37 CLASSIFICATION OF VERTEBRATES. 



In the venous system the most noticeable points are the pres- 

 ence of valves, at least in the veins of the extremities. Both 

 pre- and postcavae empty directly into the right auricle without 

 the presence of a sinus venosus. In rodents, monotremes, and 

 the elephants, two precavae occur. Rete mirabilia are frequent 

 in various situations. The red blood corpuscles are anucleate 

 and, except in tylopoda where they are oval, are circular disks. 



The lymph vessels, which contain numerous valves, empty 

 by means of a principal or thoracic duct into the precava near 

 the subclavian vein. In their course are numerous lymph 

 glands. Closely related to the lymph system are a couple of 

 masses of adenoid tissue, the tonsils, peculiar to the mammalia, 

 placed at the entrance of the pharynx. 



The entrance to the trachea (glottis) is covered by a fleshy 

 fold, the epiglottis. The larynx is well developed, with aryte- 



FIG. 353. Modifications of the aortic arch and its vessels in mammals, from 

 Wiedersheim. Ao, aorta; c, carotid; s, subclavian ; tb t truncus brachiocephalicus 

 (anonyma); the, truncus brachiocephalicus communis. 



noid, cricoid, and thyroid cartilages (p. 28), and these, moved 

 by appropriate muscles, put various tensions, etc., upon the 

 vocal cords. The cartilaginous tracheal rings are usually in- 

 complete behind ; the trachea itself is never convoluted, and it 

 divides behind into two bronchi, with occasionally a secondary 

 bronchus on the right side. Inside the lung the rule is a single 

 eparterial bronchus arising above the entrance of the pulmonary 

 artery, and nine hyparterial bronchi on either side. Air sacs 

 never occur. 



The inspiration and expiration of air is effected in part by 

 the intercostal muscles, which by their action alter the size of 

 the thoracic cavity, and in part by a transverse muscular par- 

 tition, the diaphragm, which divides the abdominal cavity of the 

 ' lower vertebrates into an anterior or pleural cavity containing 



