954 



THE INTERNAL SECRETIONS 



may be derived. Furthermore, while not in possession of a true duct, 

 they lie in close relation with definite efferent and afferent blood-vessels 

 and lymphatic channels, (6) their product can be isolated chemically 

 from their venous blood or lymph, (c) their substance or the blood 

 or lymph returned from them, may be shown to possess a specific 

 physiological action, and (d) the removal of the organ is followed by a 

 loss of a definite function whieh is absolutely essential to the health 

 and very existence of the animal. 



A. THE THYROID AND PARATHYROID BODIES 



Position and Structure of the Thyroid Gland. In the cat, dog 

 and man, the thyroid gland (Greek : thyreos, shield) consists of a right 

 and left lobe which are connected with one another by a bridge or 

 isthmus of the same tissue extending transversely across the trachea. 

 These lobes are nearly equal in size, and measure about 5 cm. in length. 



TS 



FIG. 503. FIG. 504. 



FIG. 503. DIAGRAM SHOWING THE POSITION OF THE THYROID GLAND. 

 TC, thyroid cartilage ; TG, thyroid gland ; T, Trachea. The parathyroids are indi- 

 cated in black. 



FIG. 504. DIAGRAMMATIC REPRESENTATION OF THE STRUCTURE OF HUMAN THYROID 



Their combined weight amounts to 30 or 40 grams, but these figures 

 are only approximate, because the vascularity of this organ is subject 

 to considerable fluctuations. It is generally larger in females, and 

 increases in size during the menstrual period. During adult life it 

 shows a proportion to the weight of the body of 1: 1800 and during 

 infancy a proportion of 1 : 250 ; hence, it is much larger during the latter 

 period. 



The thyroid is developed from an outgrowth of the primitive 

 pharynx and is, therefore, of hypoplastic origin. It is enveloped by a 

 layer of dense areolar tissue which also subdivides its substance into 

 small lobules of irregular size. Its tissue is composed of a large number 



