CIRCULATION AND RESPIRATION 141 



as a result of chemical reactions involved in metabolism. Water 

 has greater solvent properties than any other substance, which is 

 at least one of the reasons why it is important in cells. In 

 mammals particularly, one of the functions of the blood is in 

 connection with the regulation of body temperature, since, 

 except under conditions of hibernation, normal metabolism 

 requires that the body temperature be kept within rather narrow 

 limits. A complicated mechanism is found in these animals 

 for the maintenance of a fairly constant body temperature, only 

 the broad outlines of which can be considered here. In the 

 human body, a dilatation of peripheral blood vessels will increase 

 loss of heat by radiation. Likewise, the activation of sweat 

 glands covers the skin with perspiration, largely water, the 

 subsequent evaporation of which appreciably lowers the body 

 temperature. Contraction of the peripheral blood vessels tends 

 to conserve body heat. Animals like dogs, when overheated, 

 depend principally upon evaporation of water from the surface 

 of the lungs. The same is true of birds. 



Lymph. — Lymph consists of plasma, similar to blood plasma, 

 and relatively few corpuscles, of which lymphocytes are the most 

 abundant. Red corpuscles and polymorphonuclear leucocytes 

 are usually absent. Strictly speaking lymph is the fluid con- 

 tained in lymphatic vessels, though it differs but slightly from 

 the body fluid found in spaces between the organs and body 

 wall, or from the so-called tissue fluids occurring in microscopic 

 spaces between cells. Except in the case of the contents of the 

 lymphatics of the small intestine, which during digestion become 

 gorged with fat, the lymph generally may be considered a part 

 of the drainage system of the body, charged with excretory 

 products, which eventually reach the blood system. Its move- 

 ment is centripetal and thus comparable with the movement of 

 the venous blood. Since the lymphatic vessels begin as capil- 

 laries in the tissues, the source of the fluid component of the 

 lymph is from the blood. 



In the human body, most of the lymphatic vessels are tribu- 

 taries of the thoracic duct, which begins below the diaphragm by 

 the confluence of the lymphatic vessels from the legs, abdomen, 

 and viscera. It then passes anteriorly along the ventral side of 

 the vertebral column, receiving branches from the thorax, left 

 arm, and left side of the head, and empties into the left sub- 



