134 GENERAL ZOOLOGY 



capillaries in practically all parts of the body and end in openings 

 into the large veins near the heart. The lymphatic vessels differ 

 from blood vessels in two respects: (1) Their walls are more 

 delicate in structure and (2) the free passage of the vessels may 

 be interrupted from time to time by lymph glands, which are 

 nodules of various sizes, composed largely of lymphocytes 

 enclosed in capsules, through which the lymph slowly niters. 

 The flow of the lymph, as in the frog, is from peripheral parts 

 toward the heart. Since lymph hearts are absent in the human 

 body, the movement of the lymph is brought about principally 

 by movements of the surrounding parts exerting pressure on the 

 thin-walled lymph vessels. Other factors are involved, such as 

 the low pressure in the large veins near the heart, which would 

 encourage flow toward the heart. Lymphatic vessels of the 

 extremities are provided with valves, similar to those in the long 

 veins, which allow the lymph to be squeezed past them toward 

 the heart, but not in the reverse direction (Figs. 84 and 85). 



Blood. — The blood of vertebrates consists of a fluid, the plasma, 

 in which various kinds of cells or cell-like bodies, known as 

 corpuscles, are suspended. The principal constituent of plasma 

 is water, in which fats, sugar, and proteins are held in colloidal 

 solution or suspension, along with a number of inorganic salts. 

 In addition the plasma contains various decomposition products 

 of metabolism, formed in various parts of the body. It also 

 contains various endocrine substances, a discussion of which is set 

 forth in Chap. X. 



The principal proteins of the blood are serum albumin, serum 

 globulin, and fibrinogen. The source of these proteins is probably 

 from end products of protein digestion (amino acids) in the 

 alimentary canal that are absorbed into the blood stream. Of 

 these proteins, fibrinogen is of special interest because under 

 certain conditions it forms a solid fibrous substance known as 

 fibrin which entangles the blood corpuscles to form a blood clot. 

 The fluid left after fibrin has been formed from plasma is called 

 serum. Normally blood does not clot in the vessels, but clotting 

 may occur in the vessels from the presence of air or foreign bodies 

 produced as a result of injury or infection. There are a number 

 of factors involved in the clotting process, not all of which are 

 understood. That a certain amount of calcium is necessary for 

 clotting can be demonstrated by adding oxalic acid to the blood, 



