ON SOME HISTOLOGICAL FEATURES OF THE BRAIN. 657 



The " small pyramidal " cells have much the same features ; that is to 

 say, the cells are characterized by their pyramidal form, though this is nat- 

 urally not so distinct, by their vertical position, and by the possession of 

 branching processes which are lost in the molecular ground substance ; the 

 presence, however, of a midbasal axis-cylinder process has not been clearly 

 demonstrated. 



Other nerve-cells are more like the ordinary nerve-cells of the spinal 

 cord and of the internal cerebral gray matter ; they are branched cells, of 

 irregular, not of pyramidal, form, and for the most part small, 18 //. by 10 n. 

 They may be characterized by the relatively large size, 7 /-/, of the nucleus, 

 and do not possess an axis-cylinder process ; at least, such a process has not 

 yet been demonstrated. They are frequently spoken of as " angular " cells. 



Another kind of cell, the " fusiform cell," which is found in all regions 

 of the cortex, has a characteristic spindle-shape, the cell substance being 

 prolonged at the opposite poles into tapering, ultimately branched processes. 

 The long axis of the cell is generally placed horizontally, following the cur- 

 vature of the cortex, and being thus at the sides of the sulci vertical to the 

 surface of the brain ; it is, however, at times inclined at various angles. 



Still another kind of cell, the " granule cell," or " nuclear cell," is one in 

 which the nucleus is surrounded by a relatively small quantity of cell sub- 

 stance, 9 p by 7 />-, more or less spherical in form in ordinary preparations, 

 but probably breaking up into delicate branched processes. Cells of this 

 kind are sparsely scattered throughout the cortex generally, but in particu- 

 lar regions e. g., the occipital are crowded together into a layer, which in 

 many respects resembles the nuclear layer of the cerebellum, and has been 

 called the " granular" or "nuclear" layer. 



Lastly, throughout the cortex are found, besides indubitable nerve-cells 

 and indubitable neuroglial cells, numerous small, somewhat irregular cells, 

 concerning which it may be debated whether they are really nervous or 

 simply neuroglial in nature. Moreover, in using the names given above for 

 the various kinds of nerve-cells, it must be remembered that many transi- 

 tional forms are observed.; cells, for instance, may be seen intermediate in 

 form between pyramidal cells and " fusiform " or " angular " cells. 



The medullated nerve-fibres which take part in the cortex may be con- 

 sidered provisionally as forming two categories. In the first place, fibres 

 sweep up vertically into the cortex from the subjacent " central white 

 matter," taking at first a curved course as they enter into the gray matter, 

 and then appearing to run straight toward the surface. These are arranged 

 in the deeper levels in bundles, leaving vertical columns of the gray matter 

 between them ; but at more superficial levels the bundles spread out and are 

 gradually lost to view. Besides these distinct vertical fibres and bundles of 

 fibres of the ordinary medullated kind, which we have reason to think are 

 the ends (or beginnings) on the one hand of fibres of the pedal and tegmen- 

 tal systems, and on the other hand of fibres of the corpus callosum, or the 

 other commissural fibres spoken of as " association " fibres ( 548), an ex- 

 ceedingly large number of fibres of the peculiar fine medullated kind run 

 in various directions, forming a dense network in the ground substance of 

 the gray matter between the cells. We may add that this system of fine 

 medullated fibres is of late growth, and is not fully developed in man until 

 two or three years after birth. Many of the medullated fibres, coarse as well 

 as fine, take a horizontal direction parallel to the surface, and in certain re- 

 gions are specially developed into a layer or into two layers so. as to form a 

 horizontal streak or streaks. 



The vascular pia mater invests closely, as we have said, the whole surface 

 of the cortex, dipping down into the sulci ; and from it, as in the case of the 



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