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



58 



trunk and the extremities. Of these branches 

 the anterior one is generally much the larger. 



An exception, however, to this arrangement 

 occurs in the case of the first spinal nerve (the 

 tenth pair of Willis), to which Winslow gave the 

 appropriate name nub-occipital nerve, to indicate 

 its peculiarity of character. This nerve some- 

 times has only one root, and that corresponds to 

 the anterior. More generally it has two roots, 

 of which, unlike the other spinal nerves, the 

 anterior is the larger, containing, according 

 to Asch, from three to five or seven bun- 

 dles of filaments, whilst the posterior contains 

 two or three, or at most four much smaller 

 bundles. Very frequently the posterior fila- 

 ments of either the right or left side unite with 

 the spinal accessory, a slight enlargement or 

 knot being formed at the point of junction ; 

 from this place a bundle of filaments emerges 

 equal in size to the posterior root, and takes 

 the ordinary course of that root, a small gan- 

 glion being formed upon it at the usual situa- 

 tion. Frequently, however, this ganglion is 

 wanting. The compound nerve formed from 

 the junction of these two roots, besides giving 

 off communicating filaments to the'sympathetic, 

 divides as the other spinal nerves do into an 

 anterior and posterior root, of which, however, 

 contrary to the usual arrangement, the posterior 

 is the larger. 



The spinal nerves are arranged naturally into 

 classes according to the regions of the spine in 

 which they take their rise. We number eight 

 in the cervical region, the sub-occipital in- 

 cluded ; twelve in the dorsal region ; five in 

 the lumbar, and six in the sacral regions. All 

 the nerves, after the second, pass obliquely 

 outwards and downwards from their emer- 

 gence from the spinal cord to their exit from 

 the vertebral canal, and this obliquity gradu- 

 ally increases from the higher to the lowest 

 nerves. The roots of the nerves possess cer- 

 tain characters, of which some are common to 

 all, and others are peculiar to the nerves of 

 particular regions. 



All the spinal nerves arise from the cord by 

 separate fasciculi of filaments, which, as they 

 approach the dura mater, converge to each 

 other and are united together to constitute the 

 anterior or the posterior roots. The posterior 

 roots of opposite sides lie at a pretty uniform 

 interval, from the upper to the lower part of 

 the cord, indicating but a very trifling change 

 in the thickness of the posterior columns 

 throughout their entire course. The ganglia 

 on the posterior roots are all proportionate to 

 the size of their respective roots. 



The characters proper to the nerves of parti- 

 cular regions may be stated as follows :*- 



The cervical nerves exhibit much less obli- 

 quity of their roots than the other vertebral 

 nerves. The second cervical nerve is trans- 

 verse (the first passing a little upwards as well 

 as outwards) ; the succeeding nerves slope 

 downwards and outwards, the lowest being 



* In the succeeding statements I have followed 

 Cruveilhier's description, which I have verified, 

 excepting in a tew points which are specified. 



the most oblique ; the obliquity, however, 

 never exceeds the depth of a single vertebra. 



The roots of the nerves in the cervical re- 

 gion are of considerable size. The posterior 

 roots bear a larger proportion to the anterior 

 than in any other part of the spine. Accord- 

 ing to Cruveilhier, the ratio is as 3 to 1, and 

 this estimate is probably correct. It applies 

 not only to the entire root, but to the fasciculi 

 of filaments which enter into their formation. 



The nerves in this region increase rapidly 

 from the first to the fifth, and then maintain 

 nearly the same size to the eighth. 



The dorsal nerves, with the exception of the 

 first, which closely resembles a cervical nerve, 

 have very peculiar characters. 



There is a manifest increase in the obliquity 

 of the roots, so that the length of each root 

 within the spinal canal equals the height of at 

 least two vertebra;. And it may be remarked 

 that the apparent obliquity is less than the real,, 

 for each root remains in contact with the cord 

 for a short distance after its actual emergence 

 from the substance of it, so that the point of 

 separation is some way below the point of 

 emergence. 



The interval between the roots is greater in 

 the dorsal region than any other segment of 

 the cord. The bundles, which compose the 

 roots, are smaller than elsewhere. 



We observe a very slight disproportion be- 

 tween the anterior and posterior roots in the 

 dorsal region. The latter, however, still main- 

 tain predominance of size. 



Lumbar region. From the dorsal region to 

 its terminal extremity the surface of the spinal 

 cord is covered both on its anterior and poste- 

 rior aspects by the fasciculi of origin of the 

 lumbar and sacral nerves. They emerge very 

 close to each other upon those surfaces, and 

 the intervals between the sets of fascicles 

 proper to each root are extremely short, so- 

 that they form an uninterrupted series of bun- 

 dles on each surface. 



The proportion of the posterior to the ante- 

 rior roots in the lumbar region is as 2 to 1 ac- 

 cording to Cruveilhier, or as 1$ to 1, which 

 seems to me to be nearer the truth. And 

 there does not appear to be any material dif- 

 ference in point of size between the posterior 

 and anterior fascicles.* 



A very interesting feature in the origin of the 

 lumbar and sacral nerves may be seen by ob- 

 serving the relation to each other of the ante- 

 rior and posterior roots of opposite sides on 

 the respective surfaces of the cord. The an- 

 terior roots of opposite sides may be seen to 

 approximate the median line gradually as they 

 descend, until at the lowest points they almost 

 touch. On the contrary, the posterior roots 

 continue nearly in the same sequence all the 

 way down. It may, therefore, be supposed 

 that in the tapering of the cord the anterior 



* M. Blatidin assigns the following proportions 

 of the posterior to the anterior roots in the several 

 regions ; in the cervical region as 2 : 1 ; in the 

 dorsal region as 1 : 1 ; and in the lumbar and sacral 

 regions as li : 1. 



