Part 32

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.

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_).

The =third ventricle= (Fig. 143, _h_; Fig. 141, _v_) is a very narrow slit-like space, of considerable extent dorsoventrally, but less than a millimeter in width; it lies between the medial ends of the thalami (Fig. 141), and extends ventrad into the tuber cinereum, and thence into the infundibulum (Fig. 143, _g′_). Dorsad it extends into the corpus pineale (Fig. 143, _y_). The third ventricle communicates caudally near its dorsal border with the aqueductus cerebri (_j_); craniolaterally with the lateral ventricles, through the =interventricular foramen= (foramen of Monroe). Its cranial boundary forms in the middle line a thin plate, the =lamina terminalis= (_d_), which is, morphologically, the cranial termination of the cerebrospinal axis; it lies at the bottom of the deep fissure between the cerebral hemispheres. At the dorsal border of the lamina terminalis is a strong transverse band of fibres connecting the two sides of the brain; this is the =anterior commissure= (_c_). The cavity of the third ventricle is much encroached upon by the meeting and secondary union of the two thalami across the middle line, forming the massa intermedia (_f_).

The boundaries of the third ventricle, in order, are as follows, beginning dorsocraniad of the communication with the aqueductus cerebri: the posterior commissure (Fig. 143, _z′_), the pineal body (_y_), the choroid plexus of the third ventricle (_w_), the columns of the fornix (_v_), the anterior commissure (_c_), the lamina terminalis (_d_), the tuber cinereum with the infundibulum (_g′_), the substantia perforata posterior, and the midbrain. The lateral boundaries are formed by the thalami (Fig. 141, _t_).

5. _Telencephalon._--The telencephalon includes the two cerebral hemispheres. The name =cerebrum= is also applied to this portion of the brain; frequently, however, the name cerebrum is used as signifying the entire mass of the brain craniad of the rhombencephalon,--therefore including mesencephalon, diencephalon, and telencephalon. The term will here be used as synonymous with telencephalon.

The =cerebral hemispheres= arise as two lateral outgrowths from the cranial end of the primitive forebrain. They have undergone great increase in size in the course of evolution, so as to form the larger part of the brain; at the same time important modifications of structure have taken place. In the original condition the medial faces of the two hemispheres are not connected, the two being separate outgrowths of the ’tween-brain, and connected only through the latter. But secondary connections have been formed across the fissure between the two hemispheres, resulting in the production of the =corpus callosum= (Fig. 143, _p_; Fig. 147; Figs. 149-152, _a_), a broad transverse band of white fibres connecting the two hemispheres dorsad of the ’tween-brain. Ventrad of the corpus callosum another secondary union has resulted in the production of the =fornix= (Fig. 143, _u_).

=External Features.=--With increase in size the mass of the cerebrum shows externally a tendency to divide into three lobes, one craniad, the =frontal= (Fig. 145, _A_); one caudoventrad, the =temporal= (_B_); and one caudodorsad, the =occipital= (_C_). The two latter are not distinctly marked off from one another. The limit between the temporal and frontal lobes is marked by a short deep fissure, the =lateral fissure= (=fissura cerebri lateralis=), or fissure of Sylvius (Fig. 144, 9; Fig. 145, _a_). Each lobe is thrown up into elevations or =gyri=, which are separated by grooves or =sulci=; these are described below. The homology of the cerebral gyri and sulci of the cat with those of man is in most cases uncertain.

The sulci and gyri of the cerebral hemispheres may be described briefly in their main features as follows: On the lateral surface of the hemisphere (Fig. 145) the lateral fissure (fissura cerebri lateralis), or fissure of Sylvius, separating temporal and frontal lobes, forms the most convenient point of departure for an understanding of the fissures. The fissure of Sylvius is short but deep, and is formed early during development. In man it covers an area, the =insula=, at its bottom, which may be seen by separating the sides of the fissure. In the cat the insula is rudimentary and can be demonstrated only with difficulty.

The lateral surface of the hemisphere is marked by fissures which form three concentric arches (_b_-_c_, _d_, and _e_), irregular and incomplete, about the fissure of Sylvius. These arches all open ventrad. The first arch (_b_-_c_) (that next to the fissure of Sylvius) lacks the central part, the keystone, so that a complete arch is not formed, but only the two sides of the arch, as two separate sulci (_b_ and _c_). The one of these craniad of the fissure of Sylvius is the =sulcus anterior= (_b_); the other is the =sulcus posterior= (_c_). In the dog this arch is usually complete and the sulcus forming it is frequently known as the ectosylvian sulcus (=sulcus ectosylvius=).

The gyri included between the fissure of Sylvius on the one hand and the anterior and posterior sulci on the other are known as the =anterior= (1) and =posterior= (2) =Sylvian gyri=.

The second arch is formed by the =suprasylvian sulcus= (=sulcus suprasylvius=) (_d_). The gyrus between the anterior and posterior sulci ventrad and the suprasylvian sulcus dorsad is the =ectosylvian gyrus= (3) (because dorsad of the ectosylvian sulcus).

The third arched sulcus, next to the medial margin of the hemisphere, is variable: it forms the =lateral sulcus= (_e_). The gyrus lying between the suprasylvian and lateral sulci is the =suprasylvian gyrus= (4) (because dorsad of the suprasylvian sulcus). The lateral sulcus is sometimes broken into two (as in Fig. 137, _h_), the two ends overlapping so as to leave a small gyrus between the two ends.

The gyrus dorsomediad of the lateral sulcus, extending onto the medial surface of the hemisphere as far as the splenial sulcus, is the =marginal gyrus= (5).

The four sets of gyri included between these three sets of arched sulci and the fissure of Sylvius unite caudad and craniad to form single gyri. The caudal one is the =gyrus compositus posterior= (6). The cranial one (less regular than the caudal one) is the =sigmoid gyrus= (7). The latter curves about the cruciate sulcus (_f_) (described below).

In the cranial part of the lateral surface of the hemisphere are certain sulci and gyri which do not belong to the system above described. The =cruciate sulcus= (_f_) is a short transverse sulcus passing from the lateral surface of the hemisphere onto its medial surface, where it extends caudad about a centimeter. Curving around the lateral end of this, separated from it by part of the sigmoid gyrus, is the short =coronal sulcus= (_g_). Connected usually with the end of the lateral sulcus (_e_) and running nearly parallel with the cruciate sulcus is the short =sulcus ansatus= (_h_).

At its ventral end the fissure of Sylvius (_a_) joins a longitudinal groove passing craniad and caudad from the point of junction. That part which passes craniad is the =sulcus rhinalis= (_i_); it forms the dorsolateral boundary of the olfactory tract (8). Passing dorsocraniad from the sulcus rhinalis is a sulcus which separates a very small cranial lobe from the rest of the hemisphere; this is the =supraorbital= or =præsylvian sulcus= (_j_). The portion of the hemisphere craniad of this sulcus is the =orbital gyrus= (9).

The sulcus rhinalis (_i_) is continuous caudad with the =sulcus rhinalis posterior= (or =sulcus postrhinalis=) (_k_). This extends caudad onto that portion of the hemisphere which faces the cerebellum. It forms the lateral boundary of a large elongated oval lobe lying at the side of the ventral floor of the midbrain and ’tween-brain. This lobe has been called =tractus postrhinalis= or =lobus pyriformis= (Fig. 145, 10; Fig. 138, _f_).

In addition to the sulci and gyri above mentioned, any given specimen will usually show a number of small inconstant sulci and gyri in various regions; these inconstant structures will not be here described.

On the medial surface of the hemisphere (Fig. 146), the following arrangement of sulci and gyri seems to be typical. Some distance from the dorsal margin a long sulcus runs parallel with the margin; this is the =sulcus splenialis= (_a_). The =marginal gyrus= (1) is dorsad of the splenial sulcus, passing onto the lateral surface of the hemisphere, where it is bounded by the lateral sulcus. The splenial sulcus extends onto the caudal surface of the hemisphere. A shallow =sulcus marginalis= (_b_) occurs frequently between the sulcus splenialis and the dorsal margin of the hemisphere. Cranioventrad or the cranial end of the splenial sulcus is that portion of the =sulcus cruciatus= (_c_) that lies on the medial surface of the hemisphere. Ventrad of this is a short shallow furrow which has been called the =sulcus falcialis= (_d_).

Immediately dorsad of the corpus callosum (_h_), separating its dorsal surface from a part of the hemisphere, is the =supracallosal= or =callosal= sulcus (_e_). Between the supracallosal (_e_) and splenial (_a_) sulci is the =gyrus fornicatus= (2).

On the caudal surface of the hemisphere (that facing the cerebellum) appear the ends of the sulcus splenialis (_a_) and the sulcus rhinalis posterior (_f_). Hidden in the natural condition by the thalami and optic tract is the broad =hippocampal= sulcus (_g_), marking externally the course of the hippocampus.

A further extension of the surface of the cerebrum has taken place in connection with the sense of smell. A mass of gray matter, the =olfactory bulb= (Fig. 144, _a_), reckoned as a part of the cerebrum, is separated from the latter and lies against the cribriform plate of the ethmoid bone. From it the olfactory fibres pass through the perforations of the plate to the olfactory mucous membrane. The olfactory bulb contains a cavity, a part of the lateral ventricle.

The bulb lies against the ventral surface of the frontal lobe and projects craniad of it. It is connected to the cerebrum by a tract of fibres, the =olfactory tract= (Fig. 138, _a_), which is divisible into two roots, medial and lateral. The medial root comes from the medial surface of the frontal lobe, where it is continuous with a tract extending to the cranial end of the corpus callosum. The lateral root is traceable from an elevated gyrus-like portion of the cerebrum which lies at the side of the infundibulum and is known as the lobus pyriformis or tractus postrhinalis (Fig. 138, _f_). The lateral root is divisible into a medial white strand and a lateral gray strand.

That part of the brain comprising the olfactory bulb and the parts intimately related to it are frequently included under the term =rhinencephalon=.

In the triangular area between the two olfactory tracts and craniad of the optic chiasma appears a mass of gray matter, subdivided by a longitudinal fissure. This possesses numerous openings through which blood-vessels pass to the brain substance, and is thence known as the =anterior perforated substance= (=substantia perforata anterior=) (Fig. 138, _b_).