REGULATING DEE CONTRACTIONS OF THE ARTERIES 33 
to the sacrum, while the posterior three-tenths of the canal contain merely 
the cauda equina, including a slender filiform prolongation of the cord, which, 
though apparently composed in part of nervous matter, seems to give off no nerves. 
In the accompanying sketch of the superior aspect of the brain and cord 
of a frog, magnified two diameters, the distance from a to b represents the 
length of the spinal canal. The principal nerves for the hind 
legs spring from the cord near its extremity, but other smaller an 
branches with the same destination arise nearly as far forward ae’ 
as the middle of its length. There are also connecting fila- iA 
ments between these and some nerves for the abdominal 
parietes, taking origin slightly further forward than the middle 
of the cord. Thus the nerves for the posterior extremities 
are furnished chiefly, but not quite exclusively, from the 
posterior half of the cord. To expose the cord in its entire 
length without injury to it or any of its slender branches is 
troublesome, and also involves much loss of blood. It is 
therefore very desirable to be able to come at once on any 
part of the cord you may desire, without laying open the 
whole canal. This can be readily done from the data above 
given. The articulation between the occiput and first vertebra 
can be felt through the skin, as also can the commencement of the sacrum ; 
and the distance between these points is the length of the spinal canal. This, 
multiplied by 0-7, is the length of the cord: the requisite fraction of this 
length is then measured from the occiput and gives the place required. 
Assistance may also be derived from the circumstance that the posterior 
edges of the scapulae correspond very nearly with the mid-length of the cord, 
overlapping the posterior half by only about one-twentieth of the whole. 
To proceed with the experiments. On the 16th of April, a large frog being 
put under chloroform, the entire brain was removed about 3 o’clock p.m. without 
injury to the cord. After this operation, the arteries, which had previously 
been of pretty full size and transmitting rapid streams of blood, were found com- 
pletely contracted, so that the webs appeared bloodless except in the veins, 
and continued so for some minutes. At 3! ro™ an artery selected for special 
observation was dilating, having already attained to a diameter of 13°, and 
the circulation was returning in the web. At 3! 15™ the vessel measured 3°, 
but two minutes later was 24°, and half an hour afterwards exhibited the 
spontaneous changes in calibre commonly seen in arteries in health, the limits 
observed being 14° and 2°. It thus appeared that the removal of the brain 
had had no further effect upon the arteries than the temporary constriction 
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