THE VASCULAR GLANDS. 



387 



is enlarged after digestion, its enlargement is probably due to two causes, 

 (1) a relaxation of the muscular tissue which forms so large a part of its 

 framework; (2) a dilatation of the vessels. Both these phenomena are 

 doubtless under control of the nervous system. It has been found by 

 experiment that when the splenic nerves are cut the spleen enlarges, and 

 that contraction can be brought about (1) by stimulation of the spinal 

 cord (or of the divided nerves); (2) reflexly by stimulation of the central 

 stumps of certain divided nerves, e.g., vagus and sciatic; (3) by local 

 stimulation by an electric current; (4) the exhibition of quinine and 

 some other drugs. It has been shown by the oncometer of Koy (Fig. 

 260), that the spleen undergoes rhythmical contractions and dilatations, 

 due no doubt to the contraction and relaxation of the muscular tissue in 

 its capsule and trabeculaB. It also shows the rhythmical alteration of 

 the general blood-pressure, but to a less extent than the kidney. 



The Thymus. 



This gland must be looked upon as a temporary organ, as it attains 

 its greatest size early after birth, and after the second year gradually 



FIG. 265. 



FIG. 266. 



FIG. 267. 



FIG. QG5. Transverse section of a lobule of an injected infantile thymus gland, a, capsule of 

 connective tissue surrounding the lobule; 6, membrane of the glandular vesicles; c, cavity of the 

 lobule, from which the larger blood-vessels are seen to extend towards and ramify in the spheroidal 

 masses of the lobule, x 30. (Kolliker.) 



FIG 266. From a horizontal section through superficial part of the thymus of a calf, slightly 

 magnified. Showing in the centre a follicle of polygonal shape with similarly shaped follicles round 

 it. (Klein and Noble Smith.) 



FIG 267. The reticulum of the Thymus. a, epithelial elements; 6, corpuscles of Hassall. 

 (Cadiat.) 



diminishes, until in adult life hardly a vestige remains. At its greatest 

 development it is a long narrow body, situated in the front of the chest 



