56 ROBERT H. BROWNSON 



5,000 r had little differential effect on blood vessels. Ho\ve\er, the added 

 factor of 228 days in either low level or hiijh le\el dosa|2;e was critical, as 

 evidenced by increased severity of reaction. 



Lyman and his colleagues ( 1933) , in an early study of the effects of roent- 

 gen rays on the central nervous system of adult dogs, reported that 6 months 

 after irradiation various sized blood vessels of the brain had progressive 

 changes. These changes were quite similar to those seen in our animals, 

 with marked hyaline degeneration and obliterating sclerosis of arterioles. 



In the hypothalamus of radiated animals there was especially heavy con- 

 nective tissue, which might be thought of as an increased vascularity involv- 

 ing pial-glial extensions from the surrounding meninges. Furthermore, 

 there appeared to be an increased number of nuclei, mostly neuroglial. In- 

 creases in cellularity in animals receiving 2,000 r at 228 days following ex- 

 posure were less severe in animals which received 5,000 r after 228 days. 



Conclusions 



Rats were exposed to total head x-irradiation in doses of 1,000 r per week 

 until accumulations by groups of six animals ranged from 1,820 to 5,000 r. 



During the inter\al following each exposure to ionizing rays, animals in 

 all dosage levels demonstrated changes in behavior, physical appearance, 

 and weight. Quantitati\e food reduction had a deleterious effect on the 

 general well-being and sur\ival of the animals recei\ing the larger ac- 

 cumulated doses of x-irradiation. 



Neiuons in the cerebral cortex, brain stem, and cerebellum demonstrated 

 slight to severe alterations throughout all dose levels. Generally, cell change 

 became more frequent and in some instances more severe with increasing 

 accumulation of total roentgens and time following exposure. 



Neuroglial elements underwent variable minor alteration. Astrocytes were 

 most frequently observed undergoing hyperplasia and hypertrophy. 



Meningeal thickening with some evidence of cellular and fibrillary infiltra- 

 tion was observed throughout all dose levels. 



Endothelial and adventitial changes were noted in small and medium 

 size intramedullary vessels with perivascular accumulations of large nu- 

 cleated cells. Changes in blood vessels were directly related to increasing 

 dosage and time. 



General obser\ation indicated that changes in cells leading to necrosis 

 were similar throughout the various dose levels and days after exposure. The 

 increasing severity in reactions associated with time and dose were more 

 quantitative than qualitative. 



