478 THE SOLIDS. 



When unravelled by the removal of the interlobular cellular 

 tissue, the whole gland is seen to consist of a straight tube, 

 with the follicles arranged around it in a spiral manner. 



The central cavity, " reservoir of thymus," is lined by a 

 delicate mucous membrane, which is raised into ridges by a 

 layer of ligamentous bands situate beneath it ; these proceed 

 in various directions, an'd encircle the apertures of the pouches : 

 their use is to keep the lobules together, and to prevent the 

 injurious distention of the cavity. 



The whole organ is enclosed in a dense capsule of fibrous 

 tissue, the blood-vessels contained in which are remarkable 

 for their disposition in threes, an arrangement which is not 

 uncommon in the capsular investments of glands. (See 

 Plate LXI. fig. 7.) 



The " milky fluid " contained in the follicles and reservoir 

 is made up, to a great extent, of an immense number of 

 granular nuclei, as well as numerous cells of large size, which 

 do not appear hitherto to have been either described or figured 

 in a satisfactory manner, and which are probably to be re- 

 garded in the light of parent cells. (See Plate LXI. 

 fig. 10.) 



Many of these cells contain several granular nuclei, each of 

 which is surrounded by one or more concentric lamellae ; they 

 thus resemble the cartilage cells found in the intervertebral 

 substance, and also certain species of Microcystis, a genus of 

 Freshwater Algae. 



Mr. Simon, in his " Prize Essay," makes the following 

 observations on the above-described cells : " In specimens 

 taken from animals past that period of life when the thymus 

 is most active, I have found cells in which these dotted cor- 

 puscles occupied the relation of nuclei. The cells are at first 

 little larger than the corpuscles themselves, and contain a 

 perfectly pellucid material ; but as they grow, their contents 

 become molecular, and they develope themselves into perfect 

 fat cells, which lie in the cavities of the glands, and in some 

 instances completely fill them. During the period in which 

 these cells are being developed, the application of acetic acid 

 to the preparation as it lies under the microscope shows 



