probable Action in Health and Disease. 61 
reversed — the arterial walls relaxed and the capillaries distended 
with blood. Or suppose that any noxious materials or living germs 
of any kind were making their way from without 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, 
and the contraction of the arteries would succeed. This would be 
followed by the narrowing of the capillary vessels, and a correspond- 
ing increase of thickness in their walls, a condition likely to prevent 
or at least to retard the ingress of particles to the blood. The 
foreign body thus kept from entering the current of the circulation 
might remain till it was destroyed or altered by the fluid, and thus 
rendered harmless, or, in the case of living matter, till its death 
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 the circulating fluid. The fibres distributed 
to the capillaries are probably those which are irritated or paralyzed 
by certain substances which after being absorbed by the blood 
transude through the delicate walls of the vessels, and are thus 
brought into contact with the nerve fibres just outside. These 
are the nerve fibres which are primarily affected in cases of sudden 
death which result from the presence of certain poisons, such as 
hydrocyanic acid, for example, and which are influenced in the 
production of those slower changes which arise from other poisonous 
matters introduced into the blood. 
Influence of the Veins. 
In regulating the flow of blood through the capillary vessels, 
it must not, however, be forgotten that the veins perform an 
important part. The muscular fibres are spread over irregularly 
and ramify in all directions upon the external surface of the 
vein, instead of simply encircling the tube, as in the case of 
the arteries. The thin muscular coat of the smaller veins is 
abundantly supplied with nerve fibres, which ramify upon and 
amongst the muscular fibres in greater number than in arteries. 
By the reduction of the calibre of the vein the circulation of the 
blood in the capillaries would be retarded; and as a given degree 
of contraction of the muscular fibre cells can be preserved for a 
certain period of time, it is clear that the veins share with the 
little arteries the office of regulating the flow of blood through 
the capillaries, and form an important part of that mechanism 
through the instrumentality of which the distribution of blood to the 
