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.
In the horse its average length is about 76 to 78 inches (ca. 190 to 195
cm.), and its weight about 8½ to 9 ounces (ca. 240 to 255 grams).
It is approximately cylindrical, but more or less flattened
dorso-ventrally. There is no line of demarcation between it and the
medulla oblongata, but for practical purposes the division is usually
assumed to be at the posterior border 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 coccygeal. According to the
attachments of these series of nerves the spinal cord is divided into
=cervical=, =thoracic=, =lumbar=, and =sacral= parts.
In a horse about 16½ hands high these parts measured 65 cm. (ca. 26
in.), 86 cm. (ca. 34.4 in.), 27 cm. (ca. 10.8 in.), and 15 cm. (ca. 6
in.) respectively (Dexler).
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
vertebræ, so that the roots of the last lumbar nerve must run backward
the length of the last lumbar vertebra to reach the intervertebral
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 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 cm.
respectively. The spinal nerves are in general designated according to
the vertebræ behind which they emerge from the vertebral canal. In the
neck, however, there are eight pairs of nerves and only seven
vertebræ; 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 wide enlargements which
involve the segments with which the nerves of the limbs are connected.
The =cervical enlargement= (Intumescentia cervicalis) begins gradually
in the fifth cervical vertebra and subsides in the second thoracic. Its
maximum transverse diameter is about an inch (ca. 25 mm.) and its
dorso-ventral nearly half an inch (ca. 12 mm.). The =lumbar enlargement=
(Intumescentia lumbalis) is situated in the fourth and fifth lumbar
vertebræ. It is a little narrower than the cervical enlargement, and its
dorso-ventral diameter is also slightly smaller. Behind this the cord
tapers rapidly to form the =conus medullaris=. The tip of the latter is
continued by a delicate glistening strand, the =filum terminale=, which
is composed largely of fibrous tissue continued from the pia mater,
covered by arachnoid.
[Illustration:
FIG. 495.—VENTRAL VIEW OF MEDULLA OBLONGATA AND FIRST AND SECOND
SEGMENTS OF SPINAL CORD OF HORSE; THE MEMBRANES ARE CUT AND
REFLECTED.
_1_, Lig. suspensorium arachnoideale; _2_, right cerebrospinal artery;
_3_, _5_, digitations of lig. denticulatum; _4_, free border of lig.
denticulatum; _6_, middle spinal artery; _7_, basilar artery; _8_,
pons; _9_, arachnoidea; _10_, dura mater; _11_, _12_, ventral
root-bundles of first and second segments of spinal cord; _VI._, N.
abducens; _IX_, _X_, glossopharyngeus and vagus; _XI_, accessory,
medullary part; _XI′_, accessory, spinal part; _a_, line between
medulla oblongata and spinal cord. (Dexler, in Ellenberger-Baum,
Anat. d. Haustiere.)
]
[Illustration: