292 VERTEBRATE ZOOLOGY 



Exercise 14. Draw the ventral aspect of the heart and the thoracic 

 blood vessels just described, so far as observed. 



The Precaval System of Veins. The precaval, azygos, and 

 coronary veins have been observed. Follow the right precaval 

 forward. Note again the azygos, and also the several pairs of 

 intercostal veins which enter it at regular intervals from the inter- 

 costal spaces ; note the right anterior intercostal vein, which enters 

 the precaval immediately in front of the azygos, bringing blood 

 from the anterior four or five intercostal spaces. The precaval 

 vein will be seen to be formed anterior to this point by the union 

 of three veins, the large subclavian and external jugular veins and 

 the small internal jugular. The subclavian vein returns blood 

 from the foreleg and the shoulder. Lateral to the wall of the 

 thorax it becomes the axillary vein, and in the leg the brachial 

 vein; trace these veins and their branches. Note the prominent 

 white nerves which occupy the axillary region; they form the 

 brachial plexus, and must not be injured. 



The external jugular vein appears as a continuation of the 

 precaval and lies on the side of the neck near the skin ; it is formed 

 near the base of the head by the union of the anterior facial vein, 

 which collects blood from the lower portion of the face, and the 

 posterior facial, which collects it from the upper and hinder por- 

 tion of the head. The small internal jugular vein lies alongside 

 the trachea, the large vagus nerve, and the carotid artery ; it brings 

 blood from the brain and the base of the skull and joins the pre- 

 caval opposite the base of the subclavian. 



The left precaval vein receives branches similar to those of the 

 right precaval, with the exception of the azygos vein, which is not 

 present on the left side. Note the transverse jugular vein, a cross 

 vein which unites the right and left external jugulars a short 

 distance anterior to the point of their union with the subclavians ; 

 it may have been cut when the throat was opened. Note the large 

 thyroid gland at the forward end of the neck ; it is composed of two 

 elongated lobes which lie on each side of the trachea and larynx. 



Exercise 15. Draw a diagram showing the veins of the precaval sys- 

 tem so far as observed. 



