THE DUCTLESS GLANDS. 181 



parathyroids, the adrenal bodies, the pituitary body, the thymus, 

 and the carotid and coccygeal bodies. 



1. The Thyroid Gland (Fig. 154). This consists of a number 

 of alveoli or closed vesicles, lined with cubical epithelial cells ar- 

 ranged in a single layer upon the delicate, vascularized areolar tissue 

 which forms their walls and separates the neighboring alveoli from 

 each other. This fibrous tissue is more abundant in places, where 

 it serves to divide the gland into a number of imperfectly defined 

 lobes. At the periphery of the organ its connective tissue becomes 

 continuous with a thin but moderately dense fibrous capsule. 



The individual alveoli differ both in respect to their size and their 

 contents. Many are more or less completely filled with a nearly 

 homogeneous, glairy substance, of a slight yellowish tint, called 

 "colloid," while others appear to be occupied by a serous fluid. 



FIG. 155. FIG. 156. 



Sections of thyroid gland. (Schmid.) 

 Fig. 155. From a dog : a, colloid or secreting cells ; b, reserve cells (these differ only in their 



states of activity) ; c, cells containing less colloid than a. 

 Fig. 156. From a cat : a, daughter-cells arising from the division of an epithelial cell. 



The elaboration of this colloid material seems to be the function 

 of the organ, though it may have other less obvious duties. 



The cells lining the alveoli may be divided into two classes, 

 which differ in appearance (Fig. 155): first, those engaged in the 

 production of colloid, secreting cells ; and, second, those in which 

 no colloid is present, and which are regarded as reserve cells. 

 The latter are capable of multiplication, thereby replacing such of 



