Part 77

The side walls of the fourth ventricle (Fig. 141) are formed by the following in order, beginning caudad: the =clava= (d), the =corpus restiforme=, =brachium pontis= (l), =brachium conjunctivum= (k), and =caudal corpora quadrigemina= (p) (=colliculi inferiores=).

The roof (Fig. 143) is formed caudad by a thin layer of non-nervous matter which is closely associated with the pia mater. This thin layer is known as the =velum medullare posterius= (n). It connects the dorsal surface of the medulla with the caudal border of the cerebellum. The pia mater covering this portion of the roof is vascular and is folded in toward the floor of the ventricle, forming the =choroid plexus= of the fourth ventricle. In the middle the roof of the fourth ventricle is the cerebellum, while craniad the roof is the =velum medullare anterius= (Fig. 143, l). This is a thin layer just craniad of the cerebellum, connecting it with the corpora quadrigemina, and attached laterally to the brachia conjunctiva. Here the fourth ventricle narrows craniad and becomes continuous with the slender =aqueductus cerebri= (Fig. 143, j) (aqueduct of Sylvius). The narrowed portion of the brain is frequently known as the =isthmus rhombencephali=.

The cerebellum and the greater part of the cerebral hemispheres have been removed. a, posterior sulcus of cord; b, posterior lateral sulcus; c, fasciculus gracilis; d, clava; e, fasciculus cuneatus; f, area ovalis; g, fasciculus cuneatus lateralis; h, fossa rhomboidea or floor of fourth ventricle; i, projection formed by origin of auditory nerve; j, facial nerve; k, cut end of brachium conjunctivum; l, cut end of brachium pontis (l′, brachium pontis); m, velum medullare anterius; n, fourth nerve; o, depression marking caudal end of aqueductus cerebri (aqueduct of Sylvius); p, caudal corpora quadrigemina (colliculi posteriores); q, cranial corpora quadrigemina (colliculi anteriores); r, brachium quadrigeminum inferius; s, corpus geniculatum mediale; t, thalamus; u, striæ medullares; v, third ventricle; w, pulvinar; x, corpus geniculatum laterale; y, corpus striatum; z, outlines of olfactory bulbs. 1, boundary between hemispheres and ’tween-brain; 2, pineal body.]

The fourth cranial nerve (Fig. 141, n; Fig. 138, IV) (=N. trochlearis=) arises from the brain at the craniolateral angle of the velum medullare anterius.

B. MESENCEPHALON.

3. Mesencephalon.--The mesencephalon or midbrain includes the =corpora quadrigemina= (Fig. 141) and the =pedunculi cerebri= (Fig. 142). In the primary midbrain there is a pronounced thickening of the walls accompanied by a reduction of the central canal. The midbrain does not thus become very large and is concealed in the dorsal view by the cerebellum and cerebrum, though its floor appears in the ventral view just craniad of the pons (Fig. 138, g). Its narrow canal is the =aqueductus cerebri= (=aqueduct of Sylvius=) (Fig. 143, j). Its roof forms the =corpora quadrigemina=, and its floor the =pedunculi cerebri=.

In a dorsal view (Fig. 141) the roof is seen to be marked by two pairs of elevations, the =corpora quadrigemina= (p and q). The cranial pair (q) (known as the colliculi superiores) are circular in outline, surrounded on all sides except the cranial one by a deep groove. From the cranial side a tract of fibres (=brachium quadrigeminum superius=, or arm of the cranial corpus) extends craniad and disappears beneath the thalamus (t). Between the anterior or cranial corpora quadrigemina lies the =pineal body= or epiphysis (corpus pineale) (Fig. 143, y; Fig. 141, 2), a portion of the roof of the ’tween-brain. The caudal corpora (Fig. 141, p) are larger than the cranial ones, and ovoid in shape with the long axis vertical. They are united in the median line, and the velum medullare anterius (m) stretches between their caudal borders. The brachium of the caudal corpus quadrigeminum (brachium quadrigeminum inferius) (r) extends craniad and disappears beneath a considerable elevation, the =corpus geniculatum mediale= (s).

Crossing this brachium is seen a small tract of fibres which extends ventrad, crosses the pedunculus cerebri, and reaches the medial border of the latter. It is the =tractus transversus peduncularis= (Fig. 142, b). Ventrad of the caudal corpus quadrigeminum is seen a triangular area of oblique fibres which corresponds in position to the human lemniscus.

The =pedunculi cerebri= (peduncles of the cerebrum) form the ventral part of the midbrain. They appear in a ventral view of the entire brain (Fig. 138) as two broad tracts of fibres (g) emerging from beneath the pons and diverging from one another as they pass craniad, finally disappearing beneath the cerebral hemispheres. Each is made up of many fibre-bundles, which are apparent in surface view (Fig. 142). The peduncles are separated by a small triangular space, which is marked by a median longitudinal sulcus. In this space, just caudad of the mammillary bodies, is a small area through which a number of blood-vessels pass into the brain. This is known as the =posterior perforated area= (or substance) (Fig. 142, j). The cerebral peduncles (a) are crossed by the tractus transversus peduncularis (b) (see above).

The third cranial nerve (Fig. 138, III) (=N. oculomotorius=) leaves the brain at the medial border of the cerebral peduncle (g), just caudad of the tractus transversus peduncularis.

The =aqueductus cerebri= (Fig. 143, j; Fig. 153, d) (or aqueduct of Sylvius) is the continuation craniad of the fourth ventricle. It is a narrow passage, one or two millimeters in diameter, lying dorsad of the pedunculi cerebri and ventrad of the corpora quadrigemina.

a, pedunculi cerebri; b, tractus transversus peduncularis; c, corpus geniculatum mediale; d, optic tract; e, optic chiasma; f, optic nerve; g, mammillary bodies; h, tuber cinereum; i, opening for infundibulum (which has been removed); j, posterior perforated area; k, pons. III, V, VII, VIII, third, fifth, seventh, and eighth cranial nerves.]

C. PROSENCEPHALON.--The prosencephalon or primitive forebrain includes the =diencephalon= or ’tween-brain and the =telencephalon= or cerebral hemispheres.

4. Diencephalon.--The diencephalon or ’tween-brain includes the =thalami= and the other parts bounding the third ventricle. The diencephalon is seen in entire brains only in ventral view (Fig. 138, c, d, e, etc.)

The diencephalon may be considered as forming almost or quite the most cranial portion of the median nervous tube,--the cranial wall of the third ventricle (the =lamina terminalis=) (Fig. 143, d), ending in the median line in the deep fissure between the hemispheres of the cerebrum. Parts of the brain which extend farther craniad than this are lateral portions, due to the forward growth of the lateral hemispheres. The two hemispheres of the cerebrum may be considered as lateral outgrowths of the central ’tween-brain; these outgrowths have extended dorsad, laterad, craniad, and caudad, so as to cover almost completely the ’tween-brain.

In early stages the cerebral hemispheres are projections from the cranial end of the ’tween-brain, so that the plane of junction was nearly transverse, the cranial end of the ’tween-brain joining the caudal end of the hemispheres. With the increasing size and backward growth of the latter, the attachment to the ’tween-brain has been shifted from a cranial to nearly a lateral position, and at the same time the originally lateral surface of the ’tween-brain has become nearly caudal. This is shown in Fig. 141, the deep fissure at 1 marking the line of attachment between the ’tween-brain and the hemispheres. The dorsoventral plane of junction of ’tween-brain and hemispheres is (as Fig. 141 shows) not wholly lateral, but oblique, passing from its cranial end near the middle line caudolaterad.

A second peculiarity of the ’tween-brain lies in the thinness of its roof. The roof is exceedingly thin and is so intimately connected with the pia mater that they cannot be removed separately. The ventral thick floor of the ’tween-brain is directly continuous with the similar floor of the cerebrum; but where the roof of the ’tween-brain joins the roof of the cerebrum along the oblique plane already indicated, the roof is very thin and is intimately connected to the pia mater, and is at the same time folded into the lateral ventricles to form, together with the pia mater, the =choroid plexuses= of the lateral ventricles (Fig. 148, e). When the pia mater is removed the thin roof of the brain along the line of junction of the ’tween-brain and the cerebrum is brought away and there appears to be a direct communication between lateral ventricles and the exterior.

We may now take up the parts of the diencephalon in detail.

a. =Thalamus= (Fig. 141, t).--The two thalami are large oblique rounded ridges, forming the sides of the ’tween-brain. They lie just craniad of the cranial corpora quadrigemina (q), but separated from them by a broad groove, and are completely covered by the caudally projecting part of the cerebral hemispheres. The medial larger end of each thalamus is near the middle line; thence the thalamus extends caudolaterad, and rises at its lateral extremity into a sharply rounded projection, the =corpus geniculatum laterale= (x). From this projection the thalamus is continued ventrad and then craniomediad as a large band of fibres, the =optic tract= (=tractus opticus=) (Fig. 142, d), which extends to the optic chiasma (e), where it passes into the optic nerves (f). On the caudal border of the thalamus, near the median line, is a very faint projection, the =pulvinar= (Fig. 141, w); this lies just craniad of the lateral border of the cranial corpus quadrigeminum (q). Just ventrad of the corpus geniculatum laterale (x) is the prominent rounded =corpus geniculatum mediale= (s); this is connected by a prominent ridge, the =brachium quadrigeminum inferius= (r), with the caudal corpus quadrigeminum (p). In a similar manner the =brachium quadrigeminum superius= passes from the cranial corpus quadrigeminum (q) into the thalamus itself.

Between the two thalami there exists a groove, the =sulcus hypothalamicus=. Over this groove lies the roof of the third ventricle, forming the =choroid plexus= of the third ventricle (Fig. 143, w). The medial surface of the two thalami are flat and extend directly ventrad, forming part of the lateral boundary of the third ventricle (Fig. 143, h). The two medial surfaces meet over a considerable area across the narrow cavity of the third ventricle, and unite, forming the =massa intermedia= (Fig. 143, f) or intermediate mass of the thalamus (“middle commissure”). This connection of the thalami of the two sides is thus not a primitive one, forming no part of the roof or floor of the central cavity, but is a secondary connection due to a growing together of a part of the two sides of the ventricle across its cavity. Along the dorsal edge of the medial border of each thalamus passes a distinct white strand, the =stria medullaris= (Fig. 141, u); the two striæ meet in an arch caudad, lying beneath the pineal body (2).

The thalamus is separated craniolaterad by a groove (1) from the corpus striatum (y), on the floor of the cerebral hemisphere.

b. The =roof= of the third ventricle is thin and united with the pia mater, as already stated. The pia mater bears many blood-vessels, and the two are folded into the groove between the optic thalami, forming the =lamina chorioidea epithelialis=, or choroid plexus of the third ventricle (Fig. 143, w). The roof is attached to the dorsomedial borders of the thalami and becomes continuous with the choroid plexus of the lateral ventricles (Fig. 148, e) at the craniolateral borders of the thalami.

The =pineal body= (=corpus pineale=) or epiphysis (Fig. 141, 2; Fig. 143, y) is a small conical body which is formed as an outgrowth of the caudal part of the roof of the third ventricle. It lies on the roof of the brain between the two cranial corpora quadrigemina (Fig. 141, q). It is hollow, containing an extension of the third ventricle. From its craniolateral angles two white strands, the =habenulæ=, extend into the striæ medullares (Fig. 141, u) of the thalami.

Just ventrad of the pineal body is a transverse band of white fibres, lying in the caudal part of the roof of the third ventricle. This interconnects the two thalami, and forms the =posterior commissure= (Fig. 143, z′) (commissura posterior). From this commissure a thin sheet of tissue extends to the pineal body.

c. The =floor= of the third ventricle appears in a ventral view of the brain (Fig. 138, Fig. 142) as a somewhat diamond-shaped space craniad of the pedunculi cerebri (Fig. 142, a) and bounded along its cranial margin by the =optic tracts= (Fig. 142, d). The optic tracts come from the thalami, as already described; they converge and unite to form the optic chiasma (Fig. 142, e), from which the optic nerves (f) diverge.

Immediately caudad of the optic chiasma lies a considerable rounded gray elevation, the =tuber cinereum= (Fig. 142, h). This bears on its ventral surface in the natural condition the =infundibulum= (Fig. 138, d) with the =hypophysis= (Fig. 138, e); in cases where the two latter structures have been removed (Fig. 142) the tuber cinereum (h) bears a small longitudinal opening (i) for attachment of the infundibulum. The infundibulum (Fig. 138, d) is a hollow extension of the floor of the third ventricle, and is attached to the middle of the ventral surface of the tuber cinereum. It bears at its ventral end the hypophysis (pituitary body) (Fig. 138, e), a vascular non-nervous body of unknown function. The hypophysis is lodged in the sella turcica of the sphenoid bone.

At its caudal border the tuber cinereum bears two white elevations, the =mammillary bodies= (corpora mammillaria) (Fig. 142, g).

I, olfactory bulb; II, cerebrum; III, section of cerebellum, showing “arbor vitæ”; IV, medulla oblongata. a, tractus olfactorius; b, substantia perforata anterior; c, anterior commissure; d, lamina terminalis; e, optic chiasma; f, massa intermedia of thalamus; g′, infundibulum; g, hypophysis; h, third ventricle; i, mammillary body; j, aqueductus cerebri (or aqueduct of Sylvius); k, pons; l, velum medullare anterius; m, fourth ventricle; n, velum medullare posterius; o, continuation of canal of spinal cord; p, corpus callosum (body); q, genu of corpus callosum; r, rostrum; s, splenium; t, septum pellucidum; u, fornix; v, pillars of fornix; w, choroid plexus of third ventricle; x, stria medullaris; y, corpus pineale; z, cranial corpus quadrigeminum; z′, posterior commissure. 1, sulcus falcialis; 2, sulcus cruciatus; 3, sulcus splenialis; 4, sulcus marginalis; 5, sulcus supracallosalis (or callosalis).]