148 FUNCTIONS OF THE NERVOUS SYSTEM. 



of the lower part of the alimentary canal.) The use of sensation in the former 

 case will presently appear. The muscular fibres of the oesophagus are also 

 excitable, though usually in a less degree, by direct stimulation ; for it appears 

 that, in some animals (the Dog, for example), section of the pneumogastric 

 does not produce that check to the propulsion of the food which it occasions 

 in the Rabbit ; and even in the Rabbit, as Dr. M. Hall* has lately remarked, 

 the simple contractility of the muscular fibre occasions a distinct peristaltic 

 movement along the tube, after its nerves have been divided ; causing it to 

 discharge its contents, when cut across. t 



195. It will be desirable here to revert for a short time to the actions which, 

 in the higher animals, precede those of Deglutition. There can be no doubt 

 that, in the Human being, the motions adapted to the ingestion and mastica- 

 tion of aliment originally result, in part at least, from distinct operations of the 

 Will ; but it would appear almost equally certain, that, in time, they come to 

 be of so habitual a character, that the will only exerts a general controlling 

 influence over them, each individual act being excited through the shorter 

 channel already alluded to ( 183). Every one is conscious that the act of 

 mastication may be performed as well, when the mind is attentively dwelling 

 on some other object, as when directed to it; but, in the former case, one is 

 rather apt to go on chewing and re-chewing what is already fit to be swallowed, 

 simply because the will does not exert itself to check the action, and to carry 

 the food backwards within the reach of the muscles of deglutition, We now 

 see why sensation should be associated with the latter process. The convey- 

 ance of food backwards to the fauces is a distinctly voluntary act ; and it is 

 necessary that it should be guided by the sensation there resulting from the 

 contact which it induces. If the surface of the pharynx were as destitute of 

 sensation as is the lower part of the oesophagus, we should not know when 

 we had done what was necessary to excite its muscles to operation. The 

 muscles concerned in the Mastication of food are nearly all supplied by the 

 third branch of the Fifth pair, a large proportion of which is well known to 

 have a motor character. Many of these muscles, especially those of the 

 cheeks, are also supplied by the portio dura of the Seventh ; and yet, if the 

 former be paralyzed, this cannot stimulate them to the necessary combined 

 actions. Hence we see that the movements are of an associated character, 

 their due performance being dependent on the part of the nervous centres from 

 which the motor influence originates. If the Fifth pair, on the other hand, be 

 uninjured, whilst the Seventh is paralyzed, the movements of Mastication are 

 performed wUhout difficulty ; whilst those connected in any way with the 

 Respiratory function, or with Expression, are paralyzed. 



196. Comparative anatomy supplies us with the key to the explanation of 

 these phenomena. It has been seen that, in the lower animals, the Respira- 

 tory organs are completely unconnected with the mouth, and that a very 



* Third Memoir on the Nervous System, 201. 



f There are many cases in which this direct contractility does not manifest itself in 

 the ordinary condition of the system, but in which it becomes evident when the muscu- 

 lar structure has gained an increase of irritability by diseased action, as we frequently 

 have to notice in the intestinal canal. For example ; in many cases of disease or injury 

 of the Spinal Cord, the bladder ceases to expel its contents, through the interruption of 

 the circle of reflex actions hereafter to be described; but, after a time, it ceases to become 

 necessary to draw off the urine by the catheter; for the fluid is constantly expelled, as 

 soon as it has accumulated in small quantities. In such cases, the mucous coat is found 

 after death to be thickened and inflamed; and the muscular coat is greatly increased in 

 strength, and contracted upon itself. Here, then, the muscularicoat, which is not excited 

 to contraction as long as the mucous coat is in a healthy condition, acquires a degree of 

 abnormal irritability which is sufficient to enable it of itself to expel the urine ; but this 

 could not be the case, unless it had originally been possessed of independent contractility. 



