228 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. 



minister. If the spinal cord of a Frog be divided in its back, above 

 the crural plexus, so as entirely to cut off the nerves of the lower ex- 

 tremities from connexion with the brain, the animal loses all voluntary 

 control over these limbs, and no sign of pain is produced by any injury 

 done to them. But they are not thereby rendered motionless ; for 

 various stimuli applied to the limbs themselves will cause movements in 

 them. Thus if the skin of the foot be pinched, or if a flame be applied 

 to it, the leg will be violently retracted. Or, if the cloaca be irritated 

 by a probe, the feet will endeavour to push away the instrument. We 

 have no reason hence to believe, that the animal feels the irritation, or 

 intends to execute these movements in order to escape from it; for 

 motions of a similar kind are exhibited by men, who have suffered injury 

 of the lower part of the spinal cord, and who are utterly unconscious, 

 either of the irritation which their limbs receive, or of the actions which 

 they perform. 



393. We are not to suppose, however, that the stimulus acts at 

 once upon the muscles, without the nervous system being concerned at 

 all ; throwing them into contraction by its direct influence. For it is 

 quite certain, that unless the nervous trunks remain continuous with 

 the spinal cord, and unless the part of the spinal cord with which they 

 are connected remains sound (although cut off from connexion with 

 the parts above, and with the brain), no action will result. If the 

 trunks be divided, or either of the roots by which they are connected 

 with the spinal cord be severed, or the lower portion of the spinal 

 cord itself be injured, no stimulation will cause the muscular move- 

 ments just described. A very good example of this necessity for the 

 completeness of the nervous trunks, which convey impressions to and 

 from the central organ, is found in the movements of the iris, for the 

 contraction and dilatation of the pupil. Here the stimulus of light upon 

 the retina gives rise to a change in the condition of the optic nerve ; 

 which, being transmitted to a certain portion of the encephalon with 

 which that nerve is connected, excites there a motor impulse ; and 

 this impulse is conveyed through a distinct nerve (a branch of the 

 third pair) to the iris, occasioning contraction of the pupil. Every one 

 knows that this adjustment of the size of the pupil to the amount of 

 light, is effected without any exertion of the will on his own part, and 

 even without any consciousness that it is taking place. It is performed, 

 too, during profound sleep ; when the influence of light upon the retina 

 excites no consciousness of its presence, when no sensation, therefore, 

 is produced -by it. 



394. The class of actions thus performed, is termed reflex; and we 

 see that every such action involves the following series of changes. 

 In the first place, an impression is made upon the extremity of a 

 nerve, by some external agent ; just as when sensation is to be pro- 

 duced. Secondly, this impression is transmitted by a nervous trunk 

 to the spinal cord in Vertebrata, or to some ganglionic mass which 

 answers to it in the Invertebrata. But instead of being communicated 

 by its means to the mind, and becoming a sensation, it immediately 

 and necessarily executes a motor impulse ; which is reflected back as it 

 were to certain muscles, and, by their contraction, gives rise to a 



