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.

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.

The surface of the spinal cord is divided into two similar halves by a dorsal median groove (Sulcus medianus dorsalis) and a ventral median fissure (Fissura mediana ventralis). On either side of the former is the dorso-lateral groove (Sulcus dorsalis lateralis) at which the fibers of the dorsal nerve-roots enter the cord; it is faint except at the enlargements, and is represented by two grooves in the first cervical segment. The ventral root-fibers as they emerge from the cord do not form a continuous series, but arise from a zone (Area radicularis ventralis) 3 to 5 mm. in width a little lateral to the ventral median fissure, and no groove is found here. In the greater part of the cervical region and the anterior part of the thoracic region there is a shallow dorsal paramedian groove (Sulcus intermedius dorsalis) a short distance lateral to the median groove. These grooves indicate the division of the white matter of the cord into columns to be described later.

Examination of cross-sections of the spinal cord shows that it is a bilaterally symmetrical structure, incompletely divided into right and left halves by a ventral fissure and a dorsal septum. The ventral median fissure (Fissura mediana ventralis) is narrow and penetrates nearly to the middle of the dorso-ventral diameter of the cord. It is occupied by a fold of pia mater. The dorsal median septum (Septum medianum dorsale) is a partition which descends from the posterior median groove to about the middle of the cord. It apparently consists of condensed neuroglia with an admixture of pial tissue. The two halves of the cord are connected by commissures of gray and white matter. The gray commissure (Commissura grisea) is a transverse band of gray matter at the ventral end of the dorsal septum. It is divided into dorsal and ventral parts by the central canal of the cord. The white commissure (Commissura alba) is a bridge of white matter which connects the ventral columns of the cord over the dorsal end of the ventral median fissure, and constitutes a conducting path from one side to the other.

The central canal of the cord (Canalis centralis), the spinal vestige of the lumen of the embryonal neural tube, is a minute passage which tunnels the gray commissure. It opens at its anterior end into the posterior part of the fourth ventricle of the brain, and its terminal part in the conus medullaris forms a slight dilatation, the ventriculus terminalis. It is lined by epithelium and is surrounded by a layer of modified neuroglia (Substantia grisea centralis).

The gray matter of the spinal cord as seen in cross-sections resembles roughly a capital H the cross-bar of the letter being formed by the gray commissure. Each lateral part is considered as consisting of dorsal and ventral gray columns or horns (Columna grisea dorsalis, ventralis). In the greater part of the cord the dorsal column or horn is elongated and narrow and tapers to a point which extends almost to the surface of the cord at the attachment of the dorsal root-fibers of the spinal nerves. Its apex or tip consists of gray matter which is lighter in color and less opaque than that of the rest of the horn and is termed the substantia gelatinosa (Rolandi). The ventral column or horn is short, thick and rounded, and is separated from the surface of the cord by a thick layer of white matter, through which the fibers of the ventral roots of the spinal nerves pass. From the middle of the cervical region to the lumbar region there is a medial projection of gray matter on the lower part of the dorsal column; this is the nucleus dorsalis or Clarke’s column. In the anterior part of the cord there is an outward projection of the gray matter at the base of the ventral horn; this is termed the lateral column or horn. The demarcation between the gray and white matter is in many places indistinct; this is especially the case laterally, where processes of gray matter extend into the white substance, producing what is known as the formatio reticularis.

Cross-sections of the spinal cord present the following gross regional characters: (1) The cervical cord near the medulla is compressed dorso-ventrally, its width is about 18 mm. and its greatest thickness about 8 mm. It has dorsally a deep median sulcus and a distinct dorso-lateral sulcus. Lateral grooves are also present. The dorsal cornua are strongly everted. Each has an expanded head, which comes very close to the surface of the cord, and has an extensive cap of substantia gelatinosa. The neck is distinct. The ventral cornua are short and blunt and diverge very little. The gray commissure is about in the middle of the section and 2.5 mm. in length. According to Dexler the column between the median and lateral grooves dorsally is the funiculus cuneatus, the funiculus gracilis being very small and not showing on the surface in this region. In the middle of the cervical region the diameters are about 16 mm. and 10 mm. respectively. The ventral surface is somewhat flattened. The dorsal cornua have pointed ends and turn decidedly outward. The ventral cornua are short and thick and are directed very slightly outward; their ends are about 4 mm. from the ventral surface. The gray commissure is just above the middle of the section and is about 2 mm. long. The cervical enlargement measures about 25 mm. transversely and 12 mm. vertically. The dorsal cornua are smaller than the ventral and have a large cap of substantia gelatinosa. The ventral cornua are short and thick, curve strongly outward, and are about 4 mm. from the ventral surface. Each bears a prominence on its inner side near the base. The gray commissure is considerably above the middle of the section and is about 4 mm. long. (2) In the middle of the thoracic region the cross-section is biconvex, the ventral surface being the more strongly curved. The transverse diameter is about 15 mm. and the dorso-ventral about 10 mm. The gray columns are close together, the gray commissure being only about 1 mm. in length, and lying considerably above the middle of the section. The dorsal cornua are short and have slightly enlarged ends. The ventral cornua have a uniform diameter, turn very little outward, and end about 3 mm. from the ventral surface. (3) The lumbar enlargement is much flattened, especially dorsally. The transverse diameter is about 22 mm. and the dorso-ventral 9 to 10 mm. The cornua are very large. The ventral cornua are thick and rounded and turn sharply outward; they end about 2 mm. from the ventral surface. The dorsal cornua are smaller and shorter and do not diverge so strongly. The gray commissure is about in the middle of the section and is about 3 mm. long. In the third lumbar vertebra the cord is about 3 mm. narrower and thicker, and both surfaces are about equally convex. The dorsal cornua are smaller, considerably everted, and constricted in the middle. The ventral cornua are very short and do not turn outward. (4) In the first sacral vertebra the cord is almost round and is 5 to 6 mm. in diameter; the cornua are relatively very large and the commissure has the form of a high intermediate mass.

The ventral horn contains large cells, the axones of which emerge as the fibers of the ventral nerve-roots (Fila radicularia). The axones of many cells cross to the opposite side in the white commissure and pass out in a ventral root of that side, or enter the white matter and pass forward and backward, associating various segments of the cord. Some pass to the ventral horn of the opposite side at the same or at different levels. Others pass to the periphery of the cord, join the cerebellospinal fasciculus, and extend to the cerebellum. Scattered through the gray matter are many smaller cells with axones which pursue a short course and serve to connect different parts of the gray matter.

The white matter of the spinal cord is divided into three pairs of columns. The dorsal columns (Funiculi dorsales) lie on either side of the dorsal median septum and extend outward to the dorso-lateral groove and the dorsal gray column. The ventral columns (Funiculi ventrales) are situated on either side between the median fissure and the ventral gray columns. They are connected above the fissure by the white commissure. The lateral columns (Funiculi laterales) lie external to the gray columns on either side; their limits are indicated superficially by the dorso-lateral groove and the emergence of the ventral root-fibers. The paramedian groove (where present) indicates a subdivision of the dorsal column into two fasciculi or tracts; the inner of these is the fasciculus gracilis (Goll’s column); the outer, the fasciculus cuneatus (Burdach’s column).

The amounts of gray and white matter vary greatly in different parts of the cord both absolutely and relatively. In cross-section the absolute areas of both are greatest in the enlargements. The relative area of gray matter is smallest in the thoracic region (except at its anterior end), and increases from the lumbar enlargement backward.

Investigations have shown that in man the columns of white matter are subdivided into fasciculi or tracts, which constitute definite conducting paths of greater or less length. Our knowledge of the tracts in the domesticated animals is very limited, and it is quite unsafe to make inferences from the arrangement in man. As evidence of this it may be noted that the ventral cerebrospinal or direct pyramidal tract of man cannot be recognized as such.

The dorsal white columns consist essentially of two sets of axones. The afferent or sensory axones which come from the cells of the spinal ganglia enter as the dorsal roots of the spinal nerves and divide into two branches in the vicinity of the dorsal gray column. The anterior (ascending) branches form the direct sensory path to the brain and extend in the fasciculus cuneatus and fasciculus gracilis or corresponding tracts to nuclei in the medulla oblongata. The posterior (descending) branches extend backward for varying distances and give off numerous collaterals to cells of the gray column, thus forming part of the mechanism for the mediation of reflex action. Some collaterals cross in the white commissure to the opposite side. Many of these fibers are collected in the comma-shaped tract between the fasciculus gracilis and cuneatus. The second set of axones arises from the smaller cells of the gray column. They enter the white matter, divide into anterior and posterior branches, forming the fasciculi proprii or ground bundles of the cord. Some branches cross to the opposite side. The function of this set of axones is chiefly to associate various levels of the cord.

The lateral columns contain some axones of the dorsal nerve-roots, which (in man) are grouped in the marginal tract of Lissauer, situated just dorsal to the apex of the dorsal horn. The cerebellospinal fasciculus (direct cerebellar tract of Flechsig) extends along the periphery of the lateral column. It contains the axones of the cells of the nucleus dorsalis (Clarke’s column), which proceed to the medulla oblongata and enter the cerebellum by the restiform body. The rubrospinal tract of Monakow appears to take the place of the lateral cerebrospinal fasciculus or crossed pyramidal tract of man. It lies at the inner side of the cerebellospinal fasciculus. Its fibers arise in the nucleus ruber of the mid-brain, cross the median plane (decussation of Forel), and pass backward in the tegmentum and medulla oblongata to the lateral column of the cord. It is a path for motor impulses coming from the cerebral cortex and the cerebellum. The lateral fasciculus proprius or ground-bundle is deeply situated at the side of the gray columns. The bulk of its fibers are axones of cells of the posterior cornu which divide into anterior and posterior branches. They are intersegmental paths which associate different levels of the gray matter of the cord. The significance of the remaining fibers is not yet known.

The ventral white columns do not contain a ventral cerebrospinal or direct pyramidal tract, as in man. There is a small tract (Fasciculus intracommissuralis ventralis) above the white commissure, which separates it from the rest of the ventral column. It extends to the middle of the thoracic region. It consists of intersegmental fibers, and contains in the anterior part of the cervical cord in the sheep and goat both crossed and direct pyramidal fibers. The descending cerebellospinal fasciculus extends from the cerebellum to the lumbar region. In the cervical region it occupies a semilunar area which reaches almost to the surface ventro-laterally. Scattered fibers belonging to it lie also in the medial part of the ventral column. Posteriorly it diminishes in size and comes to occupy a position next to the ventral median fissure, corresponding to the sulco-marginal fasciculus of man (Dexler).

The Brain

The brain or encephalon is the part of the central nervous system that is situated in the cranial cavity. It is the enlarged and highly modified cephalic part of the primitive neural tube. It conforms rather closely in size and shape to the cavity in which it lies.

Its average weight without the dura mater is about 23 ounces (ca. 650 gm.), and forms about ⅐ of 1 per cent. of the body-weight.

It is desirable to examine the general external configuration of the brain before studying its various parts in detail.

When divested of its membranes and vessels (Figs. , ), its ventral surface or base presents the median brain stem, which is continuous with the spinal cord without any natural line of demarcation; it divides in front into two branches, the cerebral peduncles, each of which disappears into the mass of the corresponding cerebral hemisphere. The brain stem consists of three parts. The medulla oblongata is the posterior part which extends forward as the direct continuation of the spinal cord. The pons (Varolii) is a transversely elongated mass which appears to turn up on either side into the cerebellum. The cerebral peduncles extend forward from the pons and diverge to plunge into the ventral part of the cerebral hemispheres. The area between them is the interpeduncular space. It is largely covered by the pituitary body or hypophysis, a yellowish-brown, discoid structure, which is connected with the base of the cerebrum by a delicate tube called the infundibulum. On drawing the pituitary body gently aside, the infundibulum is seen to be attached to a slight gray eminence, the tuber cinereum. Behind this is the mammillary body, a well-marked round prominence. The posterior part of the space is perforated by numerous openings for the passage of small arteries, and hence is termed the locus perforatus posticus. Two large bands of white matter, the optic tracts, cross the anterior ends of the cerebral peduncles and unite at the optic chiasm or commissure, forming the anterior boundary of the interpeduncular space. Above and in front of the chiasm the hemispheres are separated by the median longitudinal fissure. In contact with the anterior extremity of each hemisphere is the olfactory bulb, an oval enlargement adapted to the ethmoidal fossa of the cranium. This appears as a gray swelling on a wide flat band, the olfactory peduncle, which is continued behind by two divergent bands, the olfactory tracts or striæ. The internal tract disappears after a very short course on to the inner surface of the hemisphere. The external tract is larger and longer; it runs backward, inclines at first outward and then curves inward and disappears on the concealed or tentorial surface of the hemispheres. It is separated from the lateral cerebral gyri by a distinct groove (Sulcus rhinalis). Along the inner side of the stria are two eminences. The anterior of these is the trigonum olfactorium, a gray elevation situated in the angle of divergence of the inner and outer striæ. Behind this is a depression, the fossa transversa, which is continued across the external stria and sharply limits the second and much larger eminence, the pyriform lobe.

The superficial origins of most of the cranial nerves are visible on the base of the brain.

The olfactory nerve-fibers join the convex surface of the olfactory lobe and give it a shaggy appearance in specimens which have been removed intact—a difficult proceeding.

The second or optic nerves converge to the optic chiasm.

The third or oculomotor nerve arises from the inner part of the cerebral peduncle.

The fourth or trochlear nerve may be seen emerging between the pons and the hemisphere, but its connection with the brain is not visible.

The fifth or trigeminal nerve is connected with the lateral part of the pons.

The sixth or abducent nerve arises just behind the pons and lateral to the pyramid of the medulla.

The seventh or facial and the eighth or auditory nerves arise close together just behind the pons proper on the extremity of the corpus trapezoideum.

The ninth or glosso-pharyngeal, the tenth or vagus, and the eleventh or spinal accessory nerves are connected by a linear series of roots with the lateral aspect of the ventral surface of the medulla. The spinal part of the accessory nerve comes forward along the edge of the medulla to join its medullary root.

The twelfth or hypoglossal nerve arises from the posterior part of the medulla along the lateral edge of the pyramid.

The parts that are visible when the brain is viewed from above are the cerebral hemispheres, the cerebellum, and part of the medulla oblongata. The cerebral hemispheres form an ovoid mass, and are separated from each other by the median longitudinal fissure, in which the falx cerebri is situated. Their surfaces are marked by thick folds, the gyri cerebri, separated by sulci. The upturned ends of the olfactory bulbs are seen in front of the frontal poles of the hemispheres. The occipital poles of the hemispheres overlie the anterior part of the cerebellum, from which they are separated by the transverse fissure and the tentorium cerebelli contained in it. The cerebellum is a much smaller rounded mass which conceals the greater part of the medulla oblongata. Its surface is divided into a middle lobe, the vermis, and two lateral hemispheres. It is marked by numerous gyri and narrow sulci which have in general a transverse direction. The posterior third of the medulla oblongata is not covered by the cerebellum.

The brain is developed from the expanded cephalic part of the neural tube of the embryo. The process comprises a series of thickenings, flexures, and unequal growth and expansion of various parts of the tube. In the higher animals the result is that the tubular character of the brain is not very evident, since the lumen comes to consist of four irregular cavities, the ventricles, which are connected by narrow passages. The tube is first subdivided by two constrictions into three brain vesicles, termed respectively the hind-brain or rhombencephalon, the mid-brain or mesencephalon, and the fore-brain or prosencephalon. The hind-brain gives rise to three secondary segments and the fore-brain to two. The annexed table indicates the origin of the principal structures of the fully developed brain from the primitive vesicles. It has become customary to describe the brain with reference to its embryological relations.

TABLE INDICATING THE DERIVATION OF THE PRINCIPAL PARTS OF THE BRAIN

Primary Segments. Secondary Segments. Derivatives. Cavities.

Rhombencephalon (posterior vesicle) Myelencephalon Medulla oblongata Fourth ventricle

Metencephalon Pons

Cerebellum

Isthmus rhombencephali Anterior cerebellar peduncles

Anterior medullary velum

Mesencephalon (middle vesicle) Mesencephalon Corpora quadrigemina Cerebral aqueduct

Cerebral peduncles

Prosencephalon (anterior vesicle) Diencephalon {Optic thalami } Posterior part of third ventricle

{Subthalamic tegmenta

{Pineal body

Telencephalon {Pituitary body } Anterior part of third ventricle. Lateral ventricles and olfactory continuations.

{Optic nerves and retinæ

{Cerebral hemispheres

{Olfactory tracts and bulb