i STRUCTURE AND PHYSIOLOGY OF ANIMALS 33 



A skeleton, whether internal or external, is usually composed 

 of a number of pieces which are movably articulated together, 

 and which thus constitute a system of jointed levers on which the 

 muscles act. 



The alimentary or digestive system consists of a cavity or 

 system of cavities into which the food is received, in which it is 

 digested, and through the wall of which the nutrient matters are 

 absorbed ; together with certain glands. 



In the lowest groups in which a distinct alimentary or enteric cavity 

 is present it is not distinct from the general cavity of the body ; 

 but in all higher forms there is an enteric canal which is sus- 

 pended within the cavity of the body, and the lumen of which is 

 completely shut off from the latter. It may have simply the form of 

 a sac or bag with a single opening which serves both as mouth and 

 anus ; in other cases the sac becomes branched and may take the 

 form of a system of branching canals. In most animals, however, 

 the alimentary canal has the form of a longer or shorter tube 

 beginning at the mouth and ending at the anal opening (Fig. 24). 

 In most cases there are organs in the neighbourhood of the mouth 

 serving for the seizure of food ; these may be simply tentacles or 

 soft finger-like appendages, or they may have the form of jaws, by 

 means of which the food is not only seized, but torn to pieces or 

 pounded up to small fragments in the process of mastication. The 

 alimentary canal itself is usually divided into a number of regions 

 which differ both in structure and in function. 



In general there may be said to be three regions in the ali- 

 mentary canal the ingestive, the digestive and absorbent, and the 

 egestive or efferent. The ingestive region is the part following 

 behind the mouth, by which the food reaches the digestive and 

 absorbent region. But, besides serving as a passage, it may also 

 act as a region, in which the food undergoes certain processes, 

 chiefly mechanical, which prepare it for digestion. This ingestive 

 region may comprise a mouth-cavity or buccal cavity, a pharynx, 

 an oesophagus or gullet, with sometimes a muscular gizzard which 

 may be provided with a system of teeth for the further breaking 

 up of the food, and sometimes a crop or food-pouch. 



The digestive and absorbent region is the part in which the 

 chemical processes of digestion go on, and from which takes place 

 the absorption of the digested food-substances. Into this part are 

 poured the secretions of the various digestive glands, which act on 

 the different ingredients of the food so as to render them more 

 soluble. Through the lining membrane of this part the digested 

 nutrient matter passes, to enter the blood-system. This region 

 may present a number of subdivisions ; nearly always there are 

 at least two a wide sac, the stomach, and a narrow tube, the 

 intestine. 



The egestive or efferent region of the alimentary canal is the 

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