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



659 



fibres of the antero-lateral columns separate 

 most quickly from it. 



The direction of these roots is almost vertical, 

 and their length within the canal of the dura 

 mater is very considerable. The aggregate of 

 them forms the cauda euuina. 



I have not observed that the situation in 

 which the ganglia of the sacral nerves are 

 formed is different from others. They are 

 contained, as elsewhere, in sheaths of dura 

 mater, and lie in the sacral foramina, sur- 

 rounded by fat, and from the looseness of their 

 connection with the walls of those foramina 

 they may be very easily detached. 



1 cannot confirm Cruveilhier's statement that 

 the anterior and posterior roots in the sacral 

 region together form the ganglia. 



The roots of the sacral nerves gradually di- 

 minish in size, so that the lowest are smaller 

 than any others which emerge from the spinal 

 cord. 



In connecting the peculiar anatomical cha- 

 racters of the spinal nerves in the various re- 

 gions with their physiological action, some 

 interesting points are presented to our notice. 



The great size of the cervical nerves is quite 

 in conformity with the exalted vital actions of 

 the upper extremities. And the predominance 

 of the posterior over the anterior roots, both 

 positive, and as compared with other regions, 

 corresponds with the great developement of 

 sensation in the upper limbs. 



The posterior root of the second cervical 

 nerve, as has been noticed by Longet, is con- 

 siderably larger than the anterior, as 3 : 1 ; and 

 it is from this source that the occipital and 

 mastoid nerves, the sensitive nerves of the 

 integument in the occipital region, derive their 

 filaments. 



In the dorsal region the almost equality of 

 the anterior and posterior roots and the small 

 size of both is consistent with the absence of 

 any great degree of developement either of the 

 sensitive or motor power. Or if, as there is 

 some reason for believing, many of the move- 

 ments in that segment of the body which is 

 supplied from this region of the cord be of 

 the excito-motory kind, then we might suppose 

 that each posterior root contains an excitor fila- 

 ment for each motor one in the anterior root; 

 and if the sensitive fibres are superadded to the 

 former (allowance being made for the smaller 

 size of sensitive fibres,) the slight predominance 

 of the posterior root may be accounted for. 



Lastly, the increased muscular activity of 

 the lower extremities and their greater sensibi- 

 lity as compared with the dorsal segment, ren- 

 ders necessary the increase of size which the 

 roots of the lumbar and sacral nerves experi- 

 ence. And it may be conjectured that the 

 predominance of the posterior roots has refer- 

 ence to the exalted sensibility of some parts 

 of the lower limbs. 



One of the most important problems in the 

 anatomy of the spinal cord is to determine the 

 precise relation which the roots of the nerves 

 bear to the columns of the cord and to the grey 

 matter. As far as coarse dissection enables me 

 to determine, I would venture to make the fol 



lowing statement, founded upon my own ob- 

 servations. 



The anterior roots derive their fibres wholly 

 from the antero-lateral columns. Of these fibres 

 it is probable that some are continuous with the 

 longitudinal fibres of the cord, and that others 

 pass into the grey matter. This, however, is 

 very difficult, if it be possible, of demonstra- 

 tion by the ordinary modes of dissection. The 

 posterior roots adhere to the posterior part of 

 the antero-lateral columns, and derive their 

 fibres chiefly from that source. I have never, 

 in numerous dissections, seen any thing to in- 

 duce me to believe that the posterior columns 

 contribute to the formation of the posterior 

 roots. If they do, it must be by few and ex- 

 tremely delicate fibres. It seems highly pro- 

 bable (although the demonstration of the fact is 

 attended with great difficulty) that the fibres of 

 the posterior roots have a similar disposition to 

 that described for the anterior, and that some 

 pass into the posterior horn of the grey matter, 

 and others are continuous with the longitudinal 

 fibres. 



Various conflicting statements have been 

 made by the anatomists who have written 

 upon the spinal cord, with regard to the actual 

 connection of the roots of the nerves with the 

 proper substance of the organ. Nor is this to 

 be wondered at, when we consider the great 

 delicacy of the investigation. It is very easy 

 to trace any set of filaments to the pia mater ; 

 but after they have passed beyond that covering, 

 the nervous fibres lose their main support and 

 their bond of union, and they separate from 

 each other. Their exquisite delicacy and mi- 

 croscopic size render any further dissection of 

 them extremely difficult. Mr. Grainger, in 

 his excellent treatise on the spinal cord,* re- 

 commends certain precautions which I have 

 adopted with advantage. The cord should be 

 examined immediately after death, as the delay 

 even of a few hours increases the softness of 

 the medullary substance. Great advantage is 

 derived from placing the cord, immediately 

 after its removal, in a very weak mixture of 

 alcohol and water, as by these means firmness 

 is given to the parts without rendering them 

 crisp and brittle, as happens if strong alcohol 

 be used. The parts should be dissected with 

 very fine instruments under water. " I have 

 met with most success," says Mr. Grainger, 

 " by dividing the pia mater at the median fis- 

 sure, and very cautiously raising it as far as the 

 lateral furrow, leaving its connection with the 

 fibres of the nerves intact ; it is then necessary 

 to open either the anterior or posterior lateral 

 fissure according to the root examined, at a 

 little distance above the exact place where the 

 nerve which is to be dissected is attached to 

 the cord, when by cautiously proceeding to 

 open the fissure, the threads which dip into the 

 grey matter are perceived." Mr. Grainger re- 

 commends the adoption of a similar mode of 

 dissection for the cranial nerves, care being 

 taken in every case not to disturb the connec- 



* Observations on the Structure and Functions 

 ot" the Spinal Cord, p. 37. Lond. 1837. 



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