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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 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 fœtus in the latter part of the fourth month the development of of development; B, Right cerebral hemisphere from a foetus in the fifth lip-like folds of neomonth of development; C, Right cerebral hemisphere from a foetus in the pallium-the oper
latter 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 (1.75 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 line.
the hippocampal gyrus, to the uncus and the temporal pole. The cingulum is composed of several systems of fibres which run only for short distances within it.
The fasciculus longitudinalis superior is an arcuate bundle which is placed on the lateral aspect of the foot or basal part of the corona radiata and connects the frontal, occipital, and temporal regions of the hemisphere. It lies in the base of the superior operculum and sweeps backwards over the insular region to the posterior end of the lateral cerebral fissure. Here it bends downwards round the posterior end of the putamen and proceeds forwards in the temporal lobe, to reach its anterior extremity. As it turns downwards to reach the temporal lobe numerous fibres radiate from it into the occipital lobe.
The fasciculus longitudinalis inferior is a very conspicuous bundle which extends along the whole length of the occipital and temporal regions (Fig. 577, B). Curran has recently demonstrated that the fasciculus uncinatus and the inferior longi
Optic radiation seen in a gap cut out of
the inferior occipito-frontal fasciculus
FIG. 578.-DISSECTION TO DISPLAY SOME OF THE PRINCIPAL ASSOCIATION BUNDLES OF THE CEREBRAL
The occipito-temporal extremity of the superior longitudinal bundle has been cut away in order to expose the subjacent inferior occipito-frontal bundle, parts of which in turn have been removed to expose the origin and termination of the still deeper optic radiation (coloured blue); (acoustic fibres, yellow).
tudinal bundle are merely the shorter inferior fibres of a much bigger and longer tract (Fig. 578), to which he has applied the name occipito-frontalis inferior. The arrangement of these longitudinal tracts may be put concisely by saying that fibre connexions of differing lengths link together the various cortical areas in the longitudinal direction, the deeper fibres (i.e. those furthest removed from the cortex, medial, lateral, superior or inferior) being progressively longer than the superficial. The deepest fibres extend the whole length of the hemisphere and are pushed aside by the insula (Fig. 578) and collected into two great bundles, a superior longitudinal and an inferior occipito-frontal bundle. In the occipital lobe the inferior occipito-frontal bundle is placed on the lateral aspect of the optic radiation, which takes a similar direction and from which it is distinguished by the greater coarseness of its fibres (Figs. 576, p. 649; 578; 559, p. 631). It is not present in the macaque monkey (Ferrier and Turner), but is well developed in the orang and the chimpanzee.
The fasciculus occipito-frontalis superior is a bundle of fibres which runs in a sagittal direction in intimate relation to the lateral ventricle (Fig. 576, p. 649). It may be regarded as the medial edge of the superior longitudinal bundle. It has been pointed out (Forel, Onufrowicz, and others) that, in cases where the corpus callosum fails to develop, the tapetum remains apparently unaffected, and Déjerine has endeavoured to prove that the fibres of this layer really belong to the fasciculus occipito-frontalis. The fasciculus occipito-frontalis lies on the medial aspect of the corona radiata in intimate relation to the caudate nucleus, and posteriorly it spreads out over the superior and lateral aspect of the lateral ventricle, immediately outside the ependyma, where, according to Déjerine, it constitutes the tapetum (see p. 632).
Projection Fibres. We have already seen that every part of the cerebral cortex is linked to other cortical areas, not only in its own neighbourhood (short association fibres) (Fig. 578), but also in the most distant parts of the hemisphere (long association fibres), as well as to the cortex of the other hemisphere (commissural fibres). In addition there are two large series of fibres: (i.) an ascending group which conveys to the cerebral cortex impulses coming from the thalamus and metathalamus, the corpora quadrigemina and the red nucleus, and the various other sensory nuclei scattered throughout the brain stem and spinal medulla; and (ii.) a descending group connecting the cerebral hemisphere with the corpus striatum, various parts of the diencephalon, mesencephalon and cerebellum, as well as with all the motor nuclei scattered throughout the central nervous system. These two groups of tracts, respectively passing to and from the cerebral cortex, are known collectively as its projection fibres.
While examining the general arrangement of these projection fibres of the cerebral hemisphere it is convenient to refer incidentally to certain other fibre-tracts which do not fall strictly within this group.
Post. fun. of
Post. roots of
radiation -Bulbothalamic tract
FIG. 579.-DIAGRAM OF THE SENSORY TRACTS FROM THE
The Sensory Tracts.-A certain proportion of the fibres that enter the spinal medulla by its posterior root, which are supposed to be the sensory nerves of muscles, tendons, and joints, pass upwards without interruption in the posterior funiculi throughout the whole length of the spinal medulla until they reach the medulla oblongata, where they end in the nucleus gracilis and nucleus cuneatus. From these nuclei, arcuate fibres (fasciculus bulbothalamicus) arise and, after crossing the median plane, proceed upwards in the medial lemniscus of the other side to end in the ventro-lateral nucleus of the thalamus, from which a third group of neurones arises and proceeds upwards through the internal capsule to the cerebral cortex, where the impulses conveyed by it excite a consciousness of position and movement. But other sensory fibres end in the spinal medulla near their place of entry into it, and from the cells related to the endings of these fibres a new tract (fasciculus spinothalamicus) arises, crosses the median plane to reach the antero-lateral funiculus of the opposite side, in which it proceeds upwards throughout the whole length of the spinal medulla (that lies above its origin), the rhombencephalon and mesencephalon to the thalamus, where it ends alongside the bulbo-thalamic tract in
facial, glossopharyngeal, and vagus-and become added to the great strands that are proceeding upwards to the thalamus (Figs. 579 and 494, p. 561).
Of the other great ascending tracts in the spinal medulla, such as the two pairs of fasciculi spinocerebellares, nothing further need be said; nor is it necessary to do more than remind the reader that from the nucleus dentatus of the cerebellum a great tract (brachium conjunctivum) ascends to the opposite red nucleus and thalamus, and through them establishes an indirect connexion with the cerebral cortex in the precentral and frontal regions.
The other sensory pathways to the cerebrum, auditory, vestibular, visual, gustatory, and olfactory, are described elsewhere.
The Corticifugal Projection Strands.-The fasciculus cerebrospinalis, the great motor or pyramidal tract, is composed of fibres which arise from giant pyramidal