PHYSIOLOGY OF THE NERVOUS SYSTEM. 



722c 



all continued up into the brain, and the (so- 

 called) excito-motor fibres are implanted in the 

 cord, thai segment of the cord should be the 

 largest in which the greatest number of these 

 fibres is to be found. Now the great extent of 

 excitor surface in the lower extremities, the 

 magnitude of their muscles, the importance of 

 their movements, and, at the same time, the 

 great developement of reflex actions in them, 

 would lead most reasonably to the expectation 

 that the lumbar segment of the cord to which 

 these nerves belong should exceed considerably 

 in size the cervical segment which gives nerves 

 to the upper extremities, where the excitor sur- 

 face is of less extent, where the muscles are less 

 powerful,and the reflex actions considerably less 

 conspicuous. Moreover,the lumbar region of the 

 cord would be, if Dr. Hall's views were correct, 

 the centre of those excito-motor acts connected 

 with defsecation, micturition, parturition, &c., of 

 which he speaks so much, and on this account 

 might fairly claim a greater amount of substance. 

 But the fact is, that the lumbar swelling of the 

 cord is smaller than the cervical ; and that while 

 it contains, and owes its bulk mainly to, a large 

 quantity of vesicular matter, but a small pro- 

 portion of fibrous matter is found in it. More- 

 over, it is impossible to understand the great 

 superiority of size of the lumbar portion over 

 the dorsal segment of the cord, if we are to 

 admit that this latter segment contains in addi- 

 tion to its own fibres (sensori-volitional and 

 excito-motory) the sensori-volitional fibres of 

 the lumbar swelling also, which ought to be 

 very numerous. 



It is very generally admitted that the only 

 channel by which the will can influence the 

 spinal cord is through the fibres of the anterior 

 pyramids of the medulla oblongata, the greater 

 number of which decussate each other along 

 the median line. But it is in the highest de- 

 gree improbable that these fibres, occupying so 

 small a space as they do, should form the ag- 

 gregate of the volitional fibres (still less of the 

 sensori-volitional fibres) of the trunk and extre- 

 mities. The whole of these fibres (of both 

 sides) collected together would scarcely equal 

 in bulk the anterior portion of one of the antero- 

 lateral columns of the spinal cord. 



It has been affirmed that much support is 

 given to the excito-motory hypothesis by Dr. 

 Carpenter's and Mr. Newport's supposed de- 

 monstration of the two sets of fibres in the Arti- 

 culata. But these observations are far from 

 deserving the name of demonstration. The 

 inferences from them are derived from the 

 apparent direction of certain fibres, and not 

 from any actual tracing of them by dissection 

 or by microscopic inspection. The observa- 

 tions, too, have been made with low powers, 

 which are very insufficient for determining the 

 precise disposition of the fibres and their rela- 

 tion to the vesicular matter of the ganglia. 



These writers affirm that the longitudinal 

 fibres of the ganglionic chain of Articulata 

 pass up to the brain and constitute the sensori- 

 volitional fibres, whilst other fibres pass in a 

 transverse direction and are implanted in the 

 ganglia. Were this the case, it might reason- 



ably be expected that the brain would be the 

 largest of the ganglia as containing the sum of 

 the sensori-volitional fibres of the whole body. 

 But let any one compare the size of the cere- 

 bral ganglia of the scorpion (as figured by 

 Mr. Newport*) with the size of the animal 

 and that of its cord, and it will be evident to 

 him how disproportionately small such a centre 

 is to the number of sensori-volitional fibres 

 which must be distributed over so large a sur- 

 face and to so many muscles. Anatomy, how- 

 ever, offers no objection to the hypothesis that 

 the roots of the nerves are implanted in the 

 ganglia, and that the longitudinal fibres act 

 as commissures between different segments 

 (adjacent and remote) of the cord. 



Neither do Mr. Newport's experiments on the 

 myriapods and other Articulata throw any new 

 light on the question of the existence of two 

 orders of fibres ; nor do they add anything to 

 our knowledge beyond the important fact that 

 actions take place in certain Invertebrata after 

 decapitation, which are of the same nature with 

 those which occur in Vertebrata after a similar 

 mutilation. The mechanism of these actions 

 has not been at all elucidated by these expe- 

 riments. 



The excito-motory hypothesis is sufficient for 

 the explanation of the movements of decapi- 

 tated animals, of parts in connection with small 

 segments of the spinal cord, of limbs paralysed 

 to sensation and voluntary motion from dis- 

 eased brain or spinal cord. But there are two 

 phenomena familiar to those who observe dis- 

 ease with care, which cannot be explained by 

 it ; these are the movements which may be ex- 

 cited by mental emotion in limbs paralysed to 

 the influence of the will, and the total paralysis 

 of the sphincter ani, which frequently accom- 

 panies diseased brain, whilst at the same time 

 the limbs aie only affected to a partial extent or 

 not at all. 



Cases occur sometimes in which hemiplegia 

 arises from an apoplectic clot, or other destruc- 

 tive lesion in one hemisphere of the brain. The 

 arm and leg, or either of them, are completely 

 removed from the influence of the will; yet 

 occasionally, as the effect of some sudden emo- 

 tion, fear, joy, surprise, the paralysed limb is 

 raised involuntarily. Even so slight a cause as 

 yawning (an act of emotional kind) will excite 

 the palsied limb. Every time the patient yawns 

 the arm will be raised involuntarily. 



Such phenomena as these receive no ade- 

 quate explanation from the excito-motory hypo- 

 thesis. Mental emotions probably affect some 

 part of the brain ; if the only communica- 

 tion between the brain and the limbs be by 

 fibres of sensation and volition, it is impossible 

 to understand how the emotional influence 

 could be conveyed to them through a channel 

 which has long been interrupted. If we are to 

 adopt the excito-motory hypothesis, it will be 

 necessary to suppose with Dr. Carpenter the 

 existence of certain emotional fibres to explain 

 the phenomena of this particular case. But it 

 is difficult to admit the existence of three orders 



* Phil. Trans. 1844. 



