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4b. The Mid-brain or Mesencephalon
FIG. 710 - Coronal section through mid-brain. (Schematic.) (Testut.) 1. Corpora quadrigemina. 2. Cerebral aqueduct. 3. Central gray stratum. 4. Interpeduncular space. 5. Sulcus lateralis. 6. Substantia nigra. 7. Red nucleus of tegmentum. 8. Oculomotor nerve, with 8’, its nucleus of origin. a. Lemniscus (in blue) with a’ the medial lemniscus and a" the lateral lemniscus. b. Medial longitudinal fasciculus. c. Raphé. d. Temporopontine fibers. e. Portion of medial lemniscus, which runs to the lentiform nucleus and insula. f. Cerebrospinal fibers. g. Frontopontine fibers. The mid-brain or mesencephalon (Fig. 681) is the short, constricted portion which connects the pons and cerebellum with the thalamencephalon and cerebral hemispheres. It is directed upward and forward, and consists of (1) a ventrolateral portion, composed of a pair of cylindrical bodies, named the cerebral peduncles; (2) a dorsal portion, consisting of four rounded eminences, named the corpora quadrigemina; and (3) an intervening passage or tunnel, the cerebral aqueduct, which represents the original cavity of the mid-brain and connects the third with the fourth ventricle (Fig. 710). The cerebral peduncles (pedunculus cerebri; crus cerebri) are two cylindrical masses situated at the base of the brain, and largely hidden by the temporal lobes of the cerebrum, which must be drawn aside or removed in order to expose them. They emerge from the upper surface of the pons, one on either side of the middle line, and, diverging as they pass upward and forward, disappear into the substance of the cerebral hemispheres. The depressed area between the crura is termed the interpeduncular fossa, and consists of a layer of grayish substance, the posterior perforated substance, which is pierced by small apertures for the transmission of bloodvessels; its lower part lies on the ventral aspect of the medial portions of the tegmenta, and contains a nucleus named the interpeduncular ganglion (page 802); its upper part assists in forming the floor of the third ventricle. The ventral surface of each peduncle is crossed from the medial to the lateral side by the superior cerebellar and posterior cerebral arteries; its lateral surface is in relation to the gyrus hippocampi of the cerebral hemisphere and is crossed from behind forward by the trochlear nerve. Close to the point of disappearance of the peduncle into the cerebral hemisphere, the optic tract winds forward around its ventro-lateral surface. The medial surface of the peduncle forms the lateral boundary of the interpeduncular fossa, and is marked by a longitudinal furrow, the oculomotor sulcus, from which the roots of the oculomotor nerve emerge. On the lateral surface of each peduncle there is a second longitudinal furrow, termed the lateral sulcus; the fibers of the lateral lemniscus come to the surface in this sulcus, and pass backward and upward, to disappear under the inferior colliculus. 
FIG. 711 - Transverse section of mid-brain at level of inferior colliculi. 
FIG. 712 - Transverse section of mid-brain at level of superior colliculi. Structure of the Cerebral Peduncles (Figs. 711, 712).—On transverse section, each peduncle is seen to consist of a dorsal and a ventral part, separated by a deeply pigmented lamina of gray substance, termed the substantia nigra. The dorsal part is named the tegmentum; the ventral, the base or crusta; the two bases are separated from each other, but the tegmenta are joined in the median plane by a forward prolongation of the raphé of the pons. Laterally, the tegmenta are free; dorsally, they blend with the corpora quadrigemina. The base (basis pedunculi; crusta or pes) is semilunar on transverse section, and consists almost entirely of longitudinal bundles of efferent fibers, which arise from the cells of the cerebral cortex and are grouped into three principal sets, viz., cerebrospinal, frontopontine, and temporopontine (Fig. 710). The cerebrospinal fibers, derived from the cells of the motor area of the cerebral cortex, occupy the middle three-fifths of the base; they are continued partly to the nuclei of the motor cranial nerves, but mainly into the pyramids of the medulla oblongata. The frontopontine fibers are situated in the medial fifth of the base; they arise from the cells of the frontal lobe and end in the nuclei of the pons. The temporopontine fibers are lateral to the cerebrospinal fibers; they originate in the temporal lobe and end in the nuclei pontis. (*123 The substantia nigra (intercalatum) is a layer of gray substance containing numerous deeply pigmented, multipolar nerve cells. It is semilunar on transverse section, its concavity being directed toward the tegmentum; from its convexity, prolongations extend between the fibers of the base of the peduncle. Thicker medially than laterally, it reaches from the oculomotor sulcus to the lateral sulcus, and extends from the upper surface of the pons to the subthalamic region; its medial part is traversed by the fibers of the oculomotor nerve as these stream forward to reach the oculomotor sulcus. The connections of the cells of the substantia nigra have not been definitely established. It receives collaterals from the medial lemniscus and the pyramidal bundles. Bechterew is of the opinion that the fibers from the motor area of the cerebral cortex form synapses with cells whose axons pass to the motor nucleus of the trigeminal nerve and serve for the coördination of the muscles of mastication. The tegmentum is continuous below with the reticular formation of the pons, and, like it, consists of longitudinal and transverse fibers, together with a considerable amount of gray substance. The principal gray masses of the tegmentum are the red nucleus and the interpeduncular ganglion; of its fibers the chief longitudinal tracts are the superior peduncle, the medial longitudinal fasciculus, and the lemniscus. GRAY SUBSTANCE.—The red nucleus is situated in the anterior part of the tegmentum, and is continued upward into the posterior part of the subthalamic region. In sections at the level of the superior colliculus it appears as a circular mass which is traversed by the fibers of the oculomotor nerve. It receives many terminals and collaterals from the superior cerebellar peduncle also collaterals from the ventral longitudinal bundle, from Gudden’s bundle and the median lemniscus. The axons of its larger cells cross the middle line and are continued downward into the lateral funiculus of the medulla spinalis as the rubrospinal tract (page 761); those of its smaller cells end mainly in the thalamus. The rubrospinal tract forms an important part of the pathway from the cerebellum to the lower motor centers. The interpeduncular ganglion is a median collection of nerve cells situated in the ventral part of the tegmentum. The fibers of the fasciculus retroflexus of Meynert, which have their origin in the cells of the ganglion habenulæ (page 812), end in it. Besides the two nuclei mentioned, there are small collections of cells which form the dorsal and ventral nuclei and the central nucleus or nucleus of the raphé. WHITE SUBSTANCE.—(1) The origin and course of the superior peduncle have already been described (page 792). (2) The medial (posterior) longitudinal fasciculus is continuous below with the proper fasciculi of the anterior and lateral funiculi of the medulla spinalis. In the medulla oblongata and pons it runs close to the middle line, near the floor of the fourth ventricle; in the mid-brain it is situated on the ventral aspect of the cerebral aqueduct, below the nuclei of the oculomotor and trochlear nerves. Its connections are imperfectly known, but it consists largely of ascending and descending intersegmental or association fibers, which connect the nuclei of the hind-brain and mid-brain to each other. Many of the fibers arise in Deiters’s nucleus (lateral vestibular nucleus) and divide into ascending and descending branches which send terminals and collaterals to the motor nuclei of the cranial and spinal nerves. Its spinal portion is located in the anterior funiculus and is known as the vestibulospinal fasciculus. Other fibers pass to the median longitudinal bundle from cells in the reticular formation of the medulla, pons and mid-brain and also from certain large cells in the terminal nucleus of the trigeminal nerve. According to Edinger it extends to the so-called nucleus of the posterior longitudinal bundle in the hypothalamic region, but this is uncertain and the fibers above the nucleus of the oculomotor are smaller in diameter than the rest of the bundle. According to Held fibers from the posterior commissure can be traced into the posterior longitudinal bundle, and according to the same author many of the descending fibers arise in the superior colliculus, and, after decussating in the middle line, end in the motor nuclei of the pons and medulla oblongata. These fibers from the superior colliculus probably pass into the ventral longitudinal bundle. Fibers are said to pass through the medial longitudinal fasciculus from the nucleus of the abducent nerve into the oculomotor nerve of the opposite side, and through this nerve to the Rectus medialis oculi. Fraser, however, denies the existence of such fibers. Again, fibers are said to be prolonged through this fasciculus from the nucleus of the oculomotor nerve into the facial nerve, and are distributed to the Orbicularis oculi, the Corrugator, and the Frontalis. (*124 The ventral longitudinal bundle consists for the most part of the tectospinal fasciculus, and arises from the superior colliculus, the fibers arch ventrally around the central gray matter and cross the midline in the fountain-decussation of Meynert. They then descend in the tegmentum, part of them passing through the red nucleus ventral to the medial longitudinal bundle. In the medulla oblongata and spinal cord its fibers are more or less intermingled with the medial longitudinal bundle and the rubrospinal tract. It descends in the adjoining region of the ventral and lateral funiculi. Collaterals and terminals are given off to the red nucleus and probably other nuclei of the brain stem and to the anterior column of the spinal cord. It is probably concerned in optic reflexes. (3) The medial lemniscus or medial fillet (Fig. 713).—The fibers of the medial lemniscus take origin in the gracile and cuneate nuclei of the medulla oblongata, and as internal arcuate fibers they cross to the opposite side in the sensory decussation (page 777). They then pass in the interolivary stratum upward through the medulla oblongata, in which they are situated behind the cerebrospinal fibers and between the olives. In the pons and lower part of the mid-brain it occupies the ventral part of the reticular formation and tegmentum close to the raphé, while above it gradually shifts to the dorso-lateral part of the tegmentum in the angle between the red nucleus and the substantia nigra. In the pons it assumes a flattened ribbon-like appearance, and is placed dorsal to the trapezium. As the lemniscus ascends, it receives additional fibers from the terminal sensory nuclei of the cranial nerves of the opposite side. Many of the fibers which arise from the terminal sensory nuclei of the cranial nerves pass upward in the formatio reticularis as a separate bundle, known as the central tract of the cranial nerves, to the thalamus. Many fibers either terminate in or send off collaterals to the gray matter of the medulla, the pons, and the mid-brain. Large numbers of fibers pass to or from the substantia nigra. Many collaterals enter the red nucleus and other fibers are said to run to the superior colliculus. The great bulk of the fibers, however, enter the ventro-lateral portion of the thalamus, give off collaterals to the posterior semilunar nucleus and then terminate in the principal sensory nucleus of the thalamus. 
FIG. 713 - Scheme showing the course of the fibers of the lemniscus; medial lemniscus in blue, lateral in red. In the cerebral peduncle, a few of its fibers pass upward in the lateral part of the base of the peduncle, on the dorsal aspect of the temporopontine fibers, and reach the lentiform nucleus and the insula. The greater part of the medial lemniscus, on the other hand, is prolonged through the tegmentum, and most of its fibers end in the thalamus; probably some are continued directly through the occipital part of the internal capsule to the cerebral cortex. From the cells of the thalamus a relay of fibers is prolonged to the cerebral cortex. The medial lemniscus may be considered as the upward continuation of the posterior funiculus of the spinal cord and to convey conscious impulses of muscle sense and tactile discrimination. The central or thalamic tract of the cranial nerves is closely associated with the medial lemniscus. The fibers of the spinothalamic fasciculi are continued from the spinal cord into this tract which passes upward in the reticular formation and the tegmentum to the thalamus along the dorsal side of the median lemniscus. It receives fibers from the opposite terminal sensory nuclei of the vagus, glossopharyngeal, facial, trigeminal and probably the vestibular nerves. Many of the secondary sensory fibers of the trigeminal cross the raphé from its terminal nucleus and pass upward to the thalamus by a more or less separate but closely associated pathway known as the central tract of the trigeminal nerve which also lies on the dorsal aspect of the lemniscus. These two tracts give off collaterals to the posterior semilunar nucleus of the thalamus and terminate in the anterior semilunar nucleus of the ventro-lateral region of the thalamus sending collaterals into the zona incerta. 
FIG. 714 - Transverse section passing through the sensory decussation. Schematic. (Testut.) 1. Anterior median fissure. 2. Posterior median sulcus. 3, 3’. Head and base of anterior column (in red). 4. Hypoglossal nerve. 5. Bases of posterior column. 6. Gracile nucleus. 7. Cuneate nucleus. 8, 8. Lemniscus. 9. Sensory decussation. 10. Cerebrospinal fasciculus. The fibers of the rubrospinal tract (bundle of Monakow) arise in the red nucleus, cross the midline in the decussation of Forel and pass downward in the formatio reticularis of the brainstem into the lateral funiculus of the spinal cord ventral to the crossed pyramidal tract. The lateral lemniscus (lemniscus lateralis) comes to the surface of the mid-brain along its lateral sulcus, and disappears under the inferior colliculus. It consists of fibers from the terminal nuclei of the cochlear division of the acoustic nerve, together with others from the superior olivary and trapezoid nuclei. Most of these fibers are crossed, but some are uncrossed. Many of them pass to the inferior colliculus of the same or opposite side, but others are prolonged to the thalamus, and thence through the occipital part of the internal capsule to the middle and superior temporal gyri. The corpora quadrigemina (Fig. 720) are four rounded eminences which form the dorsal part of the mid-brain. They are situated above and in front of the anterior medullary velum and superior peduncle, and below and behind the third ventricle and posterior commissure. They are covered by the splenium of the corpus callosum, and are partly overlapped on either side by the medial angle, or pulvinar, of the posterior end of the thalamus; on the lateral aspect, under cover of the pulvinar, is an oval eminence, named the medial geniculate body. The corpora quadrigemina are arranged in pairs (superior and inferior colliculi), and are separated from one another by a crucial sulcus. The longitudinal part of this sulcus expands superiorly to form a slight depression which supports the pineal body, a cone-like structure which projects backward from the thalamencephalon and partly obscures the superior colliculi. From the inferior end of the longitudinal sulcus, a white band, termed the frenulum veli, is prolonged downward to the anterior medullary velum; on either side of this band the trochlear nerve emerges, and passes forward on the lateral aspect of the cerebral peduncle to reach the base of the brain. The superior colliculi are larger and darker in color than the inferior, and are oval in shape. The inferior colliculi are hemispherical, and somewhat more prominent than the superior. The superior colliculi are associated with the sense of sight, the inferior with that of hearing. From the lateral aspect of each colliculus a white band, termed the brachium, is prolonged upward and forward. The superior brachium extends lateralward from the superior colliculus, and, passing between the pulvinar and medial geniculate body, is partly continued into an eminence called the lateral geniculate body, and partly into the optic tract. The inferior brachium passes forward and upward from the inferior colliculus and disappears under cover of the medial geniculate body. In close relationship with the corpora quadrigemina are the superior peduncles, which emerge from the upper and medial parts of the cerebellar hemispheres. They run upward and forward, and, passing under the inferior colliculi, enter the tegmenta as already described (page 792). Structure of the Corpora Quadrigemina.—The inferior colliculus (colliculus inferior; inferior quadrigeminal body; postgemina) consists of a compact nucleus of gray substance containing large and small multipolar nerve cells, and more or less completely surrounded by white fibers derived from the lateral lemniscus. Most of these fibers end in the gray nucleus of the same side, but some cross the middle line and end in that of the opposite side. From the cells of the gray nucleus, fibers are prolonged through the inferior brachium into the tegmentum of the cerebral peduncle, and are carried to the thalamus and the cortex of the temporal lobe; other fibers cross the middle line and end in the opposite colliculus. The superior colliculus (colliculus superior; superior quadrigeminal body; pregemina) is covered by a thin stratum (stratum zonale) of white fibers, the majority of which are derived from the optic tract. Beneath this is the stratum cinereum, a cap-like layer of gray substance, thicker in the center than at the circumference, and consisting of numerous small multipolar nerve cells, imbedded in a fine network of nerve fibers. Still deeper is the stratum opticum, containing large multipolar nerve cells, separated by numerous fine nerve fibers. Finally, there is the stratum lemnisci, consisting of fibers derived partly from the lemniscus and partly from the cells of the stratum opticum; interspersed among these fibers are many large multipolar nerve cells. The two last-named strata are sometimes termed the gray-white layers, from the fact that they consist of both gray and white substance. Of the afferent fibers which reach the superior colliculus, some are derived from the lemniscus, but the majority have their origins in the retina and are conveyed to it through the superior brachium; all of them end by arborizing around the cells of the gray substance. Of the efferent fibers, some cross the middle line to the opposite colliculus; many ascend through the superior brachium, and finally reach the cortex of the occipital lobe of the cerebrum; while others, after undergoing decussation (fountain decussation of Meynert) form the tectospinal fasciculus which descends through the formatio reticularis of the midbrain, pons, and medulla oblongata into the medulla spinalis, where it is found partly in the anterior funiculus and partly intermingled with the fibers of the rubrospinal tract. The corpora quadrigemina are larger in the lower animals than in man. In fishes, reptiles, and birds they are hollow, and only two in number (corpora bigemina); they represent the superior colliculi of mammals, and are frequently termed the optic lobes, because of their intimate connection with the optic tracts. The cerebral aqueduct (aqueductus cerebri; aqueduct of Sylvius) is a narrow canal, about 15 mm. long, situated between the corpora quadrigemina and tegmenta, and connecting the third with the fourth ventricle. Its shape, as seen in transverse section, varies at different levels, being T-shaped, triangular above, and oval in the middle; the central part is slightly dilated, and was named by Retzius the ventricle of the mid-brain. It is lined by ciliated columnar epithelium, and is surrounded by a layer of gray substance named the central gray stratum: this is continuous below with the gray substance in the rhomboid fossa, and above with that of the third ventricle. Dorsally, it is partly separated from the gray substance of the quadrigeminal bodies by the fibers of the lemniscus; ventral to it are the medial longitudinal fasciculus, and the formatio reticularis of the tegmentum. Scattered throughout the central gray stratum are numerous nerve cells of various sizes, interlaced, by a net-work of fine fibers. Besides these scattered cells it contains three groups which constitute the nuclei of the oculomotor and trochlear nerves, and the nucleus of the mesencephalic root of the trigeminal nerve. The nucleus of the trigeminal nerve extends along the entire length of the aqueduct, and occupies the lateral part of the gray stratum, while the nuclei of the oculomotor and trochlear nerves are situated in its ventral part. The nucleus of the oculomotor nerve is about 10 cm. long, and lies under the superior colliculus, beyond which, however, it extends for a short distance into the gray substance of the third ventricle. The nucleus of the trochlear nerve is small and nearly circular, and is on a level with a plane carried transversely through the upper part of the inferior colliculus.
Note 123. A band of fibers, the tractus peduncularis transversus, is sometimes seen emerging from in front of the superior colliculus; it passes around the ventral aspect of the peduncle about midway between the pons and the optic tract, and dips into the oculomotor sulcus. This band is a constant structure in many mammals, but is only present in about 30 per cent. of human brains. Since it undergoes atrophy after enucleation of the eyeballs, it may be considered as forming a path for visual sensations. [back] Note 124. A. Bruce and J. H. Harvey Pirrie, “On the Origin of the Facial Nerve,” Review of Neurology and Psychiatry, December, 1908, No. 12, vol. vi, produce weighty evidence against the view that the facial nerve derives fibers from the nucleus of the oculomotor nerve. [back]
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