590 THE TISSUES. 



III. THE THYMUS GLAND. 



The thymus is an organ more especially of foetal and infantile 

 life. It consists of lobules grouped around a central canal, which 

 is generally spirally convoluted. The lobules are collected into 

 lobes ; while the latter, invested by areolar tissue, constitute the 

 whole organ. 



The lobules are, however, composed of smaller hollow subdivi- 

 sions, and the latter of rounded corpuscles, like gland- vesicles, 

 which give the exterior of the lobules a delicate mosaic aspect, not 

 unlike that of the lungs. (Fig. 417.) These corpuscles are, how- 

 Fig. 417. 



A section of the thymus gland at the eighth month, showing its structure ; from a preparation of 

 Sir A. Cooper. 1. The cervical portions of the gland ; the independence of the two lateral glands is 

 well marked. 2. Secretory follicles seen upon the surface of the section ; these are observed in 

 all parts of the section. 3, 3. The pores or openings of the secretory follicles and pouches are 

 seen covering the whole internal surface of the great central cavity reservoir. The continuity of the 

 reservoir, in the lower or thoracic portion of the gland, with the cervical portion, i's seen in the figure. 



ever, not vesicles, but solid bodies, cohering intimately towards the 

 cavities, though separated from each other on the outer side. 



Each lobule is inclosed in a thin, almost homogeneous, mem- 

 brane, 33^3 to T2 <jTyo- of an inch thick. Within this, and between 

 it and the cavity of the lobule, lies a grayish-white soft substance, 

 7*2 to A f an i ncn thick, consisting of free nuclei and minute cells, 

 among which bloodvessels and a small amount of white fibrous 

 tissue are sent; and thus a structure is presented resembling that of 

 the contents of the follicles of Peyer (p. 530). 



The cells and nuclei, however, of the thymus-lobules, with a 

 small quantity of a connecting fluid, constitute the main bulk. The 



