43^ ZOOLOGY. 



veins, often running together and then rebranching, gradually approach- 

 ing the body cavity. On their route they pass through knots of special 

 tissue lymphatic glands, where colorless amoeboid cells are added. Special 

 lymphatics the lacteals gather food from the intestines and, uniting with 

 the general lymphatics, finally empty into the large veins in the neck 

 region. The escaped lymph is thus returned to the blood. 



448. The Respiratory Structures differ from those of 

 birds chiefly in the fact that they are confined to the anterior 

 or thoracic cavity, in which they hang freely, suspended by 

 the bronchi. There are no air-sacs outside the lungs, hence 

 all the air passages terminate in the alveoli, in the walls of 

 which are the pulmonary capillaries. Inspiration and expira- 

 tion of air is effected by increasing and decreasing the size of 

 the chest cavity by means of the muscles between the ribs and 

 by the contraction of the muscles of the diaphragm which is 

 normally arched forward into the chest. By its contraction 

 the viscera are forced backward and more space is given to the 

 lung, which at once fills the chest cavity as the result of air- 

 pressure on the inside of the lung. 



449. Nervous System. The special feature worthy of 

 note in the nervous system of mammals is the large size of the 

 brain, especially of the cerebral hemispheres. In the higher 

 mammals, particularly, these become complicated by folds and 

 convolutions by which the surface or cortex of the brain is 

 much increased. The brain cells, or gray matter of the brain, 

 are especially abundant in the superficial part, and therefore 

 this increase of surface means that these cells are increased in 

 amount as compared with any other vertebrates. The intelli- 

 gence of an animal is roughly proportional to the amount of 

 the cortex. The fibrous tracts connecting, the various por- 

 tions of the cortex are likewise more perfectly developed 

 among the mammals. 



The organs of special sense are similar to those in the birds. 

 The ear becomes more complicated. There is usually a well- 

 developed external ear, or pinna, in the terrestrial forms, 

 which is often movable and serves to gather the sound waves. 



