258 OF FOOD AND THE DIGESTIVE PROCESS. 



grinding teeth peculiarly adapted to its reduction ; their surface being 

 extended horizontally, and being kept continually rough, by the alter- 

 nation of vertical plates of different degrees of hardness ; and the lower 

 jaw being so connected with the skull, that great freedom of motion is 

 permitted. In Man we find an intermediate conformation, as regards 

 both the teeth and the articulation of the jaw ; for the molar teeth 

 possess broad surfaces which are covered with a continuous coat of 

 enamel, but which are raised into rounded tubercles ; and the articula- 

 tion of the jaw allows it a degree of freedom, which is much greater 

 than that possessed by the Carnivora ; although inferior to that which 

 exists in many Herbivora. The whole apparatus of Mastication is so 

 formed in Man, as to lead to the conclusion that he is destined to live 

 on a mixed diet, composed in part of animal flesh, and in part of vege- 

 table substances that are sufficiently soft to be reduced by the simple 

 act of crushing, or by the slight trituration for which the molar teeth 

 are adapted. 



452. The mechanical reduction of the food by Mastication, and the 

 incorporation of the Salivary secretion with its substance, constitute a 

 very important step in the Digestive process. We shall hereafter see 

 that the operations, to which the alimentary matter is subjected in the 

 stomach, are of a purely Chemical nature ; and this preparation is 

 exactly of the same character as that, which the Chemist finds it 

 advantageous to make, when he is operating on a substance of difficult 

 solution* For nothing is so favourable to the action of the solvent, as 

 the previous reduction of the matter to be dissolved, and its thorough 

 incorporation with the fluid that is to act upon it. We shall hereafter 

 see, that the relative properties of the Saliva and of the gastric fluid 

 are such, that, by the minute admixture of the food with the former, 

 the latter finds access to every particle of it. Hence the practice of 

 eating so rapidly, that Mastication and Insalivation are insufficiently 

 performed, is extremely injurious ; since it throws more work upon the 

 Stomach than it ought to perform, by rendering its solvent action more 

 difficult. There can be no doubt that, by the prolonged continuance of 

 it, a foundation is laid for the distressing complaint termed Dyspepsia, 

 or difficulty of digestion ; and where any form of this complaint exists, 

 too much attention cannot be paid to the efficient reduction of the food 

 in the mouth. 



453. When the aliment has been sufficiently triturated, it is con- 

 veyed into the Pharynx by the act of Deglutition or swallowing. 

 This act involves a great many distinct movements, into a minute 

 description of which we shall not here enter ; but it is desirable that 

 its general nature should be well understood. It is one of those most 

 purely reflex in its character ( 394), and is not capable of being per- 

 formed or even controlled by a voluntary effort. This statement may 

 seem inconsistent with the fact, that we swallow when we will ; but it 

 is not so in reality. The muscular movements which are concerned in 

 deglutition, are called forth by nerves that proceed from the spinal cord, 

 not from the brain ; these motor nerves are excited to action, by the 

 contact of solid or fluid matters with the mucous surface of the fauces, 

 and in no other way. The impression produced by the contact is 



