236 J. R. M. INNES AND A. CARSTEN 



and changes in beha\ior. Some animals developed paralysis of the hind legs 

 many months after irradiation. After identification of the demyelinating 

 myelopathy in a few rats, a series of experiments were planned in which, 

 by adequate shielding, only the vertebral column from about sixth cervical 

 to second thoracic segment was irradiated. The first experiments were con- 

 cerned with definition of the pathologic process and its topographic neuro- 

 anatomic distribution. 



Techniques 



White female rats were housed two in a cage and given food and water 

 ad libitum throughout the experiment. They were 3 to 6 months of age at 

 irradiation. Before and after irradiation, they were weighed and examined 

 daily. 



Rats were anesthetized with intraperitoneal injections of sodium pento- 

 barbital, 45 mg per kg of body weight. In the groups exposed to upper 

 body irradiation, the animals were marked with dye at the xiphoid process 

 and placed in 2-in. diameter lucite tubes. A ^4 -in. -thick cylindrical lead 

 shield was placed around the lower half of the tube at the dye mark, so 

 that the upper body and head of the animal were exposed to the x-ray beam. 

 In rats receixing thoracic exposure, a second lead shield was placed over 

 the head end of tlie lucite tube at the manubrium. Animals indicated as 

 spine exposed were completely surrounded by ^ in. lead shields containing 

 a small hole in a position so that only the spine from sixth cervical to 

 second thoracic segment was exposed (Table I). 



Tlie irradiations were made with a General Electric 250 kV Maxitron 

 x-ray machine. The radiation factors were: 250 k\p. 30 ma. 0.5 mm Cu 

 filter. Aluminum parabolic filter for field uniformity, target to skin distance 

 from 6.5 to 9 in., dose rate of 200 to 350 rad per minute measured in a 

 tissue ecjuixalent phantom, half \alue layer of 2.15 mm of copper. All 

 animals rats and monkeys) received 3500 rads. 



Clinical Findings 



After irradiation, a latent period varying from 5 to 9/2 months occurred 

 in rats. The first sign of untoward involvement of the spinal cord was in- 

 continence of urine, which persisted to some degree in some rats for weeks 

 before any motor weakness or incoordination of the hind legs developed. 

 Thereafter, the rats became obviously unsteady on their hind feet and were 

 ataxic, and the tail lost its tonicity. This syndrome progressed imtil the 

 animal's hind quarters were completely immobilized, without the tail or 

 limbs becoming completely flaccid. An afTected animal could move around, 



