6 



PHYSIOLOGY 



CHAP. 



of this group of glandular organs constitutes one of the finest 

 achievements of modern experimental physiology. It more par- 

 ticularly includes the physiology of the thyroid gland, the 

 parathyroids, the hypophysis or pituitary gland, the suprarenal 

 capsules, the paraganglia; to the same category belong also the 

 pineal gland, the carotid glands, and the coccygeal gland, of which 

 the functional significance is still unknown. 



III. The Thyroid Gland (more correctly glandula tliyreoidect) 

 Ts in man a single organ, in colour dark red shading into yellow, 

 which lies at the sides and in front of the larynx and the two 

 first tracheal rings. Two lateral lobes and a median isthmus can 

 be distinguished, above which rises a slender conical process 

 (Morgagni's pyramid) which is attached to the thyroid bone by a 



FIG. 1. A, Human thyroid gland, showing bifurcation of lower end of pyramidal process, one part 

 going to each ateral lobe. B, the same, with pyramidal process attached to left lobe of 

 gland ; isthmus absent. C, the same, with pyramidal process and isthmus absent. (C F 

 Marshall.) 



fibrous and muscular band (Fig. 1, A). It varies considerably 

 in size, the weight seldom exceeding 30-40 grms. It is generally 

 more developed in females than in males, and often swells at the 

 periods of menstruation. 



In the cat, rabbit, guinea-pig, and rat, the isthmus joining the 

 two lobes is represented by a very slender band of thyroid tissue ; 

 in the dog, on the contrary, the two lobes are almost always 

 separated, as, by a congenital anomaly, may also occur in man 

 (Fig. 1, B and C). The isthmus is almost always well developed 

 in the ape, as also in ruminants. 



The thyroid is invested by a transparent capsule of dense 

 areolar tissue which connects it loosely with the adjacent parts, 

 and penetrates to the interior, separating the substance into small 

 lobules of unequal form and size. When cut into, a yellow, 

 sticky fluid escapes from the surface, which had previously been 

 contained in a multitude of closed vesicles or follicles surrounded 



