2 PHYSIOLOGY OF FARM ANIMALS [CH. 



about the heat regulatmg mechanism which we possess in 

 common with other warm-blooded animals. Again, by taking 

 advantage of the fact that all the parts of an animal do 

 not die at the same time we can perform a great variety of 

 exi3eriments which teach us much about the functions of the 

 different organs of the body. Thus the excised limb muscle of 

 a frog if kept moist by a saline solution, or the heart of a mammal 

 if perfused with an artificial fluid resembling blood serum and 

 preserved at body temperature, may be induced to survive many 

 hours after the death of the animals from which they were 

 taken, and from experiments upon these it is possible to learn 

 much concerning the mechanisms of muscular contraction and 

 the heart beat. Furthermore, we can carry out experiments 

 upon entire animals, pain being avoided by the administration 

 of anaesthetics during the progress of the operation. 



Many of the functions of the body are essentially similar 

 throughout the whole animal kingdom, and among Vertebrates 

 there is a still closer likeness. Thus the processes of digestion 

 in a frog, a bird, a rabbit, a horse, and a man are broadly speak- 

 ing similar, though there are of course very marked differences 

 which are generally greater the less closely related the animals 

 are. So also the general laws governing the nature of nerve 

 impulses, the movements of the heart, or the processes of repro- 

 duction are identical in every case. Thus by investigating any of 

 these processes in a frog or in a rabbit, for example, we can 

 learn a great deal about the functional activities of a horse, or 

 a sheep, or a cow; and it is desirable that before specialising 

 upon the Physiology of farm animals we should possess a general 

 knowledge of the principles of the science obtained by studying 

 whatever animals are most convenient for this purpose. 



Furthermore, Physiology must always be studied in close 

 relation to Anatomy or Morphology (that is, the department of 

 Biology which deals with the form and structure of organisms), 

 since it is impossible to acquire an insight mto the functions of 

 the various parts of an organism without possessing some know- 

 ledge of the composition and structural relation of those parts; 

 and conversely, an intelligent comprehension of the form and 

 structure of an organ can only be gained b}^ a consideration of 

 the part which that organ plays in the general economy of the 

 individual. 



