332 



THE DUCTLESS GLANDS 



represent a functional differentiation of the epithelium. Under 

 the influence of certain common disturbances, the thymus 

 may undergo an "accidental involution" of a more or less 

 transitory nature. Puberty normally brings on an " age 

 involution," which is brought about by diminution of the 

 leucocytal part of the organ, then by increased degenerative 

 processes, by which the function of the organ and gradual 

 destruction of the parenchyma are brought about. 1 



Pigache and Worms report an " accidental involution " 

 after thyroidectomy in dogs and rabbits ; while Utterstrom 

 finds that feeding rabbits with thyroid substance has two 

 distinct and opposing actions on the thymus, the one a depressor 

 action due to the condition of reduced nutrition, the other a 



direct thymus excitation. 

 This last action explains the 

 enlargement of the thymus 

 found in cases of exopthal- 

 mic goitre. Hoskins notes 

 that this hypertrophy of the 

 thymus after thyroid feeding 

 affects only the cortex. 



It has been proved that 

 leucocytes are already pre- 

 sent in the body before 

 the lymphoid transforma- 

 tion of the gland, so that 

 the thymus cannot be the 

 original source of the leuco- 

 cytes, as suggested by Beard. 



In regard to the Hassall concentric corpuscles modern 

 investigation appears to show that these structures are not 

 to be looked upon simply as remains of the original epithelium 

 which have not become transformed into lymphocytes. Ham- 

 mar has shown that the concentric corpuscles are derived 

 from hypertrophic reticular cells. The formation of these 

 structures begins early in foetal life, and continues long into 

 the period of post-natal evolution of the organ. Wallisch 

 has pointed out that the total volume of the Hassall bodies 



1 The bursa fabricii of birds is an organ which in some respects may be 

 regarded as analogous to the thymus. Thus it undergoes leucocytal invasion 

 and a degeneration at about the age of puberty. 



FIG. 92. HassalTs concentric cor- 

 puscle from the thymus of a cat. 

 This body shows a typical con- 

 centric arrangement. (Drawn by 

 Mrs. Thompson.) 



