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NERVOUS CENTRES. (Human Anatomy. 
the centre of which lies the filiform * nape 
gation or process of the pia mater. The su- 
perior limit of the spinal cord is marked by 
the plane which lies between the occipital 
foramen and the first vertebra of the neck. A 
section made in the direction of this plane 
Separates the spinal cord from the medulla 
oblongata. Immediately above this plane the 
decussation of fibres of the anterior pyramids. 
takes place, and may be regarded as the na- 
tural inferior limit to the medulla oblongata. 
Such is the position of the spinal cord in the 
adult. In the foetus at the third month of 
intra-uterine life, it occupies the whole spinal 
canal, and extends quite to the point of the 
sacrum. At this early period the os coccygis 
consists of seven vertebre. Coincident with 
its reduction to its normal number of segments, 
__ is the retraction of the spinal cord within the 
Spinal canal. If the ascent of the cord be ar- 
rested, the foetus is born with a tail, for the 
_ changes of the coccyx become arrested also. It 
is remarkable that among the inferior animals 
there is a direct proportion between the length 
of the spinal cord and that of the tail. The 
‘Shorter the former, or the higher in the spinal 
“Canal it may be, the less will be the latter. 
In 
animals with long tails there is no cauda equina, 
as is the case in the ox, the horse, the squirrel, 
&e. and the opposite is likewise true, namely, 
that in animals with a short tail the spinal 
cord is much shorter and is placed higher up 
in the spinal canal. In the embryo of the bat, 
_ Which has a tail, the spinal cord extends down- 
wards, but when it loses its tail the cord ap- 
_ pears to occupy a much smaller portion of the 
Spinal canal. In the tadpole of the frog, like- 
Wise, the spinal cord extends into the tail, but 
when the tail has disappeared the cord occupies 
“7 * portion of the spinal canal.* 
_In point of shape, the spinal cord is cylin- 
droid, slightly flattened on its anterior and pos- 
terior surfaces, more so on the former than on 
the latter. At its inferior extremity it gradually 
to a point. Sometimes, however, we 
observe a small tubercle immediately above 
this pointed extremity, situated on the posterior 
surface. The perfect cylindrical form of the 
cord is destroyed, not only by this pointed ter- 
mination and the flattening before and behind, 
but likewise by a marked change of dimen- 
sions in certain regions. In the cervical re- 
gion we observe a distinct swelling or enlarge- 
ment, which begins a short distance beneath the 
medulla oblongata, and gradually passes into 
the dorsal portion, which is the smallest, as 
well as the most cylindrical part of the cord. 
This cervical enlargement (intumescentia cervi- 
calis ) begins opposite the third cervical vertebra, 
and ends about the third dorsal. The cord con- 
tinues of a cylindrical form as low as about 
the ninth or tenth dorsal vertebra, and then 
— into the lumbar swelling (intumescentia 
umbalis vel cruralis ), which occupies a space 
corresponding to about two vertebre. is 
swelling is both shorter and of less diameter 
* Cuvier’s Report upon Serres’ work, Sur I’ Anat. 
p- duCerveau, Par. 1824, 
Tue Sprnat Corp.) 651 
than that in the region of the neck. The inferior 
extremity of the spinal cord tapers rather sud- 
denly, and at its point is enclosed in the com- 
mencement of the filiform prolongation of the 
pia mater. 
The bulk of the spinal cord is in the direct 
ratio to that of the body throughout the verte- 
brate series. And not only is this true with 
regard to the whole cord, but with respect to 
its segments. For when any segment supplies 
nerves to a greater sentient surface, or to more 
numerous or more powerful muscles than an- 
other, it exhibits a proportionally greater size. 
It is thus that we may satisfactorily explain the 
occurrence of the cervical and lumbar enlarge- 
ments. Both supply nerves to the extremities, 
whilst the dorsal portion furnishes them only 
to the trunk. The upper extremities enjoy, in 
part, a high degree of tactile sensibility, and 
they possess great power and extent of mus- 
cular movement. That portion of the cord 
therefore from which the nerves to the upper 
extremities proceed is larger in every way than 
that which supplies the lower extremities, 
which, although provided with large and pow- 
erful muscles, do not enjoy such a range or 
variety of motion as the upper extremities, nor 
are they endowed with so exquisite a sensi- 
bility. 
There are many interesting facts among the 
lower animals which illustrate and confirm this 
law. Thus, in animals which have no limbs, 
as serpents, the cord is of equal size through- 
out, excepting at its pointed extremity. It is 
said that in the foetus, before the developement 
of the limbs, no distinction of size can be dis- 
covered in the cord, and in persons in whom 
an arrest in the developement of the upper 
extremities has taken place, there is no cervical 
enlargement. Cruveilhier refers to the case of 
the tortoise as strongly confirmatory of this law. 
That portion of the spinal cord which corre- 
sponds to the carapace, which is equally devoid 
of sensation and motion, is reduced to a mere 
thread, whilst those segments between which 
it lies, and from which the nerves of the ex- 
tremities emanate, are of size duly propor- 
tionate to their muscular activity and their sen- 
sibility. In Fishes, the enlargements corre- 
spond to the fins which are possessed of great- 
est muscular power. In the gurnard there 
exist certain very remarkable ganglionic swel- 
lings, situate on the oa se of the cer- 
vical segment of the cord. ith these swel- 
lings nerves are connected, which are distri- 
buted to organs placed immediately behind the 
head on the lower part of the body. These 
organs are endowed with much tactile sensibi- 
lity, and seem to serve the office of feelers, as 
the animal gropes along the bottom of the 
sea. 
The length of the spinal cord in the adult is 
from sixteen to eighteen inches, according to 
the statement of Cruveilhier. Its circumfe- 
rence measures twelve lines at the smallest and 
eighteen lines at the most voluminous part. 
Chaussier states that its weight is from the 
nineteenth to the twenty-fifth part of that of 
the brain in the adult, and about the fortieth 
