30 Journal of Comparative Neurology. 



Still another phenomenon may occur. The pressure may 

 be too great. In this case the various tubes will become dis- 

 tended, and at the point or points of least resistance ruptures 

 may occur. It is obvious, however, reasoning from previous 

 knowledge and experience, that no nezv capillayies can possibly 

 be formed by injection. Experience shows that the injection 

 mass pours out into the extra-vascular areas in a confused mass. 

 This is what I desire to designate over-injection. 



Perhaps I have entered into this tedious explanation at 

 greater length than the nature of the circumstances seem to 

 warrant. My defense is that I have been asked frequently, 

 " How do you knozv that those rods of colored gelatine indicate 

 the actual size, number and arrangement of the minute blood 

 streams?" The basis for the technique is outlined above, and 

 shows that we may safely draw conclusions through the appli- 

 cation of induction as to 7imnber and arrangement of capillaries, 

 but not as to caliber. 



In prosecuting these studies several injection masses have 

 been used, — gelatin, glycerine, warm, cold, and of various col- 

 ors. The mass used in making the injection from which the 

 plate was made was carmine gelatin, a formula very near Car- 

 ter's Carmine Gelatine Mass, given in Beale, page 113. 



As to methods of making the injection, the canula was 

 always introduced into the carotid, sometimes right, sometimes 

 left, and directed toward the head. T have used several forms 

 of apparatus employing hydrostatic pressure, but in my hands 

 the syringe has given better results than any other means of in- 

 jecting. I will state that the percentage of failures is always 

 high, and usually many trials must be made before obtaining a 

 complete injection. 



/ V. Observations. 



In longitudinal vertical sections of a completely injected 

 cerebellum three sets of vessels may be observed to be distri- 

 buted from the pia mater. First, a set of small arterioles pass- 

 ing through the cortex into the medulla ; second, a set of small 

 arterioles passing through the molecular layer and breaking up 



