PHYSIOLOGY OF THE NERVOUS SYSTEM. 



722o 



altogether, notwithstanding the exciting influ- 

 ence of the impression. The true explanation 

 seems to be, that the mind can exert upon the 

 vesicular matter a power which can prevent 

 the exercise of that change, or neutralise the 

 change, without which the motor fibres will 

 not be affected by a physical stimulus. 



Reflex actions are more manifest in some 

 situations than others: thus, in cases of hemi- 

 plegia from diseased brain, they are generally 

 very obvious in the lower extremity, but to- 

 tally absent in the upper. This, the advocates 

 of the excito-motory theory ascribe to a paucity 

 of excito-motory fibres in the latter limb, and 

 to a larger amount of them in the former. Or, 

 it has been attributed to the greater and 

 more enduring influence of shock upon that 

 segment of the cord from which the nerves of 

 the upper extremities arise, as nearer the seat 

 of lesion, than upon the lumbar segment. But 

 another explanation of this important fact may 

 be offered, which is equally satisfactory, and 

 more accordant with other phenomena. A 

 certain disposition of the nerves upon the tegu- 

 mcntary surface is as necessary for the develope- 

 nient of reflex actions as of sensations ; and 

 these movements will be more or less easily 

 manifested, according as this organization of 

 the nerves on the surface is more or less perfect. 



That disposition of the cutaneous nerves 

 which renders the surface easily excitable by 

 titillation seems most favourable to the deve- 

 lopement of these actions. Hence, there is no 

 place where they are more readily excited than 

 in the lower extremities by stimulating the soles 

 of the feet or the intervals between the toes, 

 both of which situations are highly susceptible 

 of titillation. At the isthmus faucium the 

 slightest touch on the surface excites a move- 

 ment of deglutition; and this touch, at the same 

 time, produces a very peculiar sensation of 

 tickling, quite distinct from that which may 

 be excited at other parts of the pharynx, or 

 mouth. When this part of the mucous mem- 

 brane is in a state of irritation as an effect of 

 coryza, this tickling sensation is present, and 

 repeated acts of swallowing are provoked. 



Two facts may be stated here, which illus- 

 trate the position above laid down respecting 

 the necessity of a certain disposition of the 

 nerves on the tegumcntal surface, for the de- 

 velopement of reflex actions. The first is one 

 which has been noticed by Volkmann, and 

 which I have repeatedly observed, namely, that 

 in frogs, and other animals, reflex actions are 

 readily excited by stimulating the feet ; but 

 irritating the posterior roots of the spinal nerves, 

 which supply those parts, is not sufficient for 

 this purpose. I have already remarked that in 

 numerous experiments upon the posterior roots 

 of the nerves movements have not been ex- 

 cited whilst they have been subjected to irrita- 

 tion, except when galvanism was employed, 

 which, being diffused, affected the cord itself: 

 the recorded statements of most modern ex- 

 perimenters agree in the main with this state- 

 ment. The second fact is this : in the male 

 frog the developement of a papillary structure 

 on the skin of the thumb seems to have refe- 



rence to the excitation of the physical power of 

 the cord, to enable the animal to grasp the 

 female without the necessity of a prolonged 

 exercise of volition. Stimulating the fingers 

 will scarcely produce reflex actions, but the 

 slightest touch to the enlarged thumb will 

 cause the animal to assume the attitude of 

 grasping. If the papillae be shaved off the 

 thumb, its power of exciting these actions 

 is instantly lost. 



When the polarity of the cord is greatly ex- 

 cited by strychnine or other substances, or 

 when tetanus exists, all parts of the surface 

 are equally capable of exciting reflex actions. 

 The least touch will cause them, not only in 

 the limb touched, but in all that side of the 

 trunk, or even throughout the whole body. 

 So general is the excitation, that the least im- 

 pression made on the peripheral extremity of 

 a sensitive nerve in any part of the body is 

 instantly converted into muscular spasm, more 

 or less general. A slight current of air, in 

 tetanus, is sufficient to excite general spasm. 

 Miiller remarks that, in such states of the 

 cord, the reflex actions excited by stimulating 

 the nerves themselves are much less than those 

 produced by excitation of the surface. 



The readiness with which a physical change, 

 induced in one part of the centre, is propagated 

 to others, whether above or below it, is due no 

 doubt to the vesicular matter. An experiment 

 made by Van Deen illustrates this statement. 

 If, in an animal poisoned by strychnine, the 

 cord be divided in its entire length along the 

 median line, leaving only a slight bridge of 

 grey matter, stimuli applied to any part of the 

 surface will exhibit as extensive reactions as if 

 the cord were entire. It is evident that the only 

 medium of communication between the oppo- 

 site halves must be the small portion of vesi- 

 cular matter left undivided. 



Impressions conveyed to the cord by the pos- 

 terior roots of any of its nerves, may be reflected 

 to the corresponding motor nerves, and cause 

 movement, or may extend irregularly along the 

 posterior horns of grey matter and stimulate 

 the nerves implanted in them, and thus give 

 rise to new sensations, which may be referred 

 to other and even distant parts of the body or 

 to new motions. 



The hypothesis under consideration affords 

 us an explanation, more satisfactory than any 

 other, of the paralytic state of the sphincter 

 ani in brain disease, already referred to, as well 

 as in that of the spinal cord. This muscle is 

 certainly chiefly under the influence of the will. 

 In ordinary cases of diseased brain, where the 

 lesion is confined to one side, the centre of 

 volition is not sufficiently impaired to affect its 

 influence upon the sphincter. In graver lesions, 

 however, although the will may still continue 

 to exert its control upon one side of the body, 

 it loses its power over the sphincter, which is 

 not excitable by any stimulus. In disease of 

 the spinal cord, there is paralysis of the sphinc- 

 ters if the lesion involve asufficient portion of the 

 cord's substance, in whatever region of the cord it 

 may exist. Even when the lesion is situate high 

 up in the neck, or in the dorsal region, leaving 



