NERVOUS CENTRES. (HUMAN ANATOMY. THE SPINAL CORD.) 



655 



might most legitimately be expected that a 

 proportionate developement of these parts 

 would exist in the cervical region. Yet a 

 comparison of the cervical with the lumbar 

 swelling demonstrates that the developement 

 of both the grey matter and the posterior co- 

 lumns, (if not absolutely, certainly relatively to 

 the bulk of the segment,) is inferior in the 

 former to that in the latter, whence nerves are 

 supplied to the inferior extremities in which 

 sensibility is much less acute, and in which 

 there is a much less perfect adjustment of 

 the voluntary power to the muscular move- 

 ments. 



The difference of the respective sizes of the 

 antero-lateral columns in those parts of the cord 

 which supply the upper and lower extremities 

 is perfectly consistent with the difference in the 

 sensibility and voluntary power of those parts.* 

 And as in the trunk these endowments are 

 at their lowest point of developement, so the 

 dorsal region of the cord is that which exhibits 

 the antero-lateral columns of the smallest 

 bulk. 



In the lower parts of the body, which re- 

 ceive their supply of nerves from the lumbar 

 swelling of the cord, there are certain peculi- 

 arities worthy of the attention of the physiolo- 

 gist. Thus the sphincter muscles of both the 

 bladder and rectum are to a great degree inde- 

 pendent of voluntary influence, and act inde- 

 pendently of consciousness. The principal 

 function of the lower extremities is that of 

 locomotion; they are the pillars of support to 

 the trunk, and the chief agents in the main- 

 tenance of its attitudes. And, although in 

 these actions the will exercises a not inconsi- 

 derable control, still the principle of purely 

 physical nervous action renders them in a 

 great degree independent of the mind.f 

 The reflex or excito-motory actions are much 

 more evident in the lower than in the 

 upper extremities ; the former are much 

 more independent of cerebral lesion than the 

 latter. And let it be remarked that these 

 phenomena are associated with high deve- 

 lopement of grey matter, and with posterior 

 columns of large size, while the antero-late- 

 ral columns are comparatively small. May 

 not the high developement of the grey matter 

 have reference to the exalted state of the phy- 

 sical nervous actions of the lower part of the 

 body, and that of the posterior columns to the 

 locomotive actions ? To these points we shall 

 have again to refer when we discuss the func- 

 tions of the spinal cord. 



Is there a central canal in the [spinal cord ? 

 Many anatomists have affirmed that the spinal 

 cord was traversed in its entire length by a 

 canal, which was continuous with the fourth 

 ventricle. If such a canal exist, it must be 

 extremely difficult to demonstrate, as I have 

 never, after numberless examinations, been 



* Weber's experiments sufficiently indicate that 

 the general as well as the tactile sensibility of the 

 lower extremities is considerably inferior to those 

 of the upper extremities. 



t See the observations at the commencement of 

 the article, p. 589. 



able to see it. In transverse sections of the 

 spinal cord, which have been dried upon glass, 

 there is sometimes an appearance which may 

 be attributed to the presence of a minute canal; 

 but I should be more disposed to ascribe it to 

 the patulous mouth of a bloodvessel which had 

 been divided in making the section, for it is 

 by no means constant even in different regions 

 of the same spinal cord. The situation which 

 some have assigned to this supposed canal is 

 between the grey and white commissures ; but 

 Stilling and Wallack* place it in the grey 

 matter. It is obvious that an artificial sepa- 

 ration of these layers, which is easily effected, 

 and more especially while the preparation is 

 being dried, would give rise to the appearance 

 of a canal upon a transverse section. It may 

 be stated, further, that the deepest part of the 

 longitudinal fissure is wider than any other 

 portion of it, and, if cut across, might appear 

 like a canal. 



The observations of Tiedemann appear to me 

 to put this question in its true light. I shall, 

 therefore, make the following quotation from 

 his learned work on the anatomy of the fcetal 

 brain, without, however, subscribing to the ac- 

 curacy of all the statements it contains. 



" The spinal marrow," says Tiedemann, 

 " represents a hollow cylinder, the thin walls 

 of which are bent backwards, the posterior part 

 representing a longitudinal opening; for it is 

 hollowed by a groove, termed tlie canal of the 

 spinal marrow. This canal exists through the 

 whole cylinder, and communicates with the 

 calamus scriptorius, with the fourth ventricle, 

 which, strictly speaking, is but a dilatation of 

 it. During the first periods we can, without 

 difficulty, separate the thin and flexed walls of 

 the spinal marrow, and thus expose the canal 

 which they contain. This canal is somewhat 

 broader in those points where the spinal marrow 

 sensibly enlarges exteriorly, as at the origin of 

 the nerves for the pectoral and abdominal ex- 

 tremities. The mechanism of its formation is 

 very simple: the pia mater, acquiring more 

 extent, is folded longitudinally backwards and 

 dips into the substance of the spinal marrow, 

 which, as we have seen, had been previously 

 in a fluid state. It is very evident that, in the 

 commencement of the second, third, and even 

 fourth months, this canal has, in proportion to 

 the thickness of the walls of the spinal marrow, 

 a much greater capacity than it subsequently 

 acquires. The contraction which it undergoes 

 in the progress of the developement of the 

 embryo, arises from the pia mater depositing 

 a new substance, the materials of which it 

 derives from the blood sent by the heart, and 

 which, augmenting the volume of the walls of 

 the cylinder, ought necessarily to diminish the 

 calibre of the central canal. This substance is 

 soft, reddish, and traversed by numerous small 

 vessels during the period of the last two 

 months. We cannot doubt, then, that the 

 grey substance of the spinal marrow has an 

 origin subsequent to that of the medullary 



* Untersuchungen liber die Textur des Rucken- 

 marks. Leipz. 1842. 



