276 ANIMAL PHYSIOLOGY. 
but through constriction of the vessels by the vaso-motor 
nerves. 
Again, an injection of fluid into the blood does not cause an 
appreciable rise of blood-pressure, so long as the nervous sys- 
tem is intact; but, if by section of the spinal cord the vaso- 
motor influences are cut off, then a rise may take place to the 
extent of 2 to 3 per cent of the body-weight, the extra quan- 
tity of fluid seeming to be accommodated in the capillaries and 
smaller veins. These facts are highly significant in illustrat- 
ing the adaptive power of the circulatory system (protective in 
its nature), and are of practical importance in the treatment of 
disease. 
We think the benefit that sometimes follows bleeding has 
not as yet received an adequate explanation, but we shall not 
attempt to tackle the problem now. Changes in the circulation 
depend on variations in the size of the blood-vessels. 
It is important in considering this subject to have clear no- 
tions of the structure of the blood-vessels. It will be borne in 
mind that, while muscular elements are perhaps not wholly 
lacking in any of the arteries, they are most abundant in the 
smallest, the arterioles, which by their variations in size are 
best fitted to determine the quantity of blood reaching any 
organ. It is well known that nerves derived chiefly from the 
sympathetic system pass to blood-vessels, though their exact 
mode of termination is obscure. 
We may now examine into the nature of certain facts, which 
may be stated briefly thus: 
1. In certain vascular areas of some vertebrates, as in the 
vessels of the ear of the rabbit and this animal’s saphena 
artery, rhythmical variations in the size of the small arteries 
may be observed; also in the veins of the bat’s wing and of the 
fins of certain fishes (e. g., caudal vein of the eel), as well as in 
certain arteries of some groups of the cold-blooded animals. 
2. Under the microscope the arterioles of various parts of 
the frog, including those of the muscles, may be seen to vary 
apparently spontaneously, and may through stimulation be 
made to depart widely from their usual size. 
3. Section of a large number of nerves is followed by red- 
dening of the parts to which they are distributed. This is well 
seen when the cervical sympathetic of the rabbit is divided; the 
ear becomes redder, owing to obvious dilatation of its blood- 
vessels; and warmer, owing to the increased quantity of blood 
in it, etc. It has also been noticed in cases of paralysis, and 
