ANATOMY AND HISTOLOGY OF THE THYMTJS GLAND 369 



cells (1), as may be seen by reference to Fig. 6. Occasionally giant cells 

 are found. 



The Medulla. In the medulla the small cells are less abundant, and 

 for this reason the tissue stains less intensely. The reticular cells occur 

 in rather larger numbers. 



Corpuscles of Hassall. The corpuscles of Hassall are formed through 

 a clumping and degeneration (hyaline, fatty, or colloidal) of the epithelial 

 cells, or of the reticular cells formed from them. Those occupying the 

 center are, judging from the appearance of their mitochondria, either 

 dead or very nearly so. Very frequently the mitochondria seem to give 

 rise to lipoid droplets of variable size. There is a marked deposition of 

 hyaline material. Nuclei are often absent. The corpuscles may become 

 infiltrated with calcium salts. Occasionally they are multiple. Fat is 

 frequenty deposited within them. Bell has shown that they are not always 

 spherical, since they frequently take the form of branching trabeculse ex- 

 tending throughout the medullary portion of the gland. Hewer claims 

 to have caused their formation in the thymus experimentally through 

 treatment with X-ray. Differential studies on the relative volumes have 

 not yielded much of value, for under normal conditions the corpuscles 

 vary greatly in size and in number. In some animals they are entirely 

 absent throughout life. Small cavities lined with ciliated epithelium are 

 occasionally seen in the medulla, and striated muscle fibers, occurring 

 singly or in clumps, have also been described, but are of very rare 

 occurrence. 



There is good reason to believe that the thymus is an organ of blood 

 formation. The small thymic cells are certainly lymphocytes and these, 

 as Danchakoff has shown, possess the power of giving rise to plasma cells 

 and polymorphonuclear leukocytes. The formation of erythrocytes, if it 

 takes place at all in the thymus, is probably restricted to the embryo. 



Mode of Secretion. Salkind's claim that formed secretion antece- 

 dents occur within the epithelial cells is interesting, but requires con- 

 firmation. According to him, they take the shape of tiny vacuoles, in 

 the center of each of which a deeply staining granule may be seen, sug- 

 gestive in some respects of the segregation granules of Kenant, 



