INFLUENCE OF THE VEINS. 331 



out into the blood, it is obvious that if the nerve fibres 

 distributed to the capillaries were healthy they 

 would be instantly affected by the contact of the 

 foreign body, and the vaso-motor nerves of the 

 arteries would as instantly respond to the disturbance 

 excited in the ganglion cell. The contraction of the 

 arteries would follow, and the narrowing of the 

 capillary vessels, and a corresponding increase in 

 the thickness of their walls would result. This last 

 condition would be likely to prevent, and would 

 certainly retard, the ingress of particles to the blood. 

 The foreigTi body thus kept from entering the current 

 of the circulation might remain till it was destroyed 

 or altered by the surrounding fluid, and thus ren- 

 dered harmless, or, in the case of living matter, till its 

 death had occurred. In this way it may be that the 

 capillary nerves of a person in perfect health protect 

 him and are directly instrumental in preventing the 

 access to his blood of substances which are with 

 great difficulty changed, or which must infallibly 

 produce disease if they find their way into his cir- 

 culating fluid. The fibres distributed to the capil- 

 laries ai-e probably those which are irritated or 

 paralyzed by certain siibstances which after being 

 absorbed by the blood at one part of the system 

 transude through the delicate walls of the vessels in 

 other situations, and are thus brought into contact 

 with the nerve fibres just outside the capillaries. 

 These nerve fibres are, I believe, primarily affected in 

 cases of sudden death resulting from the presence of 

 certain poisons, such as hydrocyanic acid, and are I 

 think influenced in the production of those slower 

 changes which arise from poisonous matters of another 

 kind being introduced into the blood. 



312. Influence of the veins. — In reffulatins" the 

 flow of blood through the capillary vessels it must 

 not, however, be forgotten that the veins perform 

 an important part. By the reduction of the calibre 



