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cells of Betz in the posterior part of the precentral cortex (p. 663) in the district immediately in front of the sulcus centralis. The fibres descend through the corona radiata into the posterior limb of the internal capsule. From this point the further course of the pyramidal tract has been traced, viz., through the central part of the basal region of the cerebral peduncle and pons, and the pyramid of the medulla oblongata. At the level of the foramen magnum it decussates in the manner already described, and enters the spinal medulla as the lateral cerebrospinal and anterior cerebro-spinal tracts. The fibres composing these end in connexion with the ventral or motor column of cells, from which the fibres of the anterior roots of the spinal nerves arise.
Similar fibres arise from the inferior part of the precentral area and proceed through the internal capsule and cerebral peduncle to all the motor nuclei upon the opposite side of the brain stem (fasciculi cerebronucleares). Hence the cerebral cortex of one hemisphere can control all the muscles of the opposite side of the body. The fronto-pontine strand is composed of fibres which arise as the axons of the cells in the cortex which covers the frontal region that lies in front of the precentral furrows. It descends in the anterior limb of the internal capsule, enters the medial part of the base of the cerebral peduncle, through which it gains the basilar part of the pons. In this its fibres end amongst the cells of the nuclei pontis, from which axons arise and establish relations with the cortex of the opposite cerebellar hemisphere.
The temporo-pontine tract consists of fibres which spring from the cells of that part of the cortex which covers the middle portions of the lower two temporal gyri. The temporo-pontine tract passes medially under the nucleus lentiformis, enters the retrolenticular part of the posterior limb of the internal capsule, and thus gains the lateral part of the cerebral peduncle. From this it descends into the basilar part of the pons, in which it ends in the nuclei pontis.
Cortico-striate and other Descending Fibres. From the fibres of the internal capsule numerous collateral branches are given off to the nucleus caudatus and nucleus lentiformis, and from these basal ganglia fibres arise which enter the cerebral peduncle as constituent elements of the great cerebro-spinal tract.
Some of the fibres from the corpus striatum, especially the nucleus lentiformis, as well as others descending from the frontal cortex, pass into the red nucleus (Fig. 571), which also receives afferent tracts from the tectum mesencephali and from the cerebellum: it emits an important efferent tract (fasciculus rubrospinalis), which crosses the median plane and descends in the brain stem and spinal medulla to the various motor nuclei (see Figs. 454 and 475).
THE SULCI AND GYRI OF THE CEREBRAL HEMISPHERES.
Fissura Cerebri Lateralis (O.T. Fissure of Sylvius). This is the most conspicuous furrow on the surface of the cerebral hemisphere. In reality it is formed, not as a furrow upon the surface of the hemisphere, but as a great fossa, the nargins of which develop into large lip-like folds that bulge over the fossa and meet to form the superficial pattern of the lateral fissure. It is composed of a short main stem, from the lateral extremity of which two or three branches or limbs radiate. The stem of the lateral fissure is placed on the inferior surface of the hemisphere. It begins at the substantia perforata anterior and passes laterally, forming a deep cleft between the temporal pole and the orbital surface of the frontal region. Appearing on the lateral surface of the hemisphere, the fissure immediately divides into two or three radiating rami. These are: (1) the ramus posterior; (2) the ramus anterior horizontalis; (3) the ramus anterior ascendens, of which the last is inconstant.
The posterior ramus is the longest and most constant of the three limbs. extends backwards, with a slight inclination upwards, on the lateral surface of the hemisphere for a distance which may vary from about two to three inches. It intervenes between the frontal and parietal regions, which lie above it, and the temporal region which lies below it; and it finally ends in the region subjacent to
the parietal tuberosity of the cranial wall by turning upwards into the parietal region in the form of an ascending terminal piece.
The anterior horizontal ramus extends horizontally forwards in the frontal region for a distance of not more, as a rule, than three-quarters of an inch, immediately above and parallel to the posterior part of the superciliary margin of the hemisphere.
The anterior ascending ramus proceeds upwards and slightly forwards, into the inferior part of the lateral surface of the frontal region for a variable distance (an inch or less). In many cases the two anterior limbs spring from a common stem of greater or less length, and not infrequently there is only a single anterior limb. Sulcus Circularis.-If the lips of the posterior ramus of the lateral fissure are pulled widely asunder from each other, the insula (island of Reil) will be seen at the
Intermediate part of inferior frontal gyrus Gyrus frontalis superior (anterior part) Anterior ascending ramus of lateral fissure
ramus of lateral
Fissura orbitalis (transverse limb)
Pars intermedia (superior temporal gyrus)
FIG. 581.-A DIAGRAM OF THE LATERAL ASPECT OF THE LEFT CEREBRAL HEMISPHERE.
The inferior frontal sulcus (the superior boundary of the inferior frontal gyrus), the middle frontal sulcus (separating the anterior and middle frontal areas), and the superior frontal sulcus (bounding the superior frontal gyrus) are not labelled.
bottom. The insular district of the cortex is completely hidden from view, when the lateral fissure is closed, by overlapping portions of the cerebral hemisphere, and, when brought into view in the manner indicated, it is observed to present a triangular outline and to be surrounded by a limiting sulcus, of which three parts may be recognised, viz., a superior part, bounding it above and separating it from the parietal and frontal regions; an inferior part, marking it off below from the temporal region; and an anterior part, separating it in front from the frontal region.
The insula consists of three areas of different structure. At the antero-inferior corner (where the sulcus circularis is deficient) the knee-like bend of the area piriformis (see Figs. 582 and 584) appears at the limen insulæ. The rest is subdivided by an oblique furrow (sulcus centralis insula) into a posterior part divided into gyri longi and an anterior part divided into gyri breves,
Opercula Insulæ.-The overlapping portions of the cerebral substance which cover over the insula are termed the insular opercula, and they form, by the apposition of their margins, the three rami of the lateral fissure. The rami of the fissure extend from the exposed surface of the hemisphere to the submerged surface of the insula, and, in this manner, separate the opercula from each other.
The temporal operculum (pars temporalis) extends upwards over the insula from the temporal region, and its superior margin forms the inferior lip of the posterior ramus of the lateral fissure.
The superior operculum is carried downwards from the parietal (pars parietalis) and frontal (pars frontalis) regions over the insula, and its inferior margin, meeting the temporal operculum, forms the superior lip of the posterior ramus of the lateral fissure.
The small triangular piece of cerebral substance which sometimes intervenes between the ascending and horizontal anterior rami of the lateral fissure is formed by the bending downwards of the front part of the upper operculum. It
FIG. 582.-PART OF A LEFT CEREBRAL HEMISPHERE WITH THE OPERCULA OF THE INSULA WIDELY
covers over a small part of the anterior portion of the insula, and is sometimes termed the pars triangularis.
The orbital operculum is, for the most part, on the inferior surface of the hemisphere. It lies below and to the medial side of the horizontal anterior ramus of the lateral fissure, and proceeds backwards from the orbital aspect of the frontal lobe over the anterior part of the insula.
Development of the Lateral Fissure and of the Insular District of the Cerebral Hemisphere. It is only during the latter half of the intra-uterine period of development that the opercula take shape and grow over the insula, so as to shut it out from the surface. In its early condition the insula presents the form of a depressed area on the side of the cerebral hemisphere, surrounded by a distinct boundary wall formed by the surrounding more elevated surface of the hemisphere (Fig. 583, A). After a time this depressed area, which is called the fossa lateralis, assumes a triangular outline, and then the bounding wall is observed to be composed of three distinct parts, viz., a superior or fronto-parietal, an inferior or temporal, and an anterior or orbital part (Fig. 583, B). The angle formed by the meeting of the superior and anterior portions of the boundary may become flattened, and a short oblique part of the limiting wall develop into a small triangular frontal operculum (Fig. 583, F). Each of these portions of the bounding wall of the fossa becomes a line of growth, from which an operculum
takes origin, and by the approximation of these opercula, as they grow over the surface of the fossa, the insula becomes closed in and the rami of the lateral fissure are formed (Fig. 583, C).
The lateral fissure is an example of the fourth category of furrows enumerated above. It is largely the result of the operation of the mechanical factors incidental to the bending downwards of the pallium in front of and behind the place where the hemisphere-wall is supported and held in position by the corpus striatum. The cortical area roughly corresponding to the surface of the corpus striatum is the insula; the temporal region extends downwards behind it, and to a less extent
FIG. 583.-RIGHT HEMISPHERES OF HUMAN FETUSES SHOWING THREE STAGES
the frontal region in front of it (Fig. 583, A). Then towards the end of the fifth month of foetal life the exuberant growth of the free frontoparietal pallium above the insula (Fig. 583, B) and the temporal pallium below and behind it leads to
A, Right cerebral hemisphere from a foetus in the latter part of the fourth month the development of of development; B, Right cerebral hemisphere from a fœtus in the fifth lip-like folds of neomonth of development; C, Right cerebral hemisphere from a foetus in the pallium-the operlatter part of the eighth month of development. In C the temporal operculum has been removed, and thus a large part of the cula-which graduinsula is exposed. The outline of the temporal operculum is indicated by a ally approach one
F.P, Superior operculum. F, Frontal operculum. O, Orbital operculum. another (Fig. 583,C) and eventually cover up the insula. Other factors come into play in determining the form and topographical relations of the fissura lateralis. For example, the posterior part of the fissure is the morphological boundary between the acoustic and tactile territories of the neopallium.
THE ACOUSTIC AREA AND FIBRE-TRACTS.
In the embryo of the fifth month (Fig. 583, B), as well as in every later stage, even up to the adult condition (Fig. 582), an area upon the superior surface of the temporal operculum can be seen to slope medially towards the upper limb of the sulcus circularis, behind the insula. This area constitutes the receptive centre for acoustic impressions-the gyrus temporalis transversus or Heschl's convolution -although the extent of this acoustico-sensory area does not coincide exactly with that of the transverse temporal gyrus. The area formed by the upper surface of the temporal operculum immediately behind this prominent transverse gyrus is also called by the same name, so that there are anterior and posterior transverse temporal gyri (Fig. 582; the posterior transverse temporal gyrus is not labelled in the figure).
In studying the brain-stem we have seen that a tract of fibres originating in the cochlear terminal nuclei (in the medulla oblongata) crosses the median plane (corpus trapezoideum) and bends upwards in the lateral lemniscus of the other side (Fig. 580) to end in the medial geniculate body of the metathalamus. From the medial geniculate body a new tract arises (composed of tertiary acoustic neurones), which passes laterally (Figs. 580 and 584) to end in the transverse temporal gyri. This tract may be called the radiatio thalamotemporalis.
The area into which this acoustic radiation is inserted occupies not only the region of the anterior transverse temporal gyrus (Fig. 582) hidden within the lateral fissure, but also extends over its inferior lip, on to the exposed surface of the superior temporal gyrus (Fig. 581). Surrounding this area there are two concentric bands,
which are also concerned with acoustic functions, but are related to the acoustic radiation only through the intermediation of the area acustica of the transverse gyrus (Figs. 581 and 582).
These areas may be distinguished as the pars intermedia and pars circumambiens, respectively, of the gyrus temporalis superior. During the sixth month of foetal life a furrow makes its appearance along the line of the inferior boundary of the superior temporal area (Fig. 590). It is called the sulcus temporalis superior.
At a much later stage of development another furrow (sulcus temporalis anterior) makes its appearance further forwards in the temporal region, as the posterior boundary of the area temporalis polaris; it often becomes confluent with
FIG. 584.-INFERIOR ASPECT OF PART OF THE BRAIN. The mesencephalon has been cut across and a great part of the cerebral hemisphere dissected away to expose the acoustic radiation (right side of figure in yellow) passing laterally from the medial geniculate body to the deep surface of the transverse temporal gyri, of which a small part is shown in section. Motor fibres in red; optic fibres, blue: olfactory, dull yellow.
the real sulcus temporalis superior, and is usually described as part of it. But it is genetically quite distinct from it (Fig. 581).
If the area acustica is cut across in a perfectly fresh brain it will be found to be composed of a thin layer (175 mm.) of cortical gray matter, in which two very dense and fairly broad bands of white matter are visible (Fig. 584). These bands are composed largely of fibres of the acoustic radiation, which have entered the cortex to terminate in it. The superior temporal area is composed of somewhat thicker cortex with two bands which are not so densely white as those of the area acustica. The cortex of the temporal polar area is composed of moderately thick, clear, gray matter in which there is a single, narrow, sharply defined white