H94 PHYSIOLOGY 



structure, and is found to conform to the invertebrate type of eye. It is 

 doubtful whether at any time in the history of vertebrates the pineal eye 

 has been functional. 



The carotid and coccygeal glands have often been grouped with the 

 collections of chromamne cells already described as associated with the 

 sympathetic system. Their structure resembles more nearly that of the 

 parathyroid bodies or the anterior lobe of the pituitary gland. They con- 

 sist of a small collection of columns or masses of cells bound together by 

 connective tissue with a rich supply of blood- capillaries. Nothing is 

 known as to their function. 



The lymph and haemolymph glands, and the spleen, are often grouped 

 with these ductless glands. The essential activity of these bodies, however, 

 lies in the production, not of a diffusible chemical substance, but of formed 

 elements, e.g. lymph-corpuscles, and they do not properly fall within the 

 scope of this chapter. As a matter of convenience, we may deal shortly here 

 with the functions of the spleen, 



THE SPLEEN 



This organ is similar in many respects to a lymphatic gland. It is 

 formed of a framework of connective tissue and unstriated muscular fibres, 

 in the interstices of which is contained the splenic pulp. This consists of a 



FIG. 551. Plethysmographic tracing of spleen (upper curve) from a dog, showing 

 the spontaneous contractions of this organ (reduced from a tracing by SCHAFER). 



fine fibrillar network, on the fibrils of which lie endothelial cells. The 

 meshes contain the cells of the splenic pulp, which are fairly large polygonal 

 cells, and leucocytes. Just as in a lymphatic gland the cellular elements of 

 the tissues are bathed by the lymph which flows through the gland, so in the 

 spleen the walls of the capillaries become discontinuous, and the blood is 

 poured out into the interstices of the tissue. The spleen is therefore the 

 only tissue in the body where the blood comes in actual contact with the 

 tissue- elements themselves. The blood from the splenic pulp is collected into 

 large venous sinuses, which run along the trabeculse to the hilum, where 

 they unite to form the splenic vein. The arteries to the spleen are beset in 

 their course along the trabeculse with small nodules of lymphoid tissue, which 

 are known as the Malpighian follicles. 



