460 MAMMALIA— PHYSIOLOGY 



lymph current in motion. The main cause for its direction from the 

 extra-vascular spaces toward the veins in the neck is the degree of 

 pressure to which it is subjected in those spaces as compared with 

 the small pressure near the ends of the great ducts. Lymph flow 

 somewhat resembles that of venous blood, but is less regular and 

 more sluggish, though not so slow as might be supposed. 



S-pleen. — While the mammalian spleen may be operatively re- 

 moved, it serves several important functions: (i) During embryonic 

 life and even after birth, in cases of anemia, the spleen forms red 

 corpuscles. (2) It contains large quantities of organic iron and 

 probably aids in the preparation of hemoglobin. (3) It destroys 

 old worn-out red corpuscles and takes up pigment and other wastes 

 from the blood. (4) It increases in size during digestion and after 

 the fifth hour slowly decreases to normal size. It is excessively 

 vascular during digestion. 



Respiratory System. — In the cat the respiratory system is not 

 greatly different from that of man. Air inhaled at the anterior 

 nares goes through the nasal passages and out from the posterior 

 nares into the naso-pharynx, Lucas ^ finds that in the monkey the 

 cilia of the septum, the middle concha {turbinate) , and the middle 

 meatus sweep the mucus downward toward the floor and hence 

 backward into the naso-pharynx (not upward as senior authors have 

 claimed). The trachea of the cat consists of forty-five cartilages 

 (16-20 in man) which are held in place by fibrous membrane which 

 embeds them. In the cat the trachea is three-fourths of an inch in 

 diameter and four and one-half inches in length. The lungs consist 

 of three lobes on each side with a fourth lobe on the right side which 

 is divided into two parts, and lies a little to the right of the middle of 

 the body. The cartilaginous rings extend two-thirds of the way 

 around the trachea, the third not supported by the rings resting 

 upon the esophagus. The lungs are supported in the chest by paired 

 bronchi and the pulmonary and bronchial arteries, veins and 

 lymphatics. The bj-onchi also have incomplete irregularly arranged 

 cartilaginous rings. Each lung is covered by a delicate serous 

 membrane, the pleura^ which is reflected over the inner surface of 

 the thorax. The lung consists of many lobules of air-cells with 

 extremely thin walls and is richly supplied with minute capillaries 

 as well as many veins and arteries. In the trachea and the bronchi, 



* Lucas, A. M., in Chapter 1 1 of Special Cytology, 2d edition, edited by E. V. 

 Cowdry, 1931. 



