SECRETION. 



455 



-4 



called vascular or ductless glands. The influence of the pancreas in regulat- 

 ing the oxidation of sugar and the influence of the liver in the maintenance 

 of the general metabolism through the production of glycogen and the 

 formation of urea, are now established facts. That the vascular or ductless 

 glands to an equal extent, though perhaps in a different way, assist in the 

 maintenance of physiologic processes, appears certain from the results of 

 animal experimentation. The explanation given for the influence of these 

 glands is that they produce specific substances, which are poured into the 

 blood or lymph and carried direct 

 to the tissues, to the activities of 

 which they appear to be essential; 

 for without these substances the 

 nutrition of the tissues declines and 

 in a short time a fatal termination 

 ensues. 



Inasmuch as these partly un- 

 known substances are formed by 

 cell activity and are poured into 

 the circulating blood, they have 

 been termed " internal secretions." 

 Though the term internal secre- 

 tions is applicable to all sub- 

 stances which arise in consequence 

 of tissue metabolism, and which, 

 after being poured into the blood, 

 influence in varying degrees and 

 ways physiologic processes, yet the term in this connection will be applied 

 only to the secretions of the thyroid and parathyroid glands, pituitary body 

 or hypophysis cerebri, and adrenal bodies. 



Thyroid Gland. The thyroid gland or body consists of two lobes situated 

 on the lateral aspect of the upper part of the trachea (Fig. 210). Each lobe 

 is pyriform 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 (Fig. 211). Each sac is composed of a thin 

 homogenous 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 lymphatics. The individual sacs are 

 united and supported by connective tissue, which forms, in addition, a cov- 

 ering for the entire gland. 



Effects of Removal of the Thyroid. The knowledge at present pos- 

 sessed as to the function of the thyroid gland, especially in mammals, is the 

 outcome of a study of the effects which follow its arrested development in 

 the child, its degeneration in the adult, and its extirpation in the human 



FIG. 210. VIEW OF THYROID BODY. i. 

 Thyroid isthmus. 2. Median portion of crico- 

 thyroid membrane. 3. Crico-thyroid muscle. 

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



