376 



Digestion. 



Vol. XI. 



Digestion. 



Hunger and thirst are the preliminary 

 steps to digestion; they constitute a law im- 

 planted in the animal economy, for the pur- 

 pose of inducing the living being to take 

 such nourishment as is required to sustain 

 that waste of the system which animated 

 nature is continually undergoing. If the 

 dictates of the sensation of hunger and thirst 

 are rationally obeyed, satisfaction and healthy 

 digestion are the result; but if, on the con- 

 trary, these important sensations are ne- 

 glected, weakness and disease must necessa- 

 rily ensue. Appetite, or, in its more ad- 

 vanced stage, hunger, teaches animals to 

 seek for solid food, and thirst suggests the 

 propriety of rendering the solid mass more 

 pulpy and dilute by the employment of drink. 

 Experience and reason, both in man and 

 brutes, must in some measure direct the se- 

 lection of the proper objects to be employed 

 for these purposes. I was some years ago 

 consulted by a worthy individual with regard 

 to the propriety of fasting as a religions ob- 

 servance. I told him that the sensation of 

 hunger and thirst constituted a most import- 

 ant law in the animal economy, destined by 

 the Creator for the most beneficent purposes; 

 that it ought to be obeyed as a matter of duty, 

 and that if infringed, some prejudicial result 

 vifould necessarily ensue; because it is no 

 argument in favour of any such experiment 

 upon human life that existence does not ter- 

 minate upon its adoption, or that the symp- 

 toms of some frightful disease are not in- 

 stantly ushered in. The seeds of future 

 mischief may be sown by one experiment, 

 and may only lie dormant until a second or 

 succeeding infringement shall cause them 

 to spring forth into living activity. In the 

 course of extensive series of experiments 

 upon cows, it was found that, when they 

 were not supplied with sufficient food during 

 one day, the product of milk was a day or 

 two in reaching its former average; thus 

 demonstrating that the animal had been 

 weakened by the abstinence, inasmuch as it 

 took a longer period to reach its ordinary 

 condition than was required to reduce it. 

 The milk, in such an experiment, corres- 

 ponds with the muscle and fatty portions of 

 the body of animals which do not supply 

 milk; hence abstinence in all animals must 

 be followed by a diminution of the weight 

 of the body. It has been well remarked by 

 Liebig, that " in the process of starvation it 

 is not only the fat which disappears, but also 

 by degrees all such of the solids as are ca- 

 pable of being dissolved. In the wasted bo- 

 dies of those who have suffered starvation, 

 the muscles are shrunk and unnaturally soft, 



and have lost their contractility: all these 

 parts of the body which were capable of en- 

 tering into the state of motion have served 

 to protect the remainder of the frame from 

 the destructive influence of the atmosphere." 

 There is no difference in this respect be- 

 tween one set of animals and another. Civ- 

 ilized and savage men, wild and domestic 

 animals, must all be classed under the same 

 category. 



In the human species a morsel of food is 

 grasped by the front teeth of both jaws, 

 which are each supplied with sixteen teeth, 

 making thirty-two in all. In those animals 

 which chew the cud, as they have only one 

 row of teeth the food is less firmly grasped 

 by the jaws, and there is, therefore, a greater 

 necessity that it should be of a sofl and pli- 

 able nature. By the assistance of the lips, 

 jaws, tongue, and auxiliary muscles, the food 

 is conveyed into the cavity of the mouth, and 

 by the aid of the tongue and lateral motion 

 of the mouth it is placed between the op- 

 posing jaws, where it is masticated or ground 

 to a proper consistence. But the action ot 

 the jaws in grinding the morsel introduced 

 between them at the same time elicits the 

 compressing power of the muscles of the 

 cheek upon the parotid gland, which is situ- 

 ated in man in front of the ear, and expels 

 its secreted fluid, the saliva, into the mouth, 

 to assist in comminuting the nutritive mat- 

 ter. Besides this mechanical action, there 

 is, however, a nervous sympathy called into 

 operation. The masticated matter acts upon 

 the tongue and adjacent parts, inducing a 

 sympathy with the glands placed under the 

 tongue, and causes them to pour out their 

 copious contents. The object of mastica- 

 tion or chewing is, therefore, to reduce the 

 food to such a consistence as shall fit it for 

 its reception and proper digestion in the sto- 

 mach. This is well illustrated in the in- 

 stance of animals which are not supplied 

 with teeth. 



The common fowl, for example, is desti- 

 tute of these grinding apparatus ; but it has 

 a muscular mechanism, termed the gizzard, 

 which powerfully compresses the introduced 

 food, and by means of pebbles and stones, 

 which are a necessary article of food with 

 the class of animals referred to, an artificial 

 substitute for the teeth is provided. In 

 graminivorous animals, we shall find that a 

 substitute for the second row of teeth is pro- 

 vided in the operation of rumination, or 

 chewing the cud. From attention to these 

 facts, therefore, we are taught that the pre- 

 paratory step of digestion consists in the 

 fine division of solid food by means of the 

 apparatus set apart in the mouth for this 

 purpose, and its mixture with a certain 



