H2 VERTEBRATE ZOOLOGY 



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. 



Without cutting any more than is absolutely necessary, trace 

 the external jugular forward. Near the hinder margin of the 

 head it is formed by the union of two veins, the lingual and 

 the internal mandibular vein; the former arises in the tongue, 

 and the latter lies along the inner margin of the lower jaw. 



The innominate vein is formed by the union of two veins,— the 

 internal jugular, which brings blood from the head, and the sub- 

 scapular, which is one of the two veins coming from the leg. 



Several small glandular bodies are present near the external 

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

 and the thymus gland are the most important. The first of 

 these, which is often wrongly called the thyroid, is a small ovoid 

 body which lies next to the inner ventral surface of the external 

 jugular. It arises during the metamorphosis of the animal as a 

 thickening on the walls of the branchial clefts. The thyroid gland 

 is a somewhat larger body, near the pseudothyroid but dorsal to it. 

 The thymus is a small ovoid body just back of the hinder margin 

 of the tympanic membrane, near the internal jugular 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 numer- 

 ous 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 17. Draw a diagrammatic sketch showing the precaval veins 

 and their branches, so far as observed. 



