In the grey matter of the ganglions we find that the vesicles are also deposited in granular matter, which surrounds each of them as a sheath (lig. 371, f, g), completely investing it on every side, and separating, it from the neigh bouring ones. Here, however, the sheath is formed not only of a finely granular matter, but also of numerous bodies which resemble nuclei or cytoblasts, and this sheath invests both the globular variety of nerve-vesicles and the cau date ones. Nerve-tubes lie in immediate con nexion with these vesicles, and sometimes en twine themselves around them, and seem to indent their sheaths (fig. 375.) Other vesicles, much more simple in form, are found in the grey matter in certain situations. The outer layer of the optic thalamus, accord ing to Ilenle, contains only small homoge neous globules, analogous to the nuclei of the ganglionic globules, in immediate apposition with each other, and towards which the tubes seem to ascend in the vertical direction. Pur kinje states, that a similar layer is met with in the cortical substance of the brain quite close to the medullary substance.* I find a layer of similar panicles in the grey matter of the cerebellic laminT. And, according to the report of ValentinA Purkinje has found the inteiior of the ventricles in the normal state covered by an oily matter, which consists of distinct, large, transparent globules, free and lying near each other. A similar layer has been found by him in the interior of the fifth ventricle. The cavity of the rhomboidal sinus in Birds likewise contains a gelatinous mass, which consists of large globules lying close to each other.t Developement of grcy matter.—In the per fect nerve-vesicle, the cell form of primitive developement is persistent. We have the nu cleolus and nucleus (cytoblast) and the cell ; and, according to Schwann, the only change which the full-grown cell exhibits consists in an increase of size and in the developement of the pigmentary granules within. The following is Valentin's description of the developement of a nerve-vesicle. In the very young embryos of Mammalia, as the sheep or calf, the cerebral mass in the course of formation contains, in the midst of a liquid and transparent blastema, transparent cells, with a reddish yellow nucleus. The wall of the cells is very thin and simple ; their contents are colourless, transparent, ho mogeneous, and manifestly liquid; the nucleus, with well-defined contour, is generally round, sometimes central, at other times excentric, solid, and nearly of the same colour as the corpuscles of the blood. Around these primi tive cells of the central nervous system, which we find likewise formed after the same type in the spinal cord, a finely granular mass becomes deposited, which probably is not at first sur rounded by an enveloping cell-membrane. At this early period of formation the primitive cell still preserves its first delicacy to such a degree that the action of water causes it to burst Im mediately. This rupture of its membrane and effusion of its contents often take place so sud denly and quickly that they can be percened only by the movement of the nucleus, which is the consequence of it. * * In proportion as the granular mass contracts itself within certain limits, (sich immer mehr abgrenzt,) a cell-mem brane probably becomes developed around it, so that the vesicle gradually acquires its precise form and size, and its contents their proper characters, which belong to a fully formed cen tral nervous corpuscule.* Valentin compares the developement of these vesicles to that of the ovum. The nucleolus of the nerve-vesicle is always first formed, then around it the primi tive cell, and around this the outer cell. This process resembles exactly that which takes place dunng the formation of the ovum, for the germ corresponds to the nucleolus, the germinal vesicle to the nucleus, the yolk to the contents of the outer enveloping cell, and the vitelline membrane to the delicate wall of this cell, sup posing that this latter membrane always exists. t The great simplicity in the form of the ele ments of the grey nervous matter is one of its most remarkable characteristics. That a tissue, which, as will be shown by-and-bye, plays so prominent a part in the nervous actions, whether they are prompted by mental change, oeare purely corporeal, should exhibit scarcely any more complexity of structure than that which is found in the simplest animal or vegetable textures, or in structures that have not passed their earliest phase of developement, is an ana tomical fact pregnant with great physiological interest. Have this simplicity of torm and de licacy of structure reference to the celerity of the nervous actions ? or to that proneness to change which must be induced by the constant and unceasing round of impressions which the grey matter must receive from the ordinary nu trient actions that are going on in the body, as well as from the continual action of thought ? lf, according to common acceptation, we ad mit that the mind is in immediate connexion with the cerebral convolutions, it may well be imagined that no part of the frame can be the seat of such active change, from its being on the one hand the recipient of impressions from the body, and, on the other, from an association with the psychical principle so intimate that probably, under ordinary circumstances, an affection of the one cannot occur without being communi cated to and producing a change in the other.
Another curious fact, in connexion with the intimate structure of the grey nervous matter, is the large quantity of pigment or colouring rnatter which exists in it, and which appears to form one of its essential constituents, more abundant in some situations than in others, but present in all. We are utterly ignorant of the design of this peculiarity of structure. If this pigment bear any resemblance of che mical composition to the colouring matter of the blood, hamatosine,—and it is not improbable that it does,—an increased interest attaches to the practical importance of minute attention, on the part of practitioners, to avail themselves of all the means which are capable of impro ving that important element of the nutrient fluid both in quantity and quality, for it is most reasonable to presutu that the pigment of the nervous matter wOlrld derive its nou rishment from that of the blood.
It may be further remarked that pigment oc curs in connexion with the nervous system in another form besides that of incorporation with its elementary particles, that is, upon the exte rior of parts of the nervous centres or of parti cular nerves. Examples of this may be re ferred to in the case of the olfactory nerve of the sheep and of other Mammalia, the bulb of which is surrounded by black pigment con-• nected with the pia mater. It is also found soinetimes on the pia mater of the spinal cord of the human subject. Valentin, who deli neates a magnified view of this pigment, states that it occurs chiefly in the cervical region. In frog,s, the whole spinal cord and encephalon are covered with a silvery pigment interspersed with black. The same occurs in fishes. The black pigment in connexion with the retina has an obvious use. On the choroid gland of fishes, which lies immediately contiguous to the re tina and surrounds the optic nerve, there is a silvery membrane which contains a quantity of the same kind of pigment as that alluded to upon their nervous centres. On some of the ganglia of the invertebrata particles of pig ment are likewise found.
Of the structure of ganglions.—The descrip tion of the minute anatomy of ganglions as well as of all other nervous centres may be regarded as the solution of the following pro blem : to determine the relation which the white substance of these centres bears to the grey matter on the one hand, and to the nervous trunks connected with them on the other hand.
The white substance of the ganglions con sists of a series of minute nerve-tubes, as well as of sotne gelatinous fibres, which are conti nuous with those which exist in the nerves themselves. If we trace a nerve into a gan glion, it is found to break up into its com ponent nerve-tubes, and it does so by a se paration of the tubules within into smaller bundles, or single tubes. Sometimes adjoining, bundles interlace, each yielding to its neigh bour one or more tubes. The nerves which emerge from the ganglia derive their component nerve-tubes from different bundles, so that the same kind of interchange of tubules, which we have noticed as taking place in plexuses, occurs also in ganglia. The emerging nerves result from a further subdivision and greater inter mixture of the bundles of nerve-tubes which enter the ganglions. The arrangement is well shown in jig. 373, where the nerve (a), which enters the ganglion, may be seen breaking up into a plexus, from which three branches (b, b, b) emerge, and it may be observed that these emerging nerves derive nerve-tubes from very different and opposite parts of the ganglionic plexus. In the meshes, which are left be tween the interlacing nerve-tubes, the gangli onic globules or nerve-vesicles are situate (figs. 373, 374). Certain fibres, according to Valen anatomy, to determine whether there am any nerve-tubes which terminate in the grey matter of the ganglion, or originate in it,—which in short are not continued through the ganglion. At present we are unable to state further than that the tubes appear to have an intimate connection with the nerve-vesicles wherever the latter may be found, and that they often appear to be con tinuous with the sheaths of the nerve-vesicles.