THYMUS 629 



some substance normally formed in the testicle which influences the 

 metabolism of the bones and the deposition of the bone salts. 



A temporary diminution in the haemoglobin and in the number 

 of the erythrocytes has been observed in castrated bitches, an 

 observation which, so far as it goes, is in favour of the view that an 

 insufficient internal secretion of the ovaries is the cause of the 

 form of anaemia known as chlorosis. 



While these effects on general metabolism and nutrition, as well 

 as the influence on the development of the sexual characters, are 

 probably to be ascribed to changes in the internal secretion of the 

 interstitial cells, there are facts which indicate that other elements 

 may be concerned. For example, evidence has been brought 

 forward that the corpus luteum is a gland with an internal secretion, 

 whose function is connected with menstruation and with the im- 

 plantation of the ovum and the subsequent growth of both ovum 

 and uterus in pregnancy (Born, Fraenkel) (Chap. XIX.). 

 / Thymus. Our knowledge of the function of the thymus is very 

 incomplete.] Even its histological structure, and especially the 

 source and nature of its cellular elements, have long been, and still 

 are, the subject of controversy. It is developed as a pair of diver- 

 ticula from the ventral part of the third and fourth, perhaps also 

 to some extent from the second, branchial cleft. These pouches 

 grow downwards into the thorax. At this stage the organ is a 

 purely epithelial structure. Soon connective tissue and blood- 

 vessels begin to grow into it, the two halves coalesce in the middle 

 line, and the thymus becomes transformed by degrees into a struc- 

 ture with a general resemblance to a big lymph gland, and con- 

 sisting mainly of small cells like lymphocytes. Some observers 

 believt that these cells are true lymphocytes, derived from the 

 mesoderm, which have migrated into and displaced the earlier 

 epithelial tissue. Others maintain that the resemblance is merely 

 superficial, and that they are simply epithelial cells diminished in 

 size and altered in shape, but derived from the original epithelium 

 by repeated division, and remaining epithelial to the end. Any 

 theory of the function of the thymus must needs depend largely 

 upon the view adopted as to its structure. For if it is in its fully 

 developed state merely a large collection of lymphocytes, it would 

 appear quite unlikely that it should possess functions very different 

 from those of other collections of lymphocytes. On the other 

 hand, if the essential elements in the organ are epithelial, they may 

 well, like the epithelial elements of the thyroid or of other glands 

 with an internal secretion, be concerned in the elaboration of sub- 

 stances which exercise an important influence upon nutrition and 

 growth. On the whole, the best histological evidence seems to 

 favour the view that the thymus cells are different from the cells 

 of lymph glands. Chemical differences also exist. For example, 



