46 ON THE PARTS OF THE NERVOUS. SYSTEM 
doubt in consequence of slight contraction of the arteries, one of which, in 
the left foot, measured 3° by micrometer, and after sixteen hours more they 
were both moderate and variable in calibre in both feet ; that in the left limb 
before noted now changing between 14° and 2°, and a principal artery in the 
right foot between 1° and 14°. The circulation meanwhile continued active, 
and remained so more than twelve hours longer ; from which circumstance 
as well as from the normal appearance of the contractions, it was evident that 
the arteries were still under the control of the cord; and I may add, that in 
another animal in which the same operation was performed upon the thigh 
after removal of the brain and cord, the arteries remained of full size and 
without variation for thirty-four hours, after which circulation ceased. 
From these facts it appears that there exists a very remarkable provision 
for ensuring the proper regulation of the arterial calibre in a part in spite of 
almost complete division of the nerves connecting the vessels with the nervous 
centre which presides over their contractions. It has been shown by recent 
discovery that sensation and voluntary motion are abolished in parts whose 
nerves have been divided, until repair has been effected by a process of fresh 
formation of the nerve-fibres. But the control of the flow of the nutrient fluid is 
not allowed to be interrupted in this manner, but continues to be exercised more or 
less perfectly, notwithstanding nearly absolute severance of nervous connexion. 
Allusion has been more than once made to the circumstance that arteries 
do not dilate so fully when the heart is very feeble as when it is in powerful 
action. This was strikingly illustrated in the case of the frog which was the 
subject of operation on April 16, 1857. Immediately after the experiments 
recorded at p. 33, the heart having ceased to cause movement of blood in 
the web, I induced complete constriction of the arteries by irritating with 
a needle the posterior part of the cord, and then thoroughly cleared the spinal 
canal of its contents. The artery under special observation did not, however, 
become dilated to a greater diameter than 14°, although during the earlier 
experiments, when the heart was acting vigorously, it had been observed to 
attain sometimes a calibre of 3°. The heart never recovered its power, and 
the vessel maintained this medium width as long as I continued to examine 
the animal, namely, three hours. 
From this and other similar observations, I infer that full dilatation of the 
arteries is a merely passive phenomenon as respects the parietes of the vessels. 
Contraction is effected by the muscular fibre-cells of their circular coat, on the 
relaxation of which the elasticity of the arteries tends to make them expand 
to a certain degree, beyond which they do not dilate, except in so far as they 
are distended by the blood. 
