The clentated ligament has to the naked eye all the characters of white fibrous tissue, of which it is chiefly composed. In its dentated processes, however, a considerable quantity of yellow fibrous tissue may be found. The simi larity of its constitution with that of the pia water evidently justifies its being regarded as a process of that membrane, and not, as soine anatomists thought, of the dura mater, with which it has a much less intimate and extensive connexion. Its anterior and posterior surfaces are uncovered by any membrane ; they are smooth, and have the glistening silvery appear ance of white fibrous membrane. It is evident that during life these surfaces must be bathed by the subarachnoid fluid.
The office of this remarkable structure seems evidently to be mechanical ; to preserve the spinal cord in a state of equilibrium ; and to prevent lateral movement of it, whilst at the same time it forms a partition between the roots of the nerves.
General remarks on the structure of the nervous centres.—lt has already been shewn in a former part of this article that the nerves properly so called are composed exclusively of one kind of nervous substance,—namely, the fibrous nervous matter, which is disposed in bundles of peculiar fibres. It is only in the nervous centres or in continuations of them that we find an union of the white and the grey nervous matter; and, indeed, it may be stated in general, that the peculiar and distinctive anatomiail character of a nervous centre con sists in this combination of the two kinds of nervous matter.
In the nervous centres the white matter exhi bits, for the most part, the same essential cha racters of structure as in the nerves; that is to say, it is disposed in tubes containing a certain pulpy matter in them. It has been found, how ever, that these tubes are much more prone to become varicose under the influence of pressure or of any other disturbing cause. 1 hey are not, as in the nerves, bound together by areolar tissue, but are disposed in bundles and on different planes, with their nutrient blood vessels ramifying among them, and in some situations the elements of the grey matter are interposed between them. Certain parts of the nervous centres are composed exclusively of white matter, as a portion of the hemispheres of the brain, and of the cerebellum, and the superficial parts of the spinal cord.
The white fibres which are found in the ner vous centres may be distinguished according to their physiological office into four different kinds. Two of these are continuations of the fibres of the nerves, and serve to connect the nervous centres with other organs or textures, either by conveying the influence of the centres to them, or by propagating impressions from them to the centres. The former are called
efferent, the latter afferent fibres. In addition to these, we find a third and large series of fibres, which serve to establish a connection between different centres, or between different portions of the same centre. These are called commissurol fibres; they form a large portion of the mass of the brain and spinal cord. And Henle suggests that the brain contains a fourth series of fibres, associated with the operations of thought.
We remark in the nervous centres, especially in the brain and spinal cord, a greater difference as regards size between the different nerve tubes, than may be observed elsewhere, and it seems to be a constant character that they diminish in size as they approach and enter the grey matter.
(11. the grey nervous matter.—The grey ner vous matter differs very materially in its ana tomical characters from the white. Its ele ments are vesicles or cells, with nuclei and nucleoli. Although this vesicular or cell form is universally prevalent, the cells present much diversity of shape, size, and colour in different centres or even in the same centre, which ap parently have reference to some peculiarity of function. The most prevalent form is that of a globular vesicle, composed of a very deli cate transparent membrane. Within this mem brane is contained a soft minutely granular sub stance, which forms the principal mass of the body, parenchymtnasse (Valentin). The grey colour of the vesicle, which becomes very ma nifest when a number of them is congregated together, is dependent on this granular matter. (See fig. 371, a, b , c.) When the vesicle bursts and its substance is broken up, the granular matter is diffused, and confuses and darkens the specimen under examination. Sometimes the outer vesicle is removed, the contained granular rnatter retaining the globular form. Within the external vesicle (a, fig. 371) there is another much smaller and adherent to a part of its wall, so as to be quite out of the centre of the containing vesicle. This is the nucleus (b, fig. 371). Its structure is ap parently of the same nature as that of the ex ternal vesicle. The nucleus contains in its centre another minute and remarkably clear and brilliant body, also vesicular in structure. This is the nucleolu.s (c, fig. 371). Sometimes it is replaced by two or three much smaller but similar bodies. The softness of the vesicle admits of its yielding, whether from the dis turbance occasioned in the necessary manipu lation or from the pressure of the neighbouring elementary parts as it lies in its proper situation. IIence it is that these vesicles exhibit a consi derable diversity of form.