Chap. XLIII.] THE DUCTLESS GLANDS. 



339 



497. II. The thyroid gland (Fig. 167). The 

 framework of this gland is in many respects similar 

 to that of other glands, there being an outer fibrous 

 capsule, thicker and thinner septa, and finally the fine 

 trabeculne forming the septa between the gland alveoli. 

 These are closed vesicles of a spherical or oval shape, 

 and of various sizes. Each vesicle is lined with a single 

 layer of polyhedral or 



columnar epithelial cells, 

 each with a spherical or 

 oval nucleus. There is a 

 cavity, which differs in size 

 according to the size of the 

 vesicle. It contains, and 

 is more or less filled with, 

 a homogeneous, viscid, albu- 

 minous fluid the so-called 

 colloid. In this often oc- 

 cur degenerating nucleated 

 lymph - corpuscles and 

 coloured blood - corpuscles 

 (Baber). 



498. The vesicles are 

 surrounded by networks of 

 blood capillaries. In the 



connective tissue framework lie networks of lym- 

 phatics; between the framework and the surface of the 

 vesicles are lymph sinuses lined with endothelium 

 (Baber). The large and small lymphatics are often 

 filled with the same colloid material as the vesicles, 

 , and it is probable that this colloid material is produced 

 in the vesicles, and carried away by the lymphatics, 

 to be finally discharged into the circulating blood. 



499. Its formation in the vesicles is probably due 

 to an active secretion by the epithelial cells of the 

 vesicles, and to a mixture with it, or maceration by 

 itj of the effused blood mentioned above. In some 



Fig. 167. From a Section 

 through the Thyroid Gland 

 of Dog. 



a, The epithelium lining the vesi- 

 cles ; b, the " colloid " contents of 

 the .vesicles ; o, a lymphatic filled 

 with the same material as the 

 vesicles ; rf, the fibrous tissue be- 

 tween the gland vesicles. 



