Part 72

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.[187]

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=.[188] 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=,[189] 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.

[Illustration:

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

_1_, Entomarginal fissure; _2_, marginal fissure; _3_, ectomarginal fissure; _4_, suprasylvian fissure. ]

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 │ DERIVATIVES. │ CAVITIES. │ SEGMENTS. │ │ Rhombencephalon │ │ │ (posterior │Myelencephalon │Medulla oblongata│Fourth ventricle vesicle) │ │ │ „ │Metencephalon │Pons │ „ „ │ „ │Cerebellum │ „ │Isthmus │Anterior │ „ │ rhombencephali │ cerebellar │ „ │ │ peduncles │ „ │ „ │Anterior │ „ │ │ medullary velum│ Mesencephalon │ │Corpora │ (middle │Mesencephalon │ quadrigemina │Cerebral aqueduct vesicle) │ │ │ „ │ „ │Cerebral │ „ │ │ peduncles │ Prosencephalon │ │{Optic thalami │Posterior part of (anterior │Diencephalon │ } │ third ventricle vesicle) │ │ │ „ │ „ │{Subthalamic │ „ │ │ tegmenta │ „ │ „ │{Pineal body │ „ │ │ │Anterior part of │ │ │ third │ │{Pituitary body │ ventricle. „ │Telencephalon │ } │ Lateral │ │ │ ventricles and │ │ │ olfactory │ │ │ continuations. „ │ „ │{Optic nerves and│ „ │ │ retinæ │ „ │ „ │{Cerebral │ „ │ │ hemispheres │ „ │ „ │{Olfactory tracts│ „ │ │ and bulb │

THE RHOMBENCEPHALON

THE MEDULLA OBLONGATA

The =medulla oblongata= lies on the basilar part of the occipital bone. It is quadrilateral in outline, but much wider in front than behind, and compressed dorso-ventrally. Its length, measured from the root of the first cervical nerve to the pons, is about two inches (ca. 5 cm.).

Its =ventral surface= is convex in the transverse direction. It is marked by a =median fissure= (Fissura mediana ventralis) which is continuous behind with the similar fissure of the spinal cord. The posterior part of the fissure is faintly marked, but in front it becomes deeper and ends in a small depression (Foramen cæcum) behind the central part of a transverse band, the =corpus trapezoideum=. On either side of the fissure is a rounded tract, the =pyramid= (Pyramis), which is bounded externally by a faint lateral groove (Sulcus intermedius ventralis). The pyramids join the pons in front; behind they become narrower and disappear into the substance of the medulla, in which their fibers intercross, forming the =decussation of the pyramids= (Decussatio pyramidum).[190]

The superficial origin of the =sixth cranial nerve= (N. oculomotorius) is just lateral to the anterior end of the pyramid. The =corpus trapezoideum= is a transverse band which extends across the surface immediately behind the pons. It is crossed by the pyramids, which cut off a small central part. The lateral part extends out to the roots of the seventh and eighth nerves on either side. Behind the outer part of the corpus trapezoideum there is a variably developed rounded eminence, the =tuberculum faciale=. The root-fibers of the twelfth or hypoglossal nerve form an oblique linear series lateral to the posterior part of the pyramid.

[Illustration:

FIG. 499.—BASE OF BRAIN OF HORSE, NATURAL SIZE. ]

The =dorsal surface= is largely concealed by the cerebellum and forms the greater part of the floor of the fourth ventricle. The =dorsal median fissure= (Fissura mediana dorsalis), the direct continuation of the corresponding groove of the spinal cord, extends forward to about the middle of the surface. Here the =restiform bodies=, which constitute the lips of the fissure, diverge to form the lateral boundaries of a triangular depression; this is the posterior part of the rhomboid fossa or floor of the fourth ventricle of the brain. The dorso-lateral fissure winds outward and forward to the lateral aspect of the medulla, where it presents the roots of the =ninth=, =tenth=, and =eleventh cranial nerves=. External to it is a distinct oval eminence on the anterior part of the lateral column, termed the =tuberculum cinereum=. The central canal of the cord is continued in the posterior part of the medulla, inclines dorsally, and opens in the posterior angle of the fourth ventricle. Hence it is customary to distinguish a closed and an open part of the medulla. The dorsal aspect of the latter, which is concealed at present, will be considered later in the description of the fourth ventricle.

[Illustration:

FIG. 500.—LINE DRAWING OF BASE OF BRAIN OF HORSE. (KEY TO FIG. 499.) ]

The lateral surface is narrow behind, wider and rounded in front. From it the root-fibers of the ninth, tenth, and eleventh cranial nerves arise in a linear series, and alongside of it the spinal part of the eleventh nerve passes forward to join the medullary root. Close inspection reveals the presence of striæ which curve obliquely downward and backward from the surface of the restiform body toward the hypoglossal root-fibers; these are the =external arcuate fibers= (Fibræ arcuatæ externæ). The recess between the lateral aspect of the medulla and the cerebellum is occupied by an irregular mass of villous projections of the pia mater, containing tufts of vessels; this is the =chorioid plexus of the fourth ventricle=, and is the lateral edge of the =tela chorioidea= of the ventricle. On raising the chorioid plexus it is seen that the tela chorioidea is attached to the dorsal aspect of the medulla, and reinforces here the wall of the fourth ventricle; also that the restiform body terminates in front by entering the base of the cerebellum, forming its posterior peduncle.