764 NERVOUS SYSTEM OF THE HORSE 
thickening, the linea splendens, along which the ventral spinal artery runs. On 
each side the pia mater gives off a strong longitudinal band, the ligamentum den- 
ticulatum, which is connected externally with the dura mater. The inner or pial 
border extends in a line between the dorsal and ventral roots of the nerves. The 
outer or dural border is denticulated and to a large extent free. The denticulations 
are attached to the dura between the nerve-roots. 
NERVOUS SYSTEM OF THE HORSE 
THE SPINAL CORD 
The spinal cord (Medulla spinalis) is the part of the central nervous system 
which is situated in the vertebral canal. It extends from the foramen magnum to 
about the middle of the sacrum. Its average length is about 76 to 78 inches (ea. 
190-195 cm.), and its weight about 8% to 9 ounces (ca. 240-255 grams). 
It is approximately cylindrical, but more or less flattened dorso-ventrally. 
There is no natural line of demarcation between it and the medulla oblongata, but 
for descriptive purposes the division is usually assumed to be at the plane of the 
foramen magnum. Its posterior part tapers rapidly to a point, forming the conus 
medullaris. This is prolonged for a short distance by the slender filum terminale. 
Forty-two pairs of spinal nerves are connected with the sides of the spinal cord. 
They are classified as eight cervical, eighteen thoracic, six lumbar, five sacral, and 
five coceygeal. According to the attachments of these series of nerves the spinal 
cord is divided into cervical, thoracic, lumbar, and sacral parts.! 
In the embryo these divisions correspond primitively to the regions of the 
vertebral column, but later, through unequal growth of the cord and spine, the 
correspondence between the two is not at all exact in the anterior regions and is 
lost in the last two. The lumbar part of the cord in the horse ends at the junction 
of the fifth and sixth lumbar vertebra, so that the roots of the last lumbar nerve 
must run backward the length of the last lumbar vertebra to reach the interverte- 
bral foramen through which it emerges. The conus medullaris reaches only to the 
anterior part of the sacral canal, so that the roots of the sacral and coccygeal nerves 
extend backward in the spinal canal for a considerable distance, forming a leash 
of bundles, in the center of which lie the conus medullaris and the filum terminale. 
This arrangement is expressively designated the cauda equina. 
Each pair of spinal nerves is attached by its root-fibers to a certain length of 
the cord, and the latter is, therefore, regarded as consisting of as many segments 
as there are pairs of nerves. It is to be noted, however, that there is no line of 
demarcation between the segments other than the intervals between the root-fibers 
of adjacent nerves. 
The segments are of different lengths; the longest are the third to the sixth cervical, which 
measure 11, 10, 10, and 8.5 em. respectively. The spinal nerves are in general designated accord- 
ing to the vertebra behind which they emerge from the vertebral canal. In the neck, however, 
there are eight pairs of nerves and only seven vertebra; here the first nerve emerges through the 
intervertebral foramen of the atlas and the eighth between the last cervical and the first thoracic 
vertebra. 
In the greater part of the thoracic region the spinal cord is fairly uniform in 
size, but there are two conspicuous enlargements which involve the segments with 
which the nerves of the limbs are connected. The cervical enlargement (Intumes- 
centia cervicalis) begins gradually in the fifth cervical vertebra and subsides in 
1TIn a horse about 1614 hands high these parts measured 65 em. (ca. 26 in.), 86 cm. (ca. 34.4 
in.), 27 em. (ca. 10.8 in.), and 15 em. (ca. 6 in.) respectively (Dexler). 
