produced by the access of air to them would introduce into the 

 experiment. The vessels in the rabbit's ear are readily measured 

 by a micrometer used with one of Briicke's magnifiers, which is 

 simply a telescope with an extremely short focus. The ear 

 should be held up so as to allow the light to shine directly 

 through it, and the magnifier placed horizontally. 



The area of the capillaries may be lessened, and the flow of 

 blood through them retarded in two ways : 1, by contraction of 

 their walls ; 2, by pressure exerted on them from without. 

 They may be made to contract by irritation, 1, of the vaso- 

 motor centres, 2, of the vaso-motor nerves, or, 3, of their mus- 

 cular walls ; and pressure may be exerted from without by the 

 motions of muscles or of organs composed of involuntary mus- 

 cular fibre such as the intestines. The movements of respira- 

 tion also, as already mentioned, exercise an important influence 

 on the pressure. 



Elimination of Ref^piration and Muscv.lar Movement — The 

 influence both of respiration and of muscular movement may be 

 eliminated by giving the animal curare, and keeping up artificial 

 respiration, before beginning to experiment with the drug whose 

 action we wish to examine. 



Elimination of Vaso-motor Centre. — For the purpose of ascer- 

 taining whether the drug has acted on the vascular walls or on 

 the vaso-motor centre, we divide the vaso-motor nerves going to 

 a part before injecting it, and see whether it acts as it would 

 have done had they been undivided. Thus, when wc are 

 observing the rabbit's ear, we divide the sympathetic in the 

 neck ; and, when looking at the mesentery, we cut the 

 splanchnics before the injection, and see whether the vessels 

 contract or dilate as we have previously seen them do under 

 influence of the poison in animals in whom the nerves were 

 intact. 



For the purpose of ascertaining whether the drug acts on all 

 the vessels in the body in the same way that it does on those of 

 the ear or mesentery, we first cut the vagi, sympathetics and 

 depressors, and then divide the spinal cord between the occiput 

 and atlas, or atlas and axis, so as to sever the connexion between 



