652 



NERVOUS SYSTEM. (NERVOUS CENTRES. THE SPINAL CORD.) 



part in the new-born infant. The actual weight 

 of the spinal cord in an adult male may be 

 stated to be a little more than one ounce. 



We may here again notice the interesting 

 fact that there is a great disproportion between 

 the size of the spinal cord and that of the ver- 

 tebral canal, and that consequently a consider- 

 able space is found between the cord and its 

 membranes which is occupied by the cerebro- 

 spinal fluid. 



The consistence of the medullary substance 

 of the spinal cord, in the fresh state, is of much 

 greater firmnes than that of the brain. This 

 lasts, however, but for a very short time, for 

 decomposition sets in quickly, and then the 

 cord acquires a pultaceous consistence, and 

 the nervous matter may be easily squeezed out 

 of the sheath of pia mater in which it is en- 

 closed. 



The pia mater adheres very closely to the 

 surface of the cord, as intimately as the neu- 

 vilemma to a nerve. In order to examine the 

 surface of the cord, the best mode of proceeding 

 is to dissect off the pia mater carefully, the 

 cord having been fixed under water. The dis- 

 sector will then perceive that numerous minute 

 vessels, accompanied by delicate processes of 

 the membrane, penetrate the cord at all points 

 from the deep surface of the pia mater, and to 

 this is due the adhesion of this membrane 

 above-mentioned. This arrangement may also 

 be shewn by dissolving out the nervous matter 

 through the action of liquor potassae. The pro- 

 longations from the deep surface of the sheath 

 may be shewn by floating the preparation in 

 water. 



The spinal cord is penetrated both on its 

 anterior and posterior aspect by fissures, each 

 of which corresponds to the median plane. 

 They are separated from each other by a trans- 

 verse bilaminate partition of white and grey 

 matter, of which the grey layer is posterior. 

 This serves to connect the equal and symme- 

 trical portions into which the cord is divided 

 by these fissures. 



The anterior fissure is very distinct and 

 easily demonstrated. A folded portion of the 

 pia mater may be traced into it down to the 

 commissure. The edges of this fold, as it 

 enters the fissure, are connected by a band of 

 white fibrous tissue, which may be traced 

 through the whole length of the cord on the 

 exterior of the pia mater, and indicates pre- 

 cisely the position of the anterior fissure, and 

 which covers the anterior spinal artery. When 

 this fold is carefully removed, the floor of the 

 fissure becomes apparent, formed of a lamina 

 of white nervous matter. This layer is per- 

 forated by a great number of minute orifices, 

 which give to it quite the cribriform character, 

 and are for the reception and transmission of 

 bloodvessels. In many parts the layer appears 

 to be composed of oblique and decussating 

 fibres, as if the same kind of decussation 

 which occurs at the lower part of the medulla 

 oblongata extended through the whole length 

 of the cord. There is not, however, any real 

 decussation : the appearance of it results from 

 the foramina not being always on the same 



level. For, in those places where they lie quite 

 on a level with each other, no one could sup- 

 pose that such an arrangement of fibres ex- 

 isted. Here, as elsewhere, the fibres assume 

 the transverse direction. The depth of the 

 anterior fissure is not the same all down the 

 cord; it gradually diminishes towards its lowest 

 point ; its deepest part, however, corresponds to 

 the cervical enlargement, and here it is about 

 one-third of the thickness of the cord measured 

 from before backwards. 



The whole cribriform layer which forms 

 the floor of the anterior fissure constitutes a 

 commissure between the lateral halves of the 

 cord in their whole length. It is called the 

 anterior or white commissure of the cord. 



The posterior fissure is very much finer and 

 more difficult to demonstrate than the anterior. 

 It is not penetrated by a. fold of the pia mater : 

 a single and very delicate layer of that mem- 

 brane is continued from its deep surface down 

 to the floor of the fissure. It is at this situa- 

 tion that the spinal pia mater assumes the 

 appearance and character of that of the brain. 

 Here and there, within the fissure, the pia mater 

 appears interrupted and the vessels extremely 

 few, and in such situations the fissure becomes 

 very indistinct and difficult to recognise. The 

 process of pia mater becomes extremely deli- 

 cate towards the lowest extremity of the cord. 

 The posterior fissure is deeper than the an- 

 terior. Through a great part of its course it is 

 equal to fully one-half of the thickness of the 

 cord ; in the lumbar region, however, its depth 

 is very much less. Its floor is formed by a 

 layer of grey matter, which connects the cine- 

 ritious matter of each lateral half of the cord, 

 and which is called the grey commissure. In 

 the lumbar region, however, it does not appear 

 to reach the grey commissure. 



Arnold denies the continuity of the posterior 

 fissure through the greater part of the cervical 

 and dorsal regions. According to his figures it 

 ceases on a level with the second cervical nerve 

 and reappears about the second dorsal vertebra. 

 This does not at all accord with my observation, 

 nor is it confirmed by any anatomist that I 

 know of. It appears to me that the continuity 

 of the fissure might be more properly ques- 

 tioned at the lowest third of the cord, where 

 it is often so feebly developed as to elude de- 

 tection. In three out of four specimens now 

 before me, the fissure is sufficiently distinctly 

 marked down to quite the lowest extremity of 

 the cord, and the posterior columns separate 

 readily from each other along it. In the fourth, 

 which is quite recent, the fissure at the lowest 

 part of the cord is only to be distinguished 

 here and there by a solitary red vessel pass- 

 ing to the grey commissure, and is most 

 distinct in the cervical region. Those spe- 

 cimens which shew it well have been hard- 

 ened in a preserving liquid, which, by con- 

 stringing the substance of the posterior columns, 

 renders the fissure much more distinct. There 

 seems little doubt that the posterior columns 

 have no connexion with each other as far down 

 as the lumbar region. Below that, however, it 

 is not improbable that they may be united acruss 



