THYROIDS AND PARATHYROIDS 



645- 



life in mammals, and that its removal is always followed by a characteris- 

 tic cachexia (Klose) has been overthrown by later experiments on the 

 effects of extirpation of the gland (Pappenheim, Howland, etc.). The 

 thymus of young mammals can be readily auto-grafted into the subcu- 

 taneous or subperitoneal tissues. Such grafts involute at sexual 

 maturity with the main thymus. Homceo-grafts are not permanently 

 successful in mammals, except that they take and survive for two or 

 three weeks (Marine and Manley). 



The chief effect of intravenous injection of extract of human or ox 

 thymus is a lowering of blood-pressure ; but there is nothing specific 

 in this, a similar effect being given by thyroid extract and the extracts 

 of many other tissues. The heart may be at the same time accelerated. 

 When thymus substance is fed to tadpoles, growth is markedly stimu- 

 lated and metamorphosis delayed (Gudernatsch), the opposite of the 

 effect produced by thyroid substances. This direct evidence supports 

 the observations on mammals above referred to that removal of the 

 thymus hastens sexual differentiation ; that delayed sexual differentia- 

 tion, as in myxcedema, lymphatism, etc., is associated with enlarged 

 thymus; and that castration delays the involution of the thymus. 



Thyroids and Parathyroids. The thyroid consists of two lobes 

 connected by an isthmus across the middle line in man and some 

 animals, but often separate. 

 In the neighbourhood of the 

 thyroid, or embedded in its 

 tissue, are certain bodies 

 called parathyroids, consist- 

 ing of solid columns of epi- 

 thelial cells. The number 

 and situation of the para- 

 thyroids are not constant. 

 As a rule, there are four in 

 mammals, two on each side, 

 but this number is subject 

 to variations in different 

 individuals of the same 

 species. The variability in 

 their anatomical relations to 

 the thyroid is of greater 

 significance. For much of 

 the uncertainty in which the 

 whole question of the symp- 

 toms following extirpation 



Fig. 202. Parathyroid (Vincent and Jolly). 

 A small portion of parathyroid of cat em- 

 bedded in thyroid tissue. It consists for 

 the most part of solid columns of epithelial 

 cells (3, 5, 8) with strands of vascular con- 

 nective tissue (6). A thyroid vesicle (n)' 

 and portions of two others (i, 10) are seen 

 in the lower part of the figure, separated 

 from the parathyroid by a fibrous capsule 

 (2). 4, 7, bloodvessels; 9, lower boundary 

 of the parathyroid tissue. ( x 500.) 



of the thyroids was until 

 lately involved arose from 

 ignorance or insufficient re- 

 cognition of this variability. 

 In most animals the inferior, anterior, or external pair of para- 

 thyroids is more or less distinctly separated from the thyroid. 

 The separation is especially evident in the herbivora, in the monkey, 



