(fifth and sixth layers), where they take part in the formation of the inner line of Baillarger; some extend into the layer of superficial medium-sized pyramids (third layer), where they form a very dense plexus around the cells, which is also contributed to by the axons of cells of Golgi's type ii. belonging to this and to adjacent layers, and by collaterals from the ascending axons of the cells of Martinotti from the deeper layers. The total effect of this mass of fibres is, as already stated, to produce the whitish line in the macroscopic appearance of the section of the fresh cortex which is known as the outer line of Baillarger (or the line of Gennari of the visual cortex). The corticipetal fibres probably originate mainly in the thalamus, but some come from other parts of the cortex itself. In the brain of man, as compared with the lower mammals, there is a large preponderance of small nerve-cells of Golgi's type ii., especially in certain parts of the cortex. STRUCTURE OF SPECIAL PARTS OF THE CEREBRAL CORTEX. The results of recent researches have shown that there is a large amount of variation in the structure of the cortex in different regions. The variations manifest themselves (1) in the thickness of the grey matter, (2) in the relative number of medullated fibres passing into and away from the cortex, (3) in the number, breadth, and distinctness of the white striæ, (4) in the character of the nerve-cells, (5) in the relative time at which the medullation of the fibres occurs after birth. All these features have been employed by different observers to map out the various areas of the cortical surface, and it is found that some of the areas so mapped out correspond to areas which, on experimental grounds, have been considered to be concerned with definite functions. Adjacent areas of different structure do not, as a rule, pass by a gradual transition into one another, but the change is more or less abrupt. In many cases the line of demarcation is coincident with a cerebral fissure; but this is by no means constant, and it may, indeed, happen that the junction occurs at the convexity of a gyrus. It is impossible within the limits of this work to give an account of the differences of structure of all the areas which have been thus variously mapped out, nor do all observers agree precisely as to the number of separate areas and as to their exact limits. It will suffice for the present to exhibit, in the form of diagrams, the results which have been obtained by three different methods, leaving it to the reader to notice the points of variation and of correspondence which are brought out in them. Figs. 333 A, and 333 B, show the areas into which Flechsig finds it possible to subdivide the surface according to the relative date of myelination of the fibres. It will be seen that altogether thirty-six areas are marked out, arranged chronologically. They form three main groups: a primary, which includes the areas numbered from 1 to 10; an intermediate group, from 11 to 31, and a late or final group, from 32 to 36. The primary areas are myelinated at or soon after birth; the intermediate do not begin to myelinate until a month after birth; and the final even later; these may not be fully myelinated eight months after birth. According to Flechsig, projection-fibres are confined almost entirely to the areas of the primary group, which includes the so-called motor cortex (precentral gyrus) and the portions of cortex which are believed to be receptive of the impressions of special sense, such as the visual (visuo-sensory, Bolton), the acoustic (audito-sensory), and the olfactory; the areas which are receptive of the impressions derived from the other special senses are not yet ascertained. The intermediate areas lie, for the most part, adjacent to the receptive; they include the areas around the visual receptive cortex corresponding with the visuo-psychic area of Bolton, those adjacent to the auditory receptive cortex (audito-psychic), and so on. The late myelinating, final or terminal, areas lie between the intermediate zones, and these two occupy the greater part of the surface of the human brain, but a much smaller proportion of the brain of lower mammals, even of Primates (except the anthropoid apes). They are regarded as being concerned with the higher functions of the brain and as being linked to the motor VOL. III. BB FIG. 333.-DIAGRAMS OF CORTICAL AREAS AS DETERMINED BY THE MYELOGENETIC METHOD. (Flechsig.) A, external surface; B, mesial surface. The numerals indicate, in a general manner, the order of myelination. Visuo-psychic Prefrontal Visuo-sensory FIG. 334.-DIAGRAMS (ORTHOGONAL) OF CORTICAL AREAS AS DETERMINED BY THE DISTRIBUTION AND ARRANGEMENT OF FIBRES AND CELLS. (A. W. Campbell.) I., lateral surface; II., mesial surface. (Where a fissure appears to bound adjacent areas the real boundary is in the depth of the fissure.) A, B, C (in II.), parts of the limbic lobe. BB 2 Visuo sensory 372 THE CEREBRAL CORTEX and receptive areas by association-fibres; hence the name associated areas,' which was originally applied to them by Flechsig. Several observers have employed the characters of the cell-layers for the topographical analysis of the cortex (cyto-architectonic method). This method is the one which promises. in the long run, to yield the most accurate results, and which will have to be used ultimately to check the results obtained by other methods. But up to the present it has not been worked out in sufficient detail to enable us to map out every different portion with accuracy, although in the hands of Vogt and his collaborators considerable progress has been made in this direction. With regard to certain of the cortical areas, a mass of accurate information relating to the form of the cells and their arrangement is already accumulated, thanks to the labours of Bevan Lewis, Bolton, Mott, Vogt, Brodmann, Campbell, and Cajal. It has proved more easy to obtain evidences of structural differences of the several areas by a study of the relative coarseness and fineness and the number of vertically running medullated fibres entering and issuing from the grey matter. With this is combined the observation of the manner in which they are distributed in the planes, parallel to the surface, which are known in section as the lines of Baillarger and of Gennari. This method has been employed by Kaes and by Campbell, in conjunction with the study of the cell-layers. The areas which Campbell has thus succeeded in defining are shown in fig. 334. FIG. 335.-DRAWINGS SHOWING THE NAKED-EYE APPEARANCES OF SECTIONS OF THE FRESH CEREBRAL CORTEX IN DIFFERENT REGIONS. (Elliot Smith.) (The regions from which the several sections are taken are shown in fig. 336.) But even this method-for which Weigert-Pal preparations of the cortex are almost exclusively employed, assisted in the study of the cell-layers by sections stained by the methylene-blue method of Nissl-necessitates the preparation of an enormous number of microscopic sections and the expenditure of a prodigious amount of time and labour. These objections apply to all methods requiring histological investigation, owing to the great extent of surface which the brain presents. They are, however, largely obviated by a mode of examination which has been described by Elliot Smith. This observer, in place of using microscopic sections of fixed and stained brain-tissue, examines and measures the extent of the thickness of the grey matter and the character of the lines of Baillarger in the perfectly fresh or, failing this, in the formalin-injected cortex. The accompanying diagram (fig. 335) exhibits no fewer than twenty-eight different types of cortex, all, according to Elliot Smith, recognisable with the naked eye. The topographical diagrams, which are also here reproduced (fig. 336, I. and II.), serve to show the manner in which this method can be employed to map out the several areas. In illustration of its value the structural appearances of sections of the mesial occipital cortex may be taken. In this region the area striata, or visual cortex proper, characterised by the very distinct line of Gennari (see diagram, fig. 335), is found to be confined to the lips of the calcarine fissure. Immediately outside it is an area, the area parastriata, which no longer has the single white line of Gennari, but exhibits a double-lined |