THE FROG 23 



oxygen necessary for this process is obtained from the atmosphere 

 by respiration. The oxidation of the various substances results in 

 the production of waste materials, which must be removed from the 

 body or excreted. For the carrying on of these processes we find 

 a collecting and distributing agent in the blood, which carries oxygen 

 and food to the places where they are wanted, and also collects up the 

 waste matter so that it can be eliminated by the kidneys and lungs. 



In order to perform all the various operations in as efficient a 

 manner as possible, we find that the body is divided up into a large 

 number of separate parts, each specially modified to perform one or 

 two functions ; such parts are termed organs. Thus the ovaries 

 and testes are reproductive organs, the pancreas and liver digestive 

 organs, and so on. The maintenance of life depends on the correla- 

 tion of the activities of the various organs, and when an important 

 one of them is put out of action by disease the other vital phenomena 

 of the animal are brought to a standstill, or, as we say, the 

 animal dies. As a rule an organ is not homogeneous throughout, 

 but is composed of various kinds of living material ; each separate 

 kind is spoken of as a tissue. Again, a number of organs are often 

 linked together, and their functions are contributory to one main end. 

 A group of such organs constitutes a system. The alimentary canal 

 with its various parts, the liver and pancreas, form the digestive 

 system for example. 



We can thus recognise in the higher animals nine such systems, 

 as follows : (i) the skeletal, (2) the muscular, (3) the integumentary, 

 (4) the alimentary or digestive, (5) the -respiratory, (6) the circulatory, 

 (7) the excretory, (8) the reproductive, and (9) the nervous and 

 sensory. In addition to these there are a number of more or less 

 isolated organs collectively termed the ductless glands, whose 

 functions in some cases are but imperfectly understood, and they 

 may conveniently be dealt with together, although they do not 

 constitute a system. We shall now pass on to consider the various 

 systems in Rana from the point of view of their structure, both 

 gross and minute, and their physiology. 



Skeletal System. 



Certain portions of the hard framework, or skeleton, within 

 the body have already been noticed, and it is now necessary to make 

 a more detailed study of its constituent parts. This may be done 

 on prepared skeletons, or, preferably, on one you have made for 

 yourself.* 



* A skeleton may easily be prepared by removing as much as possible of 

 the flesh from a frog and then leaving it to go bad or macerate in water. The 

 muscles can then be picked off or removed with a stiff brush. It may be done 

 more quickly, but not so well, by dipping the frog repeatedly into hot water. 



