b Nov., 1907.] Elements of Animal Physiology. 693 



rectum excites the centre for defsecation. But the centres are excitable 

 by other afferent nerves and by impulses coming down from the brain. 

 Thus, increased intestinal peristalsis may be caused by fright, and vomiting 

 in the human being mav be due to many factors other than those connected 

 with digestion. 



Voluntary control is possessed by the animal over the first stage of 

 swallowing and over the relaxation of the s-phincicr ani but all the other 

 nerve mechanisms are independent of the will ami are effected bv the 

 autonomic system. 



THE FUNCTIONS OF THE ALIMENTARY CANAL.— The first 

 important function of the alimentary canal is the digestion of certain 

 ingredients of the food by rendering them soluble, and soluble in such 

 a form that they can pass into the columnar epithelium of the mucous 

 membrane and so be absorbed. The second important function is the 

 absorption of the digested food. These are the fundamental functions 

 but thev do not exhaust the list. Thus the lower part of the alimentary 

 canal in all animals has excretory powers, i.e., it gets rid. to some extent, 

 of waste matter or foreign materials from the blood. Further, in many 

 animals, we find some part of the canal functioning as a reservoir of water 

 which can tide the animal over long marches between one drinking place 

 and another. 



DIGESTION IN THE MOUTH.— The shape and arrangement of 

 the teeth vary in different animals according to the food they are fitted 

 to eat and their manner of eating. The differences in the method of 

 chewing and mastication are fairly obvious and need not be described here. 

 In all mammals the tongue is a muscular organ covered with a rough and 

 thick layer of stratified epithelium ; it is richly supplied with end-organs 

 for touch, and at its back has special t-nd-organs for taste. 



The saliva is the mixed secretion of a number (generally three pairs) 

 of salivary glands. Its composition \aries according to the composition 

 of the substance in th.e mouth which has excited the flow. Thus sand or 

 an irritating chemical in the mouth excites a flow of very watery saliva 

 suitable for washing the substance away ; water excites the flow of a very 

 thick saliva rich in mucin, whereby the water is rendered semi-gelatinous — 

 generally speaking the saliva excited is the best for that particular sub- 

 stance which enters the mouth. Saliva keeps the mouth moist, and by 

 wetting the food can help the teeth and tongue to grind or roll the food 

 into a mass fit for swallowing. In the saliva of omnivores and herbivores, 

 especially w'hen these are eating foods containing starch, the saliva contains 

 an enzyme diastase or ftyalin which can convert the starch into the sugar 

 maltose. The main ingredients of saliva are water, over 99 per cent., 

 mucin to make the saliva adhesive and viscid, salts, and particularly lime 

 salts, so that the saliva shall not absorb away the lime of the teeth (in 

 fact lime salts are often deposited on the teeth as tartar) and diastase as 

 mentioned. In saliva are also found some round white cells or salivary 

 corpuscles, the function of which is not known. The functions of the 

 mouth as regards digestion are therefore the prehension of food ; the 

 mastication of food, by which the surface of the food is increased and the 

 food made into masses fit for swallowing; the selection of food by taste; 

 and the mixing of the food with saliva. 



No absorption of food occurs in the mouth. 



{To be continued.) 



