to a system of minute capillaries, through the very thin 

 walls of which there is an exchange of products between 

 the blood and the tissues. From the capillaries the blood 

 flows into the veins, by which it is returned to the heart. 

 All of the blood vessels with the exception of the capilla- 

 ries are provided with coats of unstriated muscle, by the 

 contraction and relaxation of which their caliber may be 

 diminished or increased. 



The arterial system begins in the truncus arteriosus, which, 

 after its bifurcation, splits up into three pairs of arteries which 

 are symmetrically disposed on either side of the middle line. 

 The anterior of these three arteries, the common carotid, 

 soon divides into the lingual, or external carotid, and the 

 larger internal carotid. The lingual runs forward, giving 

 branches to the thyroid, pseudo-thyroid, various muscles 

 of the hyoid apparatus, and tongue. At the junction of 

 the internal and external carotid, but principally upon the 

 former, there is an oval enlargement known as the carotid 

 gland. Internally the carotid gland contains a spongy net- 

 work which serves as an impediment to the flow of the 

 blood. As this organ becomes somewhat distended after 

 each pulsation of the heart and then slowly contracts 

 (Hyrtl, Sabatier), it also serves to equalize the blood flow, 

 especially in the internal carotid. The carotid gland is 

 developed in the larva through the anastomoses of vessels 

 connecting the afferent and efferent arteries of the first gill 

 arch ; between the blood vessels are cells derived from the 

 epithelium of the gill slits (Maurer). 



The internal carotid proceeds outward and dorsally from 

 the carotid gland, and then forward and somewhat toward 



