102 APPLIED PHYSIOLOGY 



clinical significance. When the discharge of blood from 

 the splenic vein is interfered with, the organ becomes 

 passively overfilled. This is probably the reason for the 

 enlargement of the spleen which occurs in cirrhosis of 

 the liver. Oncometer tracings show that the spleen 

 exhibits independent rhythmical contraction, apparently 

 by means of the muscular tissue in its capsule and 

 trabeculse. In consequence of this it has been looked 

 upon as endowed with a propulsive power as regards the 

 blood in the portal circulation, and has even been spoken 

 of as the ' portal heart.' The thickening of the capsule 

 and trabeculae of the spleen, which undoubtedly occurs 

 when the portal circulation is obstructed, has been 

 regarded as evidence of an attempt on the part of the 

 ' portal heart ' to overcome the obstruction by hyper- 

 trophy. 



The Thymus. The only thing we know with certainty 

 about the functions of the thymus is that, like adenoid 

 tissue generally, it is a place where lymphocytes are 

 born. Beard,* indeed, regards the thymus as the start- 

 ing-point of all adenoid tissue in the body. In his view, 

 lymphocytes are first formed by a transformation of the 

 primitive epithelial cells of the embryonic thymus, and 

 until the latter appears there are no white cells in the 

 blood. Later on secondary depots or colonies of adenoid 

 tissue are started in various parts of the body by 

 lymphocytes which have emigrated from the thymus, 

 and when these have been fairly established the thymus 

 itself disappears. With its disappearance, however, as 

 he justly remarks, adenoid tissue no more vanishes from 

 * Lancet, 1899, i. 144. 



