ON THE SPINAL MARROW. 
129 
the lower cervical vertebrae it swells out again, for it has to fur¬ 
nish these plexuses of nerves which bestow sensibility and the 
power of voluntary motion on the fore extremities. After it has 
passed the first dorsal vertebra, and the muscles of the spine and 
the ribs alone are to be fed, it contracts again, and to less than 
its former dimensions : it loses, as it were, the lateral column 
whose function has been discharged, and it becomes diminished 
to a mere cylindrical chord ; but, arriving at the lumbar verte- 
brse, where the femoral plexuses begin to be formed in order to 
call into action the muscles of that part of the frame in which 
the power of the horse seems peculiarly to reside, it once more 
thickens and widens, and attains a bulk to which it had not 
reached before :—and, finally, the lumbar vertebrae being tra¬ 
versed, it rapidly and strangely shrinks, until it becomes a mere 
point, for it has the tail alone to supply. Before the spinal marrow 
quite disappears, it divides into a great number of chords, some 
of which soon terminate, while others extend into the coccvx : 
this portion of the marrow is called the cauda equina , from its 
supposed resemblance to a horse’s tail. In every part of the 
canal the bulk of the canal has reference to the size or the func¬ 
tion of the neighbouring muscles; and so the animal principle 
preponderates in the inferior part of the creation, while the extent 
of intellectual power in the human being more than compensates 
for the deficiency of brute strength. 
The Columns of the Spinal Chord .—It does not require any 
very minute inspection of the spinal marrow to observe that 
there is a mesian furrow running through its whole extent. It is 
more evident on the inferior than the superior surface, but suffi¬ 
ciently plain on both. It is a very convenient sulcus for the 
transmission and defence of the bloodvessels of the chord; but 
it is more than this—it constitutes a longitudinal division of the 
chord into two equal parts. Observe how readily, in this har¬ 
dened brain, the chord separates in the direction of this furrow. 
You see the lacerated portions of minute medullary fibres running 
across, and, quite as plainly, an exceedingly fine cellular mem¬ 
brane. In fact, there are two nervous chords placed in apposition 
with each other through the whole extent of the spinal canal; 
connected with yet distinct from each other, and each discharg¬ 
ing its proper function with regard to the side of the body on 
which it is placed. Some of the paralytic affections of the hu¬ 
man frame are illustrations of this; hemiplegia, however,is a dis¬ 
ease that rarely occurs in the quadruped. The medullary bands 
between the two columns are more evident than in the human 
being, and preserve, perhaps, a greater sympathy between the two 
divisions of the chord. 
VOL. VII. 
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