THYMUS 6 43 



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) (Chapter XIX.) . Removal 

 of the corpora lutea occurring during the first half of pregnancy 

 causes abortion. 



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, mainly from the ventral part of the third branchial cleft, and 

 to a slight extent from the fourth. These pouches grow downwards 

 into the thorax. At this stage the organ is a purely epithelial 

 structure. Soon connective tissue and bloodvessels begin to grow 

 into it, the two halves coalesce in the middle line, and the thymus 

 becomes transformed by degrees into a structure with a general 

 resemblance to a big lymph gland, and consisting mainly of small 

 cells like lymphocytes. Some observers believe 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 con- 

 cerned in the elaboration of substances which exercise an important 

 influence upon nutrition and growth. On the whole, the best histo- 

 logical evidence seems to favour the view that the thymus cells are 



