DUCTLESS GLANDS AND INTERNAL SECRETIONS 295 



The theory is very plausible, though by no means definitely 

 proved ; at the same time there is great difficulty in getting 

 away from the fact that the spleen appears in every way to be 

 admirably suited to act the part of a blood filter. 



The lymphoid tissue of the spleen, like that of lymphoid 

 tissues in general, is capable of forming a substance from which 

 uric acid may be readily produced, and the spleen has in conse- 

 quence been regarded by many as the seat of active metabolic 

 changes with the formation of uric acid. The evidence, how- 

 ever, is not sufficiently conclusive to warrant uric acid being 

 regarded as a special product of the spleen. 



Some physiologists have suggested that the spleen produces 

 an enzyme which converts trypsinogen into trypsin. There 

 is no reason why the spleen might not do so, but it by no means 

 follows that this is normally its function, nor would there appear 

 to be any necessity for this action in face of the fact that it is one 

 of the special duties of the intestinal juice. 



In connection with all these theories it is well to remember 

 that the spleen may be removed completely, and no ill effects 

 follow. 



Thyroid and Parathyroid Glands. — Some of the most interest- 

 ing work on the ductless glands has been carried out on the 

 thyroid, and it is largely to this body that such little knowledge 

 as we as yet possess of internal secretion is mainly due. 



For years it had been observed that atrophy or absence of 

 this gland in the human subject was associated with arrested 

 development both mental and physical ; the man so affected 

 remained a child both in intelligence and appearance. This 

 stimulated experimental inquiry, and the thyroids were removed 

 in many animals, the majority of carnivora dying as the result, 

 while half of the herbivora recovered from the operation. So 

 contradictory were the results obtained by different observers 

 on the gland and its uses, that the whole question was submitted 

 to very close inquiry, which revealed the fact that the ordinary 

 thyroid consists of two distinct portions, one part the thyroid 

 proper, the other the parathyroids. Considerable variation exists 

 as to the arrangement of the thyroids and parathyroids. As 

 a rule there are four parathyroids — an anterior and posterior 

 pair. The latter, in the herbivora, are closely embedded in 

 the capsule of the thyroid, but there are great variations in 

 arrangement even in the same species. In most animals much 

 the same results are obtained when both parts are removed, but 

 when the parathyroids alone are excised, death rapidly ensues, 

 preceded by convulsions. The removal of the thyroid only 

 gives rise to a train of symptoms accompanied by chronic wasting 

 and malnutrition, much slower in development than in the case 



