6 PHYSIOLOGY OP FARM ANIMALS [CH. 



changes in the surroundings; the inner layer lining the gut 

 becomes adapted for the digestion and assimilation of food ; while 

 between these are developed the skeleton and general framework 

 of the body, and all the other tissues which assist in performing 

 the vital functions. 



These functions may now be considered more closely. The 

 parts concerned with each function are usually called systems, 

 and the subsidiary parts which compose these systems are known 

 as organs. The following are the jnincipal sj^stems of the body 

 in a higher animal : (1) the digestive system ; (2) the circulatory ' 

 system ; (3) the resy)iratory sj^stem ; (4) the excretory system ; 

 (5) the muscular system ; (6) the nervous system ; and (7) the 

 reproductive system. In addition to these are the organs of 

 special sense, such as the eye and ear, and the various organs of 

 internal secretion. 



Digestive System. In such lowly forms of Hfe as the amoeba^ 

 food is taken in at any point on the surface, and is then assimi- 

 lated, the indigestible residue being cast out at some other part 

 of the surface. But in man and in all the other higher animals 

 there is a differentiation of function, food being taken in only at 

 a definitely located mouth, whence it passes down an alimentary 

 canal which is divided into several portions. In some of these 

 the digestible material is absorbed, passing through the wall of 

 the canal and into the neighbouring lymph vessels or blood 

 vessels, whence it is distributed throughout the body. The 

 indigestible residue is expelled at a definite anus, which is placed 

 at the hind end of the body. The actual digestive process takes 

 place through the action of various glands such as the salivary 

 glands, the gastric glands, the liver and the pancreas, all of which 

 pour out juices having a digestive or solvent action on the food 

 stuffs. 



Circulatory System. To enable the food which is absorbed 

 from the alimentary canal to be distributed to all the organs of 

 the body a circulatory medium is provided, and this medium is 

 the blood. In all higher animals the blood is kept in motion by 

 a central organ of propulsion, the heart. In a mammal the 

 heart is divided into four chambers — a right and a left auricle, 

 and a right and a left ventricle. Each chamber communicates 

 with the adjoining ones by valves, which only permit of the 

 blood passing in one direction. The circulation is maintained by 



