994 RADIATION BIOLOGY 



although he is careful to point out that the response of the diseased gland 

 may he quite different. The relative resistance of the normal pituitary 

 to direct irradiation is also apparent from more recent studies. Single 

 dosages of 200 to 500 r applied to the pituitary region of adult female rats 

 do not lead to degenerative changes in the pituitary or in the ovaries, 

 adrenals, and thyroids (Kotz et al., 1941). A transient effect of X rays 

 on pituitary function is indicated by the reappearance of physiological 

 signs of heat when dosages ranging from 5 to 300 r are applied to the 

 pituitary of sexually mature female rats on the second day of their 

 estrous cycle (Freed et al., 1948). This reaction lasts for 6 to 8 hours and 

 is accompanied by a rather definite increase in pituitary weight and a 

 slight increase in uterine weight while ovarian weight is unchanged. 

 Comparable irradiation of immature female rats or ground squirrels does 

 not alter the development of their sexual systems (Freed et al., 1948; 

 Denniston, 1942). Although some increase in size and weight of the 

 pituitaries has been described in the infantile animal 11 to 14 weeks after 

 low-dose irradiation, there is no evidence of an increase in function 

 (Baidens et al., 1946). The early transient physiological effects of 

 pituitary irradiation are regarded by Freed et al. (1948) as secondary to 

 increased vascularity or altered permeability rather than as primary 

 stimulation of hypophyseal activity. While injurious effects are not 

 evident with small dosages of radiation, exposure of the pituitary region 

 to an air dose of 1000 r or more reduces the rate of growth of young rats 

 (Denniston, 1942). On the other hand, evidence of hypopituitarism is 

 not apparent in man with localized dosages of 8100 to 10,000 r (Kelly 

 etal., 1951). 



The functional integrity of the pituitary during the first few days after 

 total-body irradiation is indicated by the typical pituitary-adrenal 

 cortical response to stress (Patt et al., 1947). Adrenal changes are not 

 seen in the irradiated hypophysectomized animal and there is some evi- 

 dence that pituitary ablation increases sensitivity to total-body irradia- 

 tion (Patt et al., 1948). It is not known whether the pituitary discharge, 

 which apparently takes place within several hours of exposure, is a 

 direct consequence of irradiation or of neural or humoral stimuli such as 

 epinephrine and tissue breakdown products. Many of these considera- 

 tions have already been discussed in connection with the effects of radia- 

 tion on the adrenal. It is worth recalling that the adrenocorticotrophic 

 hormone, in the one experimental study in which it was employed, did 

 not modify X-ray toxicity (W. W. Smith et al., 1950). 



NERVOUS SYSTEM 



The radioresistance of nervous tissue, indicated in the early experi- 

 ments of Kanoky (1907), has been confirmed in numerous investigations. 



