FUNCTIONS OF THE SPINAL CORD. 645 



rated by the hand, they will be found presently to return. Here, as in study- 

 ing the respiratory and other movements, we are led to perceive that it is 

 the Brain alone which is torpid during sleep, and whose functions are affected 

 by this torpidity. As Dr. M. Hall very justly remarks, "the Spinal system 

 never sleeps;" it is constantly in activity ; and it is thus that, in all periods 

 and phases of Life, the movements which are essential to its continued 

 maintenance are kept up without sensible effort. The closure of the pupil 

 against a strong light, is another movement of the same protective tendency. 

 The contraction of the pupil is immediately caused by the Third pair, or 

 Motor Oculi, as is easily shown by irritating the trunk of that nerve and 

 observing the result ; but the stimulus which excites it is conveyed through 

 the Optic nerve. Yet although the contraction of the pupil is usually in 

 close accordance with the sensation occasioned by the impression of light 

 upon the retina, yet there is evidence to prove that the sensation of light is 

 not always necessary ; for even when the sight of both eyes has been entirely 

 destroyed by amaurosis, the normal actions have been witnessed in the pupil, 

 in accordance with the varying degrees of light impinging on the retina. 

 Such cases seem to indicate that the motion results from an impression upon 

 the retina, which impression being conducted to the Sensorium ordinarily 

 produces a sensation ; but that even where no sensation is produced, on ac- 

 count of a disordered state of the part of the ganglionic centre in which the 

 Optic nerve terminates, if the central tract which connects that nerve with 

 the Third pair retains its integrity, the reflex contraction of the pupil may 

 still be excited through it. The rarity of the occurrence is easily accounted 

 for, by the fact that in most cases of amaurosis the disease lies in the retina 

 or in the trunk of the nerve, and thereby checks both its spinal and its 

 encephalic actions. Although we are not at present acquainted with any 

 similar protective movements in the Human being, designed to keep the 

 organ of Hearing from injury, yet there can be little doubt that those which 

 we are constantly witnessing in other animals possessed of large external 

 ears, are reflex actions excited by the irritation applied to them. In regard 

 to the Nose, we find a remarkably complex action that of Sneezing- 

 adapted to drive off any cause of irritation ( 304). The stimulus is con- 

 veyed, in this case, not through the Olfactory nerve, but through the Fifth 

 pair ; so that it is not dependent upon the excitement of the sensation of 

 Smell. The act of Coughing, also, may be regarded as of a protective char- 

 acter ; being destined to remove sources of irritation from the air-passages. 

 Many of the automatic movements performed by the limbs of Frogs and 

 other animals, when their connection with the brain has been cut off ( 501) 

 appear destined to remove these parts from sources of irritation or injury ; 

 and they may thus be rightly placed under the same category. 



509. The fact that Sensation is very commonly associated ivith the reflex 

 action we have been considering, being produced by the impression that ex- 

 cites them, has led many to suppose that it necessarily participates in them; 

 a doctrine which we have seen to be untenable. But the question not 

 unnaturally arises, why Sensation should so constantly participate in these 

 operations, if not essential to them ; and the answer to this question is to be 

 found in the fact, that it is only through sensation that a higher set of ac- 

 tions, mental and bodily, is called into play, which is essential to the con- 

 tinued maintenance of those belonging to the present category. Illustrations 

 of this truth might be drawn from any of the functions already noticed ; 

 but the iugestion of food will supply us with one of the most apposite. 

 We have seen that the act of Deglutition is in itself independent of sensa- 

 tion ; anything that comes within the grasp of the pharyngeal constrictors 

 being conveyed downwards by their reflex contraction, just as anything 



