296 A MANUAL OF VETERINARY PHYSIOLOGY 



of the parathyroids. The convulsions attending removal of the 

 parathyroids are said to be abolished by injections of extracts 

 of the gland. Recently it has been shown that injection of calcium 

 salts effects a cure of all the symptoms produced by removal of 

 these bodies. The colloid substance constitutes the internal 

 secretion of the thyroid, but forms no part of the secretion of the 

 parathyroids ; and histologically while the former consists of 

 vesicles lined by a single layer of cubical epithelium, the para- 

 thyroid is composed of columns of epithelium-like cells. The 

 gland contains a nucleo-protein and colloid substance ; the latter 

 is not a nucleo-protein, and is remarkable for containing iodine 

 in organic combination with the proteidh The iodine-containing 

 substance is termed iodothyrin ; it is a brdwn amorphous material, 

 containing phosphorus and 10 per cent, of iodine. The para- 

 thyroids contain no iodine. 



As to the use of these bodies, it is now generally accepted that 

 the internal secretions of the parathyroids and thyroids are not 

 the same. It is believed that the parathyroids are intended to 

 neutralise some poisonous substance produced in the body during 

 metabolism ; hence, with their removal, the toxic substances 

 accumulate in the blood. It would also appear that in some 

 way or other they may be connected with calcium metabolism. 

 The functions of the thyroid are equally obscure ; they appear 

 to be connected with the metabolism of the central nervous 

 system. The colloid iodine-containing substance is probably 

 the active principle, as therapeutically it takes the place of 

 thyroid tissue. Whether this is due to its containing iodine is 

 not decided, but extracts of the gland which are rich in iodine 

 give better results in treatment than glands poor in that 

 substance. 



Thymus. — This body, composed of modified lymphoid tissue, 

 is mainly of use in fcetal and very early life ; later on it atrophies. 

 Nothing is known of its function, though it is observed that 

 castration appears to have an effect on its disappearance. The 

 process of atrophy is much slower in the castrated as compared 

 with the uncastrated animal, while its early absorption has been 

 observed to be associated with a rapid growth of the testicles. 



Adrenals. — The experimental removal of the adrenals in any 

 animal is rapidly followed by death, preceded by symptoms of 

 great muscular prostration and diminution of vascular tone. 

 In Addison's disease in man these bodies are affected, and give 

 rise to much the same symptoms as above, and in addition 

 bronzing of the skin is present. Like the thyroids, the adrenals 

 consist of two distinct tissues, a medulla, which can be shown 

 to be derived during the process of development from the sym- 

 pathetic nervous system, while the cortex is formed from the 



