Very frequently we observe that, besides the granular substance above described, there are certain pigment particles of large size and dark colour, which are collected into one or two roundish or oval groups, situate at or towards one or both sides of the vesicle (fig. 371, e). These masses of colouring rnatter sometimes occupy considerable space, and enable the ob server readily to detect the position of such vesicles as contain them. W hen the mass of pigment is placed at one side, we may com pare the containing vesicle, as Volckmann has done, to a fruit which is coloured only on that side which is exposed to the sun. The aggre gation of many such vesicles at any one spot gives the nervous matter there a peculiarly dark colour. A remarkable example of this is found in that portion of the crus cerebri which is known by the name of locus niger.
A very interesting form of nerve-vesicle is that which exhibits the greatest departure from the globular shape by the prolongation of the wall of the outer cell into one or more tail-like processes. These bodies may, from this pecu liar character, be designated caudate nerve vesicles. They possess the nucleus and nucleo lus, as in the more simple form, and contain one or more of the masses of colouring matter; indeed, in them the quantity of pigment is generally much more considerable than in any other form. I have noted an observation which shewed two nuclei in one vesicle. They vary much in size and shape, and so also do the processes. The largest nerve-vesicles are found among those of this description. The variety in shape may depend in some degree upon the situations from which the caudate processes take their rise. In some (fig. 371, d) they proceed from opposite poles of the vesicles; in others they arise near each other from the same region of the vesicle, and when numerous, give to it somewhat the form of a cuttle-fish with extended tentacles. In examining the structure of one of these processes, we find it evidently exactly similar to that of the matter contained in the outer vesicle, exhibiting the same minutely granular appearance. The pro cesses are implanted in the surrounding sub stance, and firmly connected with it, so as to be with great difficulty separated from it. They exhibit much strength of cohesion, but are fre quently broken off quite close to their points of origin, and the broken ends present a dis tinctly lacerated edge (d, fig. 371). More
rarely we are able to trace these processes to a considerable distance, and then we observe them to bifurcate or even to subdivide further, and to terminate in exceedingly fine transpa rent fibres, the connexion of which with the other elements of the nervous matter has not yet been ascertained.* It is in vain, in the present state of our knowledge, to speculate upon the use of these caudate processes. Do they constitute a bond of union between the nerve-vesicles and certain nerve-tubes? or are they commissural fibres serving to connect the grey substance of different portions of the nervous centres ? Until a more extended research has made us better acquainted with the peculiarities of these vesicles in various localities, it would be premature to offer any conjecture concerning their precise relation to the other elements of the nervous centres. They exist, with different degrees of developement, in the locus niger of the crus cerebri, in the laminw of the cerebellum, in the grey matter of the spinal cord and medulla oblongata, and in the ganglions, and in the grey substance of the cerebral convolutions, in which latter situa tion they are generally of small size.
When a portion of grey matter from a con volution of the brain is examined with a high power in the microscope, we observe it to con sist chiefly of a mass of granular matter, in which nerve-vesicles are imbedded with consi derable intervals between them. Henle states, with much truth, that the superficial part of the grey matter of the convolutions seems almost entirely composed of finely granular substance, in which lie, scattered here and there, several clear vesicles which, as he renaarks, look almost like openings (fig. 372). In the middle por tion the vesicles appear larger, and the gra nular matter becomes less abundant, and on the most deep-seated plane the nerve-vesicles are much increased in size and lie in closer juxtaposition, being, however, covered by a thin layer of granular matter, which forms a sheath to each vesicle. Nerve-tubes are found throughout the whole depth of the grey matter. Those in the most superficial layer are ex tremely fine and varicose, and seem to corre spond in number and situation to the vesicles. 1 or wherever there is a nerve-vesicle, we find an extremely fine varicose nerve-tube apparently adherent to it.