412 



TEXT-BOOK OF PHYSIOLOGY. 



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 homogeneous membrane lined by cuboid epithe- 

 lium. The interior of the sac in adult life contains a transparent, 

 viscid fluid containing albumin and termed "cofloi(^V substance. 

 Externally, the sacs are surrounded by a plexus of capillary blood- 

 vessels and lymphatics. The individual sacs are united and sup- 

 ported by connective tissue, .which forms, in addition, a covering 



for the entire gland. 



Function of the Thy- 

 roid. The knowledge at 

 present possessed as to the 

 function of the thyroid 

 gland, especially in mam- 

 mals, is the outcome of a 

 study of the effects which 

 follow its arrested develop- 

 ment in the child, its de- 

 generation in the adult, 

 and its extirpation in the 

 human being as well as in 

 animals. The results, how- 

 ever, which follow its ex- 

 tirpation are not always 

 uniform in all animals, 

 though sufficient reasons 

 for the lack of uniformity 



. . can not always be assigned. 



Cretinism, a condition characterized by a want of physical and 

 mental development, is associated with, if not directly dependent on, 

 a congenital absence of the thyroid or its arrested development dur- 

 ing the early years of childhood. 



Myxedema, a condition of the skin in which there is a hyperplasia 

 of the connective tissue, of an embryonic type, rich in mucin, is gener- 

 ally regarded as one of the effects of degenerative processes in the 

 thyroid. Partly in consequence of this change in the skin the face 

 becomes broader, swollen, and flattened, giving rise to a loss of ex- 

 sssion. At the same time the mind becomes dull, clouded, even 

 approximating the idiotic type. This supposed infiltration of the 

 i with mucin was termed myxedema by Ord, who at the same 

 time associated* with a change in the structure of the thyroid as a 

 result of which it became functionally useless 



FIG. 187. VIEW OF THYROID BODY. i. Thy- 

 roid isthmus. 2. Median portion of crico- 

 thyroid membrane. 3. Crico-thyroid 



muscle. 4. Lateral lobe of thyroid body 

 (After Morris.) 



