250 VERTEBRATES. 



mucous membrane, or inside skin (which lines all the cavi- 

 ties of the body that have open communication with the 

 exterior), strengthened by slender elastic ligaments. They 

 are attached posteriorly to the arytenoids, by rotation of 

 which they are stretched across the tube of the larynx. 

 When fully stretched, the opening between them is but 

 a narrow vertical cleft. In this position they vibrate 

 readily in a passing current of air expelled from the lungs, 

 and they thus produce voice. A pair of folds of similar 

 appearance, at the anterior boundaries of the ventricles, 

 are the false vocal cords. 



The Lacteal System. The thoracic duct is a slender, 

 thin- walled tube which extends through the thorax length- 

 wise, close to, but a little above, the dorsal aorta. It is 

 formed by the union of numerous small vessels, called 

 lacteals, in the anterior part of the mesentery, in the 

 abdominal cavity. It perforates the diaphragm, and 

 passes directly forward to the anterior end of the thoracic 

 cavity, where it widens somewhat, and turns inward, to 

 open into the left preecava, near to its subclavian branch. 

 It receives in its course other lymphatic vessels from 

 nearly all parts of the body. These vessels form a sort 

 of drainage system for the tissues. The lymph they 

 bring, and the liquid products of digestion brought by 

 the lacteals from the intestine, are carried by the thoracic 

 duct, and emptied into the venous system, to mix and 

 commingle with the blood. 



The pale, soft, glandular-appearing body, through which 

 the curved anterior end of the thoracic duct passes before 

 entering the prsecava, is the thymus. 



Spinal and Sympathetic Nerves. The spinal nerves 

 may be seen through pleura and peritoneum, in the dorsal 

 wall of the body cavity. 



A double chain of sympathetic ganglia extends along 



