400 OF FOOD, AND THE DIGESTIVE PROCESS. 



liable to derangement. Such derangement we continually meet with; for there 

 is not, perhaps, a more frequent source of Dyspepsia than imperfect mastication, 

 whether resulting from the haste with which the food is swallowed, or from the 

 want of the instruments proper for the reducing operation. The mechanical dis- 

 integration of the food is manifestly aided by Insalivation ; but the admixture 

 of Saliva also exerts, as we shall hereafter see ( 439), a very marked influence 

 on the chemical composition of certain of its constituents. The movements of 

 Mastication, still more than those already adverted to, although under the com- 

 plete control of the Will, and originally dependent upon it for their excitation, 

 come at last to be of so habitual a character, that they continue when the direct 

 influence of the will is withdrawn, the influence of the " guiding sensation," 

 however, being essential to their performance. 1 Every one is conscious that the 

 act of mastication may be performed as well when the mind is attentively dwell- 

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

 are rather apt to go on chewing and rechewing what is already fit to be swal- 

 lowed, 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. This 

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

 is necessary that it should be guided by the sensation which there results from 

 the contact 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 sup- 

 plied by the Facial nerve ; and yet, if the former be paralyzed, the latter can- 

 not stimulate them to the necessary combined actions. Hence we see that the 

 movements are of an associated character, their due performance being depend- 

 ent on the part of the nervous centres from which the motor influence origi- 

 nates. 3 If the Fifth pair, on the other hand, be uninjured, whilst the Portio 

 Dura is paralyzed, the movements of Mastication are performed without diffi- 

 culty; whilst those connected in any way with the Respiratory function, or with 

 Expression, are paralyzed. If, again, the sensory portion of the Fifth pair be 

 paralyzed, the act of Mastication is very imperfectly performed, even though 

 the motor power be not in the least impaired ; for the muscles cannot be made 

 to perform the requisite associated movements without the guidance of sensa- 

 tions; so that the morsel lodges between the teeth and the cheek, or beneath the 

 tongue, and can with difficulty be kept in the appropriate position. 



1 Thus, in the curious case formerly referred to ( 373, note), food can only be admin- 

 istered by carrying back the spoon containing it, until it touches the fauces and thus ex- 

 cites an act of deglutition. Sensation being here entirely deficient, there is nothing to 

 excite or to guide the movements of the muscles of the mouth and tongue. 



2 Comparative Anatomy furnishes the key to these phenomena, which seem at first sight 

 to be somewhat strange. Among Invertebrate animals generally, the Respiratory organs 

 are completely unconnected with the mouth ; and a very distinct set of muscles is provided 

 to keep them in action. These muscles have separate ganglia as the centres of their opera- 

 tions ; and these ganglia are only connected indirectly with those of the sensori-motor 

 system. The same is the case, in regard to the introduction of the food into the digestive 

 apparatus. The muscles concerned in this operation have their own centres the Stomato- 

 gastric and Pharyngeal ganglia, which are not very closely connected with the cephalic, 

 or with the respiratory, or with those of general locomotion. Now in the Vertebrata, the 

 distinct organs have been so far blended together, that the same muscles serve the purposes 

 of both; but the different sets of movements of these muscles are excited by different 

 nerves ; and the effect of division of either nerve is to throw the muscle out of connection 

 with the function to which that nerve previously rendered it subservient as much as if 

 the muscle were separated from the nervous system altogether. 



