300 FUNCTIONS OF THE NERVOUS SYSTEM. 



4 



experiment, that the continuity of the cesophageal branches of the Par Vagum 

 with the Spinal Cord, is necessary for the rapid propulsion of the food; so 

 that it can scarcely be doubted, that an impression made upon the mucous 

 surface of the oesophagus, conveyed by the afferent fibres of these nerves to 

 the Medulla Oblongata, 'and reflected downwards along the motor fibres, is 

 the real cause of the muscular contraction. If the Par Vagum be divided in 

 the rabbit, on each side, above the cesophageal plexus, but below the pharyn- 

 geal branches, and the animal be then fed, it is found that the food is delayed 

 in the oesophagus, which becomes greatly distended. Further, if the lower 

 extremity of the par vagum be irritated, distinct contractions are seen in the 

 O3sophageal tube, proceeding from above downwards, and extending over the 

 cardiac extremity of the stomach. We have here, then, a distinct case of 

 reflex action without sensation, occurring as one of the regular associated 

 movements in the natural condition of the animal body ; and it is very inte- 

 resting to find this following upon a reflex action with sensation (that of the 

 pharynx), and preceding a movement which is altogether unconnected with 

 the Spinal Cord (that 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 remarked, the simple contractility of the muscular fibre occa- 

 sions a distinct peristaltic movement along the tube, after its nerves have been 

 divided; causing it to discharge its contents, when cut across. Such a move- 

 ment, indeed, seems to take place in something of a rhythmical manner (that 

 is, at short and tolerably regular intervals), whilst a meal is being swallowed; 

 but as the stomach becomes full, the intervals are longer, and the wave-like 

 contractions less frequent. These movements are reversed in Vomiting; and 

 this reversion has been .observed, even after the separation of the stomach 

 from the oesophagus, as a consequence of the injection of tartar emetic into 

 the veins. 



a. 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 Mastication 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 con- 

 trolling influence over them, each individual act being directly excited by sensation. 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 rechewing what is already fit to be swallowed, 

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

 wards within the reach of the muscles of deglutition. We now see why sensation should 

 be associated with the latter process, though not essential to it. The conveyance of food back- 

 wards to the fauces is a distinctly voluntary act; and it is necessary that it should bo guided 

 by the sensation, which there results from the contact it induces. If the surlace 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. It' t!u> Fifth pair, on the other hand, he uninjured, whilst the Portio 

 Dura is paralyxed, the movements of Mastication are porliirmod without difficulty; whilst 

 those connected in any way with the Respiratory function, or with Expression, are para- 

 lyzed. 



* Third Memoir on the Nervous System, 201. 



