224 



OF ABSORPTION AND SANGUIFICATION. 



(as concerns their development) of those bodies and of each other. The 

 cortical and medullary portions have a distinct origin. From an early 

 period their minute structure bears a close resemblance to that of the spleen, 

 consisting of the same elements as that gland, excepting in the existence of 

 more numerous dark granules, which give to the organ at a later period an 

 opaque and darkly-granular texture; and the general history follows a veiy 

 similar course: the Suprarenal capsules, however, acquiring their character- 

 istic structure, and attaining their largest relative size, so early in foetal life, 

 as to surpass the Kidneys in dimension up to the tenth or twelfth week of 

 Human embryonic development; though they afterwards diminish so much 

 relatively to the Kidneys, as to possess in the adult condition only ^gth 

 part of their bulk. 



162. The general structure of the Thymus Gland may be best understood 

 from the simple form it presents when it is first capable of being distin- 

 guished in the embryo. It is then solid, 1 but soon breaks down in the central 

 part so as to form a single tube, closed at both ends, and filled with granular 

 matter ; and its subsequent development consists in the lateral growth of 

 branchiug-off shoots from this central tubular axis. In its mature state, 

 therefore, it consists of an assemblage of hollow glandular lobules, united 

 together by connective tissue ; and their cavities all communicate with the 

 central reservoir, from which, however, there is no outlet (Figs. 96, 97). 



FIG. 96. 



FIG. 97. 



N 



Fi<;. 9fi. Portion of Thyinns of Calf, unfolded: a, main canal; &, glandular lobules; c, isolated 

 gland-granules seated on the main canal. 



I ii.. 97. Section of Human Thymus, showing the large cavity in the wide portion, and numerous 

 orifices lending to its loludar eavities. 



Each lobule is bounded externally by an indistinctly fibrous or almost 

 homogeneous membrane (Fig. 96, a), which sends prolongations (/) into its 



1 Jemlnissik, Sitxunysberidit d. Wion AUacl., 185G, Bd. xxii, p. 75. 



