170 



HISTOLOGY. 



be seen if the gland is pulled apart. The medullary substance extends 

 from the cord into the lobules (Figs. 191 and 192) where it is partially sur 



Thymic 

 corpuscles. 



Connective 

 tissue. 



>* Transverse 

 fr-*, section of 

 blood vessel. 



"H Medullary cord. 



FIG. 191. FROM .A CROSS SECTION OP THE THYMUS op A CHILD, i YEAR AND 9 MONTHS OLD. X 21 



Cortical substance. 



Medullary substance. 



Blood vessel. 



Thymic 

 corpuscle. 



rounded by a denser cortical substance. In places the medulla is in con- 

 tact with interlobular connective tissue. The cortex and medulla are not 



sharply separated from 

 one another. 



The cells of the thy- 

 mus have been variously 

 interpreted. According 

 to a recent investigation 

 (by Dr. E. T. Bell) the 

 thymus is at first a com- 

 pact mass of entodermal 

 cells. By vacuolization 

 the cells form a reticulum, 

 and certain of them be- 

 come lymphocytes. The 

 lymphocytes pass into 

 the cortex where they are 

 most abundant, and enter 

 the vessels. The lym- 

 phoid transformation of 

 the thymus " is noticeable 



in pigs of 3.5 cms. and is well advanced at 4.5 cms." It has already been 

 stated that lymphocytes are first recognizable in the blood and in the 

 lymph glands of pigs of 8 cms. The possible first appearance of lympho- 



Tangential sections of lobules. 



FIG. 192. PART OP A SECTION OF THE THYMUS FROM A 5 

 MONTHS' HUMAN FETUS. X 50. 



