SECRETION. 467 



of sugar in the blood under these circumstances, would be a quickly avail- 

 able source, of energy-yielding material for the continued muscle activity 

 that would attend either flight or defense. 



The Spleen. The spleen is a soft bluish-red organ, oval in shape, from 

 twelve to fifteen centimeters long by eight broad and four thick. It is situ- 

 ated in the left hypochondrium between the stomach and the diaphragm. 

 In this situation it is held in position by a fold of the peritoneum which passes 

 from the upper border to the diaphragm. 



Structure. A section of the spleen shows that it consists of connective 

 tissue, blood-vessels, lymph-corpuscles, and lymphoid tissue. The surface 

 of the spleen is covered by a capsule composed of dense fibrous tissue, 

 from the inner surface of which septa or trabeculae pass inward toward the 

 center of the organ. In their course they give off a series of processes which 

 unite freely, forming a spongy connective-tissue framework. The capsule 

 and the main trabeculae in some animals contain numerous non-striated 

 muscle-fibers. In man they are relatively few in number. The blood-ves- 

 sels which enter the spleen are supported by the connective-tissue septa. 

 As they pass toward the center of the organ they divide very rapidly and 

 soon diminish in size. In their course small branches are given off, which 

 penetrate the inter-trabecular tissue and become encased with spheric or 

 cylindric masses of adenoid tissue known as Malpighian corpuscles. These 

 corpuscles are composed largely of leukocytes. In some animals the leuko- 

 cytes, instead of being arranged in masses, are distributed along the walls 

 of the artery as a continuous layer. Within the corpuscles the arteries pass 

 into capillaries; whether the artery passes directly to the splenic pulp or indi- 

 rectly by way of the corpuscles, its ultimate branches terminate in capillaries 

 which open into the spaces of the splenic pulp. From these spaces a net- 

 work of venules gathers the blood and transmits it to the veins. It is a dis- 

 puted question as to whether the spaces are lined by epithelium, thus form- 

 ing a continuous blood channel, or whether they are wanting in this histologic 

 element. 



The Splenic Pulp. The spaces of the connective- tissue framework 

 are filled with a dark red semifluid mass known as the splenic pulp. When 

 microscopically examined, the pulp presents a fine loose network of adenoid 

 tissue, large numbers of leukocytes or lymph-corpuscles, red corpuscles in 

 various stages of disintegration, and pigment granules. Chemic analysis 

 reveals the presence of a number of nitrogen-holding bodies, e.g., leucin, 

 tyrosin, xanthin, uric acid; organic acids, e.g., acetic, lactic, succinic acids; pig- 

 ments containing iron, and inorganic salts. 



The Functions of the Spleen. Notwithstanding all the experiments 

 which have been made to determine the functions of the spleen, it can not be 

 said that any very definite results have been obtained. The fact that the 

 spleen can be removed from the body of an animal without appreciably inter- 

 fering with the normal metabolism would indicate that its function is not very 

 important. The chief changes observed after such a procedure are an en- 

 largement of the lymphatic glands and an increase in the activity of the red 

 marrow of the bones. The presence of large numbers of leukocytes in the 

 splenic pulp and in the blood of the splenic vein suggested the idea that the 

 spleen is engaged in the production of leukocytes, and to this extent contrib- 



