77//!,' lif.OOl) SYSTEM 27 



The Veins. — Alllmimh (lie iiijcctccl inlcrit's of (lie fr(»n .studied 

 lire decidedly larger than uiiinj<'('l.ed veins, the hitter in their 

 normal state are larger than arti-ries. As a rule, the veins lie 

 nearer the surface than do tlie arteries. 



Keep in mind the fact that tlie ventricle is the compartment of 

 the heart which forces blood out through the arterial system to 

 all parts of the body. It receives blood from the auricles only. 

 Four veins return blood to the heart; three open into the sinus 

 venosus and thence into the right auricle; while one, the pul- 

 monary vein, opens directly into the left auricle. 



As in the case of arteries in their relations to general regions 

 which they supply, one finds similar relations of the larger veins 

 antl regions which they drain back toward the heart. The vein 

 regions differ slightly in the combination of parts from the artery 

 regions. There are three, as follows: (1) the head, arms, and 

 skin, (2) the lungs, (3) the trunk and legs. Two of these regions 

 may be subdivided. The head-arm-skin region according to the 

 relations of the main vessels draining them is divisible into (a) 

 head and (6) skin-arm divisions. The trunk-leg region similarly 

 comprises (a) dorsal-trunk and (6) ventral-trunk divisions. 



Throughout the study of the veins make comparisons of the 

 dissections with Fig. 1 in order to discover for yourself the struc- 

 tural plan of body regions and the vessels draining them and the 

 general course and interrelations of the vessels themselves. 



Veins Entering the Si7ius Venosus. — The right precava is a large 

 vessel opening into the right side of the sinus venosus. Its 

 branches return blood from the right side of the head, the right 

 forelimb, and the skin of the right side of the back. Gently 

 take hold of the tip of the ventricle with the forceps, raise the 

 heart, turn it to the frog's left, and observe the short precava 

 lying close to the auricle. Follow this short vessel latero-cepha- 

 lad from the heart to a point just outside the peritoneal membrane 

 where it is formed by three tributaries: 



a. The external jugular is the inner of these three divisions 

 and drains the floor of the mouth and the tongue. Close to the 

 ventral surface of each external jugular vein is a small, round, 

 highly vascular body, the thyroid gland. 



h. The middle of the three tributaries of the precava is the 

 innominate vein, coming from a lateral direction and earlier from 

 abruptly dorsad, receiving as it turns a small vein from the back 

 of the arm and shoulder. The main vein iufore the entrance of 



