THE MEDULLA OBLONGATA. 441 
on to its destination. Each of these nerves has two distinct origins ; one 
from the upper part of the grey central matter 
(the sensitive root), the other from its inferior 
surface, which is the motor portion. The 
superior has an accession of grey matter around 
it, soon after the union of its nervelets, 
called its ganglion, beyond which the two 
divisions unite to form a large nerve, which 
soon begins to subdivide again for supplying 
the several parts of the body. The termi- 
nating branches reunite in loops, so that 
these nerves may be considered to form a 
complete circle, those of sensation receiving 
impressions from the parts on which they are 
distributed, and conveying them to the central 
ganglion, while the motor nerves cause the 
muscles which they supply to contract on 
receipt of the proper stimulus from the centre. 
The spinal nerves are from forty-two to forty- 
three in number on each side ; namely, eight 
pairs of cervical, seventeen pairs of dorsal, 
six pairs of lumbar, five pairs of sacral nerves, 
and six or seven coccygeal. Each of these 
nerves divides at once into a superior and 
inferior branch, the latter giving off a small ; 
nervelet to communicate with the sympa- “A 
thetic, and then going on to supply the lower By 
parts of the body and the extremities. The 3 
cord varies somewhat in size in the several in 
regions of the spine. It commences at the 
occiput of full size, then diminishes to the 
fifth cervical vertebra, where there is a slight 
swelling, after which it is gradually reduced 
in dimensions to the loins, where it spreads 
out into a wide but thin plate, after which J 
it divides into its terminal branches, which qs 
have been compared to a horse’s tail. 
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THE MEDULLA OBLONGATA. 
THE MEDULLA OBLONGATA is the anterior 
enlarged portion of the spinal cord, of a 
conical shape, which extends to the pons i 
Varolii. On its inferior face it presents two 
pyramidal bodies, and on its superior two 
Hattened cords, the corpora restiformia, while 
between the two are the corpora olivaria. On 
making a section of this part, the corpus 
olivare is seen to be chiefly composed of grey 
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Fia. 2.—-SPpINAL CORD REMOVED FROM ITS CANAL. 
a a. a. a. Roots of the spinal nerves. 
b. b. b. The filaments going to unite to form each of these 
nerves. 
¢ ¢. ¢. Clear spaces between the roots of the nerves. 
