VASCULAR OR DUCTLESS GLANDS. 159 



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 unknown substances are formed by cell 

 activity and are poured into the interstices of the tissues, they have 

 been termed " internal secretions." Though the term internal se- 

 cretions is applicable to all substances which arise in consequence 

 of tissue metabolism, and which after being poured into the blood, 

 influence in varying degrees and ways physiological processes, yet 

 the term in this connection will be applied only to the secretions 

 of the thyroid gland, 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. 

 Each lobe is pyriform in shape, the base being directed down- 

 ward 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. In color the gland is reddish, and it 

 is 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 epi- 

 thelium. The interior of the sac in adult life contains a trans- 

 parent viscid fluid, containing albumin and termed " colloid " sub- 

 stance. 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 covering 

 for the entire gland. 



Function of the Thyroid. The knowledge at present possessed 

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

 the outcome of a study of the effects which follow its arrest of de- 

 velopment in the child, its degeneration in the adult, its extirpation 

 in the human being as well as in animals. The results, however, 

 which follow its extirpation are not always uniform in all animals ; 

 sufficient reasons for which lack of uniformity can not always be 

 assigned. 



Cretinism, a condition characterized by a want of physical and 



