128 OF FOOD, AND THE DIGESTIVE PROCESS. 



of "guiding sensations" for their performance is made evident by one of Sir 

 C. Bell's experiments, the wrong interpretation of whose results originally 

 led him to an erroneous view of the functions of the Fifth pair of nerves. 

 He found that an Ass, in which the infraorbital branch of this nerve had 

 been divided, made no attempt to pick up oats with its lip, although the 

 animal saw them, bent down its head with the obvious purpose of ingesting 

 them, and brought its lip into absolute contact with them; hence he con- 

 cluded that the power of motion, was destroyed in the lip, when it was in 

 reality only the guiding sensation that was deficient, the motor power being 

 supplied by the Facial nerve or Portio dura. 



86. The food thus introduced into the mouth, is subjected (unless it be 

 already in a state which needs no further reduction) to the process of Mastica- 

 tion. This is evidently an operation of great importance, in preparing the sub- 

 stances to be afterwards operated on for the action of their solvent; and it 

 exactly corresponds with the trituration to which the Chemist would submit 

 any solid matter, that he might present it in the most advantageous form to a 

 digestive menstruum. The complete disintegration of the alimentary matter 

 is, therefore, of great consequence ; and, if imperfectly effected, the subsequent 

 processes are 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 food is 

 swallowed, or from the want of the instruments proper for the reducing 

 operation. The mechanical disintegration of the food is manifestly aided 

 by Insalivation ; but the admixture of Saliva also exerts, as we shall here- 

 after see, 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 complete control of the Will, and originally 

 dependent upon it for their excitation, come at last to be of so huhitnal a 

 character, that they continue when the direct influence of the will is with- 

 drawn, the influence of the guiding "sensation," however, being essential to 

 their performance. 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, we are rather apt to 

 go on chewing and rechewing what is already fit to be swallowed, simply be- 

 cause 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 neces- 

 sary 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 sen- 

 sation, 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 

 depressing the lower jaw are the genio-hyoid, the mylo-hyoid, and the anterior 

 belly of the digastric. The omo-hyoid, sterno-hyoid, and thyro-hyoid muscles, 

 also aid. The upward movement is effected by the combined action of the 

 masscter, temporal and internal pterygoid muscles. The forward movement 

 is due to the action of both pterygoids, and the lateral or grinding movement 

 to the external pterygoid of one side. The Tongue is moved downwards and 

 forwards by the hyo-glossus and geuio-glossus, upwards and backwards by the 

 stylo-glossus, whilst its changes of form are due in great measure to its intrinsic 

 longitudinal and transverse fibres. The more important muscles concerned in 



(Lancet, Nov. 15th and 29th. ISir, i, which fully confirms the view here advanced; 

 the movements, not merely of the lips and jaws, but those by which food was con- 

 veyed to the mouth, having been carried on <tit1t>inti<'.<illy, when once (so to speak) 

 the spring was touched by which they were set in action. 



