40 OF DIGESTION. 



the urine, and 8.33 are re-excreted into the faeces. As a general result, 

 it follows, from these experiments, that an adult man oxidizes, on an 

 average, 289 grammes of carbon, and 18.6 grammes of hydrogen in 

 twenty-four hours. 



CHAPTER III. 



OF DIGESTION. 



TISSUE-MAKING OR HISTOGENETIC DIGESTION. 



Nature of Digestion. The Mouth, Teeth, Stomach. The Salivary Glands. Different Kinds of 

 Saliva. Properties of mixed Saliva : its Quantity, Composition, and Functions. Relation of 

 the Salivary Glands and Kidneys. The digestive Tract. The Stomach. Gastric Juice. 

 Organs for its Preparation. Manner of producing Chyme. Influence of the Nerves. Artifi- 

 cial Digestion. Preparation and Properties of Pepsin. Regional and functional Divisions of 

 the Stomach in Animals and in Man. Object of Stomach Digestion. Peptones. Use of Salt. 

 Digestibility of various Articles of Food. 



BEFORE the food can be absorbed and carried to all parts of the sys- 

 Nature of t em it must be submitted to certain preparatory operations, 

 digestion. gi nC e it is either to be dissolved in the blood or transported as 

 chyle through the lacteal vessels, it is absolutely necessary to bring it 

 into a condition of solution in water, or at least into a state of minute 

 suspension in that liquid. Eeceived in masses of a certain size, it is 

 first cut and crushed into smaller portions by the teeth, and then brought 

 from an insoluble into a soluble or suspended state by the chemical ac- 

 tion of the digestive juices. 



In the mouth the food is submitted to a twofold preparation. It is 

 Functions of divided by the mechanical action of the teeth, and also simul- 

 the mouth, taneously mingled with liquids secreted from the salivary 

 glands. 



The animal series present us with numberless contrivances for accom- 

 plishing this comminution. The teeth, though of a bony nature, are not 

 to be regarded as appertaining to the skeleton, but rather to the digestive 

 mechanism. Their structure, number, and position differ veiy much in 

 different tribes. In certain fishes the mouth is almost lined with them. 

 In crabs they extend to the stomach, but in other cases they are restrict- 

 ed to the pharynx, or are wholly absent ; this being the case, for instance, 

 among the ant-eaters. Those insects whose food is of a fluid nature have 

 Instruments of no need of teeth ; but those which use solid material are ac- 

 in^ariou^ani- comm dated with suitable instruments of abrasion, such as 

 mals. borers, chisels, saws, nippers, the particular mechanism re- 



