A BY, Blood Capillaries in Cerebellar Cortex. 29 



membranes in living animals that there are certain minute tubes 

 called capillaries through which blood may be seen to flow. 

 Having observed the size and arrangement of these capillaries in 

 the living animal while the blood is still flowing through the tubes, 

 we may next proceed to find the artery leading to that region, 

 or the vein leading from it, introduce a canula into the vessel, 

 pointing it toward the region under consideration. Next we 

 may inject some colored fluid into the vessel, and by actual in- 

 spection may see this fluid fill every capillary which was ob- 

 served during the life of the animal. More than this, if the 

 pressure be exactly equal to the normal blood pressure, other 

 conditions remaining the same, the size of the capillaries should 

 be equal to their size in the living animal. As the caliber of 

 capillaries may vary from moment to moment in the living ani- 

 mal, so may the caliber vary at the time of injection, giving us 

 apparently a capillary of large size, or a capillary of small size. 

 Under the circumstances quoted above, the tubes may all re- 

 main free from emboli, the pressure may be about equal to the 

 normal blood pressure, and every tube which contained blood 

 in the living animal, be that tube large or small, may be exact- 

 ly filled. These are the ideal conditions, the result an ideal re- 

 sult. It is to an injection of this kind that I apply the term 

 complete injection. It is very clear to the practical histologist 

 that this ideal result, a complete injection, is not a very frequent 

 occurrence. 



If now a small embolus plug up an artery or an arteriole, 

 and the pressure is not sufficient to drive the injection mass 

 around through the collateral circulation, it is clear that there 

 will be areas where every capillary is not filled. Even if no 

 embolus interfere, the pressure may be insufiicient, and certain 

 capillaries are not filled. Still again, embolism may be absent, 

 the pressure approximate the normal blood pressure, but some 

 or all the vessels may be contracted because of post mortem 

 changes, and in this case again certain capillaries may remain 

 empty, or partly filled. Any one or any combination of these 

 factors may result in something less than a complete injection, 

 and to this result I apply the term under-injection. 



