INTERNAL SECRETION 



495 



in shape, the base being directed downward and on a level with the fifth or 

 sixth tracheal ring. The lobe is about 50 mm. in length, 20 mm. in breadth, 

 and 25 mm. in thickness. As a rule, the lobes are united by a narrow band 

 or isthmus of the same tissue. The gland is reddish in color, and abundantly 

 supplied with blood-vessels and lymphatics. 



Microscopic examination shows that the thyroid consists of an enormous 

 number of closed sacs or vesicles, variable in size, the largest not measuring 

 more than o.i mm. in diameter. Each sac is composed of a thin homogen- 

 ous membrane lined by cuboid epithelium. The interior of the sac in adult 

 life contains a transparent, viscid fluid containing albumin and termed 

 "colloid" substance. Externally, the sacs are surrounded by a plexus of 

 capillary blood-vessels and lymphat- 

 ics. The individual sacs are united 

 and supported by connective tissue, 

 which forms, in addition, a cover- 

 ing for the entire gland. Non- 

 medullated nerve fibers derived from 

 the middle and inferior cervical gan- 

 glia pass into the gland and are dis- 

 tributed to the walls of the smaller 

 blood-vessels and to the epithelium 

 lining the vesicles (Berkley). 



The role played by the thyroid 

 gland and its secretion, especially in 

 mammals in the regulation of the 

 functional activities of various 

 organs and tissues of the body has 

 been determined from the examina- 

 tion of the effects that follow its 

 arrested development in the early 



years of childhood, its degeneration in adults, its surgical removal in human 

 beings when this is necessitated by pathologic conditions, its extirpation in 

 animals, and the ingestion of extracts of the gland or the gland itself. 



The Effects of Arrested Development. A congenital absence of the 

 thyroid or an arrested development in early childhood is followed by a de- 

 fective physical and mental development characterized by a group of phe- 

 nomena termed cretinism. The most characteristic of these phenomena are, 

 a small and irregular- shaped body; a swollen face; puffy eyelids; abroad flat 

 nose; a large tongue, inclined to hang out of the mouth; a swollen abdomen; 

 short thick legs; mental dullness and stupidity, and even more or less idiocy. 

 The thyroid, as shown on post-mortem examination presents an atrophied 

 appearance and consists mainly of hard fibrous tissue. 



The Effects that Follow Degeneration in the Adult. The degen- 

 erative processes that occur in the thyroid in the adult give rise to a group 

 of phenomena the most striking of which is a swollen condition of the skin, 

 the result of the hyperplasia of a subcutaneous connective tissue, of an 

 embryonic type and rich in mucinoid material, to which the term myxedema 

 is given. Partly in consequence of this change in the skin, the face becomes 

 broader, swollen and flattened, giving rise to a loss of expression. The fea- 

 tures in some instances become coarse and irregular. With the progress of the 



FIG. 218. VIEW or THYROID 

 Thyroid isthmus. 2. Median portion of crico- 

 thyroid membrane. 3. Crico-thyroid muscle. 

 4. Lateral lobe of thyroid body. (After Morris.) 



