438 EDWARD HORNE CRAIGIE 



capillaries also would be so, since the capillary and the cerebral 

 venous pressures are the same, according to Hill. The diameter 

 of the capillaries in every case, however, was found to be from 

 about 2 jj. to about 4.5 n, a size which one would not consider 

 at first sight to be natural. The diameter of the erythrocytes 

 in the wild Norway rat has been determined by three observers 

 to be 6.5 or 7 ^ (Donaldson, '15). Some of the erythrocytes 

 in the albino rats used in this study were measured (in the fresh 

 condition) and were found to average about 6 fx in diameter. 

 The average diameter of the capillaries in the human subject, 

 where the corpuscles are just a little larger than in the rat, is 

 stated to be about 12 m (Cajal, '11 : Halliburton, '14), so that one 

 would expect those of the rat to measure at any rate 9 or 10 ju. 

 Sterzi ('04) finds the capillaries in the spinal cord of the horse 

 to measure from 70 to 80 fx. According to Koelliker, however, 

 the finest capillaries in the human spinal cord are 5 /x in diameter, 

 in the brain 4.5, which, like my measurements, would be smaller 

 than the erythrocytes. Moreover, a recent paper by Krogh 

 ('19 a) makes it appear not improbable that the vessels in the 

 preparations used for this study are about their natural size. 

 Krogh finds that in living, resting muscle of the frog, the average 

 diameter of the capillaries is about 4.5 n, while in the guinea-pig 

 it is only 3.5 /x, although the red corpuscles of the frog measure 

 about 22 x 15 x 4 /x and those of the guinea-pig about 7.2 /x 

 diameter by 2 n in thickness. In passage, the corpuscles become 

 greatly deformed and the walls of the capillaries yield somewhat 

 where the corpuscles lie. Even in working muscle, where the 

 vessels are found to expand greatly, the average diameter of the 

 capillaries is still less than that of the corpuscles. It may be 

 noted in passing that the same author, while demonstrating that 

 a large number of the capillaries in resting muscle are completely 

 closed, found that all the capillaries in the brain seemed to 

 remain permanently open, so that there is not much danger of 

 error from that source in the present case. 



One factor in the apparent small size of the capillaries is the 

 shrinkage of the gelatin injection mass. An attempt was made 

 to measure this in some of the larger vessels, but it was found 



