i82 PHYLUM CHORDATA 



Three large caval veins are present, which enter the sinus 

 venosus; two of these, the right and left precavals, bring 

 blood from the anterior half of the body, including the forelegs ; 

 the other, the postcaval, brings blood from the posterior half 

 of the body. 



Turn the apex of the heart forward and observe the two large 

 precaval veins which enter the sinus venosus at its forward end. 

 Each precaval is formed by the union of three veins, which meet 

 immediately in front of the heart. These are the external jugular, 

 the most anterior of the three, which brings blood from the head ; 

 the innominate vein, the middle one, which brings blood from the 

 brain, the shoulder, and the forearm ; and the subclavian vein, 

 the largest and hindermost, which brings blood from the arm and 

 the skin. 



Several small, glandular bodies are present near the external 

 and internal jugular veins, of which the parathjrroid, the thyroid, 

 and the thymus glands are the most important. The first of these 

 is a small, ovoid body which lies next to the inner ventral surface 

 of the external jugular. The thyroid gland is a somewhat larger 

 body near the parathyroid but dorsal to it. The thymus is a small, 

 ovoid body just back of the hinder margin of the tympanic mem- 

 brane near the innominate vein. 



The subclavian vein is formed by the union of two veins, the 

 brachial vein, which is one of the two veins returning blood from 

 the foreleg, and the great cutaneous vein, which returns it from 

 the skin. Follow the former vein and its branches. The latter 

 vein occupies a peculiar position in that it is partly respiratory in 

 function. It lies on the inner surface of the skin, receiving nu- 

 merous branches, and may be traced forward into the head, where 

 it receives branches from the mucous membrane of the mouth 

 and the pharynx. It is, however, not wholly respiratory, as it 

 also receives branches from muscles. ^ 



Exercise 14. Draw a diagrammatic sketch showing the precaval veins 

 and their branches so far as observed. 



The postcaval is a large median vein which enters the posterior 

 end of the sinus venosus. It arises between the kidneys and^uns 



