PHYSIOLOGY OF THE NERVOUS SYSTEM. 



720X 



the means of giving to the anterior root the 

 slight degree of sensitive power which Majendie 

 attributes to it. 



From the determination of the office of each 

 root of the spinal nerves we obtain the further 

 important result, that the nerve, which is formed 

 by the junction of these two roots, is sensitive and 

 motor, and that nervous fibres of different en- 

 dowments may be bound together in the same 

 sheath constituting one nerve, which is com- 

 pound in its functions. And the anatomical 

 distribution of spinal nerves, both in man and 

 the inferior animals, to the muscles and sen- 

 sitive surfaces of the trunk and extremities, is 

 entirely confirmatory of the results thus derived 

 from experiment. 



By the use of the various means for deter- 

 mining the functions of nerves, above detailed, 

 and aided by the determination of the law dis- 

 covered and developed by Bell and others, 

 as to the motor nature of the anterior and the sen- 

 sitive endowment of the posterior roots, and the 

 subsequent binding together of these fibres in one 

 sheath to form a compound nerve, physiologists 

 have made great advances in determining the 

 functions of the various encephalic nerves, and 

 our knowledge on this subject may be said to 

 have approached more to perfection than" that 

 of any other physiological questions. The main 

 facts connected with the anatomy and physio- 

 logy of each of these nerves will be found under 

 the articles headed by their names. 



Of the Junctions of the nervous centres. 

 In examining into the functions of the va- 

 rious parts of the cerebro-spinal axis I shall 

 adhere to the definitions already adopted in the 

 previous part of this article, and use the term 

 spinal cord as denoting the nervous cylinder 

 within the spinal canal, and the encephalon as 

 the intra-cranial mass, consisting of medulla 

 oblonguta, mesocephale, cerebellum, and cere- 

 brum. 



Of the functions of the spinal cord. It was 

 long held that the spinal cord was no more 

 than a bundle of nerves proceeding from or to 

 the brain, and emerging at various points of 

 the vertebral canal to be distributed to their 

 destined regions.* 



The anatomy of the organ, however, suffi- 

 ciently exposed the error of this opinion. The 

 existence of a large quantity of vesicular matter 

 in it varying in quantity according to the bulk 

 of its segments showed that it was more than a 

 mere fasciculus of nerves. Although the true 

 office of the spinal cord was known to physio- 

 logists long before, to Prochaska for example, 

 Gall appears to have been the first who ad- 

 duced the best proofs from anatomy to show 

 that the spinal cord was not a mere appendage 

 to the brain, but a special centre in itself. His 

 principal arguments were derived from the 

 want of any constant proportion in bulk be- 

 tween it and the brain, the spinal cord being 

 small with a large brain, as in man, and large 

 with a small brain, as in the inferior mammalia 

 and in other vertebrata, from the fact that it 

 does not taper gradually in proportion as it 



* That this was Willis's view a perusal of chap- 

 ters xviii. and xix. of his Ccrebri Anatome will 

 shew. 



gives off nerves, but on the contrary is alter- 

 nately large or small according to the number and 

 volume of the nerves which are given off from 

 its various segments; and, lastly, from the ana- 

 logy which he indicated between the spinal 

 cord of vertebrata and the ganglionic chain of 

 articulata, the former consisting of a series of 

 ganglia fused together, the latter remaining 

 separate by reason of the peculiar disposi- 

 tion of the bodies of these animals in distinct 

 segments. 



The determination of the functions of the 

 nerves which are intimately connected with or 

 implanted in the spinal cord affords some clue 

 to the solution of the problem as to its own 

 office. There can be no doubt that as the 

 nerves of sensation as well as those of motion 

 of the trunk and extremities are all, to say the 

 least, intimately connected with the cord, this 

 organ must be the medium of the reception 

 and propagation of the sentient impressions 

 made upon the one, and of the mental or phy-. 

 sical impulses which excite the others. 



If, moreover, we look to the results of expe- 

 riments on the lower animals, or to the effects 

 of injury or disease in the human body, we 

 obtain the following important facts : 1st, that 

 the perfect connexion of this organ, in all its 

 integrity, with the encephalon is the essential 

 condition for the full and complete exercise of the 

 nervous force, whether for sensation or voluntary 

 motion, as far as regards the trunk and extremi- 

 ties; 2nd, that division of the cord, so as com- 

 pletely to separate the lower from the upper 

 segment, causes paralysis both of sensation 

 and voluntary motion in the parts supplied with 

 nerves from the lower segment; 3rd, if the 

 section be made high up in the neck so as to 

 separate the cord from the medulla oblongata, 

 all the parts supplied by spinal nerves will be 

 paralysed in the same way ; by such an expe- 

 riment the spinal cord remains entire, but its 

 continuity with the encephalon is interrupted. 



In cases of injury to the vertebral column 

 it may be laid down as the rule that the higher 

 the seat of injury the more extensive will be 

 the paralysis. A man who has received exten- 

 sive injury of the spinal cord high up in the 

 neck is like a living head and a dead trunk, 

 dead to its own sensations, and to all voluntary 

 control over its movements. The same rule 

 prevails with regard to the effects resulting from 

 disease of the vertebra or from any intra-spinal 

 growth, or from a morbid state of the cord itself, 

 there being only this difference, that where the 

 morbid change is chronic, the paralytic effects 

 are less marked than in injury or acute disease. 

 In all cases the extent of the paralysis affords 

 a correct indication of the seat of the solution 

 of continuity. 



If the spinal cord be divided partially in 

 the transverse direction, there will be paralysis 

 of parts on the same side with the injury. 

 Dr. Yellowly has put on record an experi- 

 ment of Sir Astley Cooper's, in which he 

 divided the right half of the spinal cord in 

 a dog just above the first vertebra. The effect 

 was paralysis of the motions of the ribs on the 

 right side, and of the right posterior and pos- 

 terior extremities, with irritation of those of 



