42 ROBERT H. BROWNSON 



tion of gray and white matter, as well as profound gliovascular alterations. 

 The authors believe that their morphologic analysis supports the assumption 

 that the vascular changes were primary and delayed; lesions in the brain 

 weie essentially the same type after fractionated as single dose exposure. 



Materials and Methods 



In this study approximately 120 male 9-month-old rats were used. Of 

 these, 54 were placed in a group for long-term studies whose postirradiation 

 sacrifice dates were selectively extended. These animals were grouped in six 

 large cages, 9 rats per cage. Dining the first week of x-irradiation, a total of 

 36 animals were exposed; at random, 6 animals per cage received 1,000 r 

 total-head irradiation. The remaining 18 animals, 3 in each cage, were 

 maintained as controls. The rats received 1,000 r until the total accumulated 

 dosage of the last remaining experimental group had reached 5,000 r. 



Control animals and experimental animals scheduled for radiation were 

 fed during a 45-minute period each day for 6 days. On the 7th day, each 

 animal was placed in a Skinner box for 45 minutes. The animals' first 

 experience in the box resulted in learning to press a bar for food reward. 

 For the next two periods in the box, the animals were placed on short 

 aperiodic reward schedules. The third time, all animals were placed on the 

 aperiodic schedules and bar presses recorded during a 45-minute interval. 

 After establishing a desirable level of indi\idual performance, all of the 

 \arious groups of animals underwent the scheduled exposure to x-irradiation 

 and 5 weeks of testing. 



The head of each experimental animal was exposed to x-irradiation from 

 a 1,000 kvp x-ray imit filtered through 2 mm of aluminum and 56 cm of 

 air. A Victoreen Chamber R-Meter was used at each operation to obtain 

 equal and exact data of the total roentgens delivered in a prescribed 

 interval of time. The rats body was protected in a lead-lined box which 

 was it.self shielded by a wall of lead bricks 2-3 in. thick from which the 

 animal's unshielded head protruded vertically above the body shield. Each 

 unanesthetized animal was secured in the box by means of a modified 

 burette clamp that formed a collar and limited the animal's movement. 

 The lead-lined box containing the rat was rotated clockwise through 360° 

 at 33 rpm by a turntable. Each rat's head was continuously exposed to 

 x-irradiation from multiple angles until the animal accumulated at one 

 exposure a level of 1,000 r, delivered at the rate of 237 r per minute and 

 J/a r per minute to a partially exposed neck. Each animal remained approxi- 

 mately 4 minutes in the radiation chamber. As previously mentioned, this 

 procedure was repeated every 7 days on the selected group of animals until 

 the desired cumulative dosage was attained. The animal groups were then 



