THE THYROID. 213 



positive results, but chemical investigation shows that the parenchyma contains a 

 large number of products indicating considerable metabolic activity. The volume 

 of the gland undergoes variations both in health and disease.] 



III. The Thyroid. 



Structure. In a connective tissue net-work rich in cells there lie numerous 

 completely closed sacs (O04 O'l mm. in diameter), which in the embryo and the 

 newly-born animal are composed of a membrana propria lined by a single layer of 

 nucleated cubical cells (Fig. 94). The sacs contain a transparent, viscid, albuminous 

 fluid. [Not unfrequently the 

 sacs contain many coloured c 

 blood-corpuscles (Baber). As 

 in other glands, there are 

 lobes and lobules.] Each sac 

 is surrounded by a plexus of 

 capillaries which do not pene- 

 trate the membrana propria. 

 There are also numerous 

 lymphatics. At an early 

 period the sacs dilate, their 

 cellular lining atrophies, and 

 their contents undergo colloid 

 degeneration. When the 

 gland vesicles are greatly 

 enlarged, "goitre" is pro- 

 duced. 



The Chemical Composi- 

 tion of this gland has not 

 been much investigated. In 

 addition to the ordinary con- 

 stituents, leucin, xanthin, 

 sarkin, lactic, succinic, and 

 volatile fatty acids have been 

 found. 



Functions. Its functions are quite unknown. Perhaps it may be an apparatus 

 for regulating the blood supply to the head (?). It becomes enlarged in Basedow's 

 disease, in which there is great palpitation, as well as protrusion of the eyeball 

 [Exophthalmos], which seem to depend upon a simultaneous stimulation of the 

 accelerating nerve of the heart, and the sympathetic fibres for the smooth muscles 

 in the orbital cavity and the eyelids, as well as of the inhibitory fibres of the 

 vessels of the thyroid. In many localities it is common to find swelling of the 

 thyroid constituting goitre, which is sometimes, but far from invariably, associated 

 with idiocy and cretinism. 



Fig. 94. 



Section of the thyroid gland ( x 250) or, small 

 closed vesicles lined by low columnar epithe- 

 lium ; 6, colloid masses distending the vesicles ; 

 c, connective tissue between the vesicles. 



IV, The Supra-Renal Capsules. 



Structure. These organs are invested by a thin capsule which sends processes 

 into the interior of the organ. They consist of an outer (broad) or cortical layer 

 and an inner (narrow) or medullary layer. The former is yellowish in colour, firm 

 and striated, while the latter is softer and deeper in tint. In the outermost zone 

 of the cortex (Fig. 95 3 b), the trabeculte form polygonal meshes which contain the 



