RAT, A REPRESENTATIVE MAMMAL 131 



Because of the presence of the substances fibrinogen (a protein), 

 throiJihin, and calcium salts, in the blood, it clots upon exposure to 

 air. The solid fibrous material formed in clotting is fibrin. The 

 thin, watery straw-colored liquid which separates from the clot is 

 sei^um. 



Lymphatic System. — As the blood passes through the tissues of 

 the various organs, it loses a portion of its fluid base. This liquid 

 which seeps from the capillaries is lymph and is collected in the 

 Ijmiph spaces surrounding the cells in most tissues. It is quite simi- 

 lar to plasma, and it distributes food and other necessities to the in- 

 dividual cells and collects carbon dioxide and other waste products to 

 be returned to the blood when it finally returns to it by way of the 

 thoracic lymphatic duct and the subclavian veins. The thoracic duct, 

 the largest in the body, lies in the dorsal part of the trunk between 

 the dorsal aorta and the vertebral column. It receives the various 

 Ijonphatic vessels. These vessels have ultimately been formed by the 

 union of the lymph spaces between the cells in the tissues. Along the 

 lymphatic vessels at certain points are glandular masses, the lymph 

 glands or nodes. These bodies produce Ij^mphocytes, a type of white 

 cell carried in the IjTnph and blood. In certain portions of the body 

 these glands are abundant, as the inguinal (groin), axillary (armpit), 

 popliteal (knee), cubital (elbow), tracheal, and submaxillary regions. 



Respiratory System 



In mammals the special organs of respiration are the lungs, 

 wherein the carbon dioxide is taken from the blood and oxygen is 

 taken from the air by the blood. The nasal passages lead through 

 the nasopharynx to the pharjmx and from here through the glottis 

 to the larynx (voice box), thence by trachea (windpipe) to the 

 bronchi, and into the lungs. The air is warmed as it is passed 

 through the nasal chambers and pharynx. The turbinated bones 

 form extensive convolutions in the lining of the inner surface of 

 the nasal chamber. The tear ducts lead from the eyes into the nasal 

 chamber while the Eustachian (auditory) tubes lead from the 

 middle ear into the naso-pharynx. Air passes from the pharynx, 

 around the fingerlike epiglottis which guards the slitlike glottis lead- 

 ing into the larynx or voice box. The epiglottis closes the entrance 

 of the glottis only during swallowing in order that food or other 

 material may not enter. The vocal cords are located on the inner wall 



