THE DUCTLESS GLANDS. 409 



contraction and relaxation of the muscular tissue in its capsule and 

 trabeculre ; this rhythmic contraction, which takes place about once a 

 minute, can be recorded by enclosing the spleen in a plethysmograph 

 connected, with a tambour. The muscular fibres are supplied with 

 nerves from the sympathetic system, and the direct or reflex stimulation 

 of these nerves, or the injection of adrenalin, produces contraction of 

 the muscle and diminution of the volume of the spleen. 



The functions of the spleen are not fully known, though it is hot 

 essential to life and can be removed without serious after-effects. The 

 presence of partially disintegrated red blood corpuscles in the phagocytic 

 cells of the pulp indicates that it is concerned in the destruction of 

 red cells, but the extent to which this takes place is not known. The 

 spleen normally contains a relatively large amount of iron, and when 

 the destruction of red cells in the body is excessive this amount is in- 

 creased. Further, the Malpighian bodies undoubtedly form lympho- 

 cytes. In all probability the spleen also takes part in the production 

 of uric acid, since it contains enzymes which can convert xanthine 

 and hypoxanthine into uric acid. 



In many infective diseases the spleen is enlarged, and it seems to 

 play a part in the protection of the body against disease by removing 

 micro-organisms from the blood, possibly also by destroying the poisons 

 formed by such organisms. 



THE THYMUS. 



The thymus is composed of lobules united by connective tissue ; 

 each lobule consists of an outer, denser cortex and an inner medullary 

 part, the cortex being subdivided by strand^ of connective tissue into a 

 number of compartments. Both the cortex and medulla are composed 

 of a network of fibrils covered with endothelial cells, the meshes being 

 occupied by lymphocytes. In the medulla are found a number of 

 small masses of flattened epithelial cells arranged concentrically ; they 

 are called Hassall's corpuscles, and represent the remains of part of 

 the epithelium of the third visceral pouch. The gland is abundantly 

 supplied with blood-vessels. In man it reaches its maximum size 

 during the first two or three years of life, and then becomes smaller, 

 being almost completely absent in the adult. After its removal in 

 animals, the testes develop more rapidly, and conversely castration 

 delays the atrophy of the thymus. When young animals receive fresh 

 thymus with their food sexual maturity is delayed, and, in the male, 

 degeneration of the testis takes place. 



