INTERNAL SECRETK )NS 



r>19 



extract. For example, if the depressor nerve be stimulated 

 before and after excitation of the thyroid nerves (or injection of 

 thyroid extract), the fall of blood-pressure is greater in the second 

 case (Fig. 256). Likewise, if adrenalin be injected, the rise of blood- 

 pressure is greater in the second case (Fig. 238). 



The injection of any of the commercial preparations of thyroid 

 extract produces these effects, but not the separated product, " iodo- 

 thyrin." As to the exact chemical nature of the active body, nothing 

 definite is known. Its iodine content, no doubt, is of great importance, 

 but is not the sole factor. 



jgygz ijiH 



vjv;ra: .-. <r- .^-r-M^- 



-Vlrf.'i - , ^ > 



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mm 





ILc. 



H.c. 



FIG. 257. PORTION OF THYMUS GLAND or A MONKEY. Low POWER. 

 Vincent, drawn by Mrs. Thompson.) 



(From Swale 

 c., Cortex; H.c., Hassal's concentric corpuscles; m., medulla. 



The Carotid Body is a minute structure situated at the bifurcation 

 of the common carotid artery. It is richly supplied with nervous 

 elements, and probably belongs to the group of chromophil tissues, 

 with a function akin to that of the medulla of the suprarenal gland. 



The Thymus Gland. In its development, the thymus gland arises 

 from the gill-clefts, and may be apparently entodermal or ectodermal 

 in origin, or both (Fig. 257). In man and the rabbit it is ento- 

 dermal, in the mole it is ectodermal, in the guinea-pig and pig it has 

 a dual origin. At birth it weighs about ounce, and is relatively a 

 large organ. It increases in size and weight for some years after 

 birth, probably until puberty, and then atrophies slowly. 



It is subdivided by connective tissue into lobes, and each of these 

 is made up of several lobules, which are divided into a darker cortex 



