Part 199

1, Dorsal column; 2, lateral column; 3, ventral column; 4, dorsal horn; 5, ventral horn; 6, gray commissure; 7, white commissure; 8, central canal; 9, dorsal septum; 10, ventral fissure; 11, dorsal nerve-root; 12, ventral nerve-root; 13, spinal ganglion; 14, nerve; 15, intervertebral foramen; 16, arch of vertebra; 17, epidural space; 18, dura mater (represented a little too thick); 19, ligamentum denticulatum; 20, 20, longitudinal venous sinuses; 21, middle spinal artery. The subdural and subarachnoid spaces (black) are traversed by delicate trabeculæ. The outer part of the arachnoid and the pia mater are not shown. ]

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. 499, 500), 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.

FIG. 498.—BRAIN OF HORSE, DORSAL VIEW, ABOUT ⁵⁄₇ NATURAL SIZE.

1, Entomarginal fissure; 2, marginal fissure; 3, ectomarginal fissure; 4, suprasylvian fissure. ]