VASCULARITY IN THE NERVOUS SYSTEM 439 



to vary greatly, so that other factors are evidently concerned 

 also. Where the shrinkage appeared greatest, the diameter of 

 the mass 'was only about one-half of the diameter of the vessel 

 in which it was contained. "What changes the reagents may have 

 produced in the caliber of the vessel itself could not, of course, 

 be determined. 



It was originally intended that the volume of the capillaries 

 in a given volume of brain tissue should be measured, but it is 

 evident from the above that measurements of the diameter of 

 the vessels were of doubtful value. Moreover, there was found 

 to be generally no very marked difference in the caliber of the 

 capillaries in different regions, practically all sizes between the 

 limits mentioned above occurring in each in similar numerical 

 proportions, so that the ratio would be given just about as well 

 by considering the lengths instead of the volumes. 



Finally, the possibility of error in measuring the vessels is 

 considerable. It is no easy matter to make an accurate estimate 

 of the length of a vessel which is twisting about in the thickness 

 of the section and to make correct allowance for foreshortening, 

 etc. When it is considered, however, that the result for each 

 part represents the sum of from about 60 to about 350 measure- 

 ments in each brain studied, this source of error may probably 

 be neglected. 



The particulars regarding the ten animals from which the 

 results about to be detailed were obtained are as follows: nos. 

 12 and 16 were female albinos and no. 14 was a female hooded 

 rat. These animals were obtained from a local dealer in Toronto 

 and seemed to be in good condition. Nos. 23 and 24 were male 

 albinos and no. 26 was a female albino. These were procured 

 from the stock of the Department of Pathology of the University 

 of Toronto. They were badly infested with lice and did not seem 

 very healthy. All these animals were adults, not senescent, 

 but of unknown age. They were not weighed or measured. 

 Nos. 31, 55, 56, and 58 were albinos from the standard colony 

 of The Wistar Institute, and full data concerning them were, 

 recorded. 



THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 31, NO. 5 



