HISTOLOGY 103 



highly developed reticular zone, which has specific variations under certain 

 conditions, the X zone. The X zone is present in the male until about the 

 time sexual maturity is reached. It persists in castrated males at least four 

 months longer. In the female it continues to develop until at 4-5 weeks it 

 occupies a much larger area than in the male of the same age. The zone 

 disappears during first pregnancy, but in virgin females it persists for a 

 longer period, gradually degenerating between the third and seventh month. 

 The gradual degeneration leads to hyperemia and widespread vacuoliza- 

 tion and the separation of the medulla and cortex by connective tissue. 

 In a later article (52) the same author states that: "Mice of different strains 

 show considerable variation in the amount of adrenal X zone tissue which 

 they normally develop." 



The cells of the medulla are arranged in rounded groups and wide 

 reticular cords, separated from each other by sinusoidal blood spaces. The 

 cells and consequently the reticular cords are considerably larger than those 

 in the zona reticularis of the cortex. The nuclei are large and central!}' 

 located and the cytoplasm is pale staining. If the gland is treated with 

 potassium dichromate, small brown granules are visible in the cells. Chro- 

 mic acid stains the cells evenly brown, giving the so-called chromaffin 

 reaction. 



Arteries enter the gland from the capsule. They form the capillaries 

 of the cortex. The sinusoidal blood spaces of the medulla drain into the 

 tributaries of a large central vein and leave the gland at the hilus. 



The presence of small accessory adrenals in the vicinity of the gland is 

 not infrequent. 



Thymus. — The thymus is situated in the thorax ventral to the aortic 

 arch. It consists of two lobes of unequal size lying close together. The 

 lobes are covered by fibrous connective tissue from which septa project in 

 and produce lobulation without dividing the gland into distinctly separate 

 lobules. In a cross section of the thymus a light staining medulla and a 

 dark staining cortex can be differentiated (Fig. 44). In both parts reticular 

 cells form a supporting framework. In the cortex densely packed, small, 

 round cells are present. These cells are considered identical morphologically 

 with small lymphocytes by some authors, while others consider them of 

 epithelial origin and call them small thymocytes. They have dark staining, 

 slightly eccentric, round nuclei with dark chromatin granules and 

 prominent nucleoli, surrounded by a very small amount of cytoplasm.' 

 Because of the dense arrangement of these cells, the reticular cells are 

 difficult to see. 



