26 ELEMENTARY PHYSIOLOGY less. 



rapidity to the seat of each local waste. The circulatory 

 system is the commissariat of the physiological army. 



Again, if tlie body is to* be maintained at a tolerably 

 even temperature, while tliat of the air is constantly vary- 

 ing, the condition of the hot-water ai)paratus must be 

 most carefully regulated. 



In other words, a coordinating' organ must be 

 added to the organs already mentioned, and this is found 

 in the nervous system, which not only possesses the 

 function already described of enabling us to move our 

 bodies and to know what is going on in the external 

 world ; but makes us aware of the need of food, enables us 

 to discriminate nutritious from innutritions matters, and 

 to exert the nuiscular actions needful for seizing, killing, 

 and cooking ; guides the hand to the mouth, governs all 

 the movements of tlie jaws and of the alimentary canal, 

 and determines the due supply of the juices necessary for 

 digestion. By it, the working of the heart can be 

 properly adjusted and the calibres of the distributing 

 pipes can be regulated, so as indirectly to govern the 

 excretory and oxidational processes, which are also 

 additionally and more directly affected by other actions 

 of the nervous system. 



The nervous system has often been compared to a 

 telephone system with its exchange (the brain and spinal 

 cord) and its wires (the tibres) which go to and fro in 

 communication with the various instruments in which 

 the messages are given or heard (sensory or motor nerve 

 endings). In addition to this system there is another 

 coordinating system which may be compared to a wireless 

 system of communication. The messages do not run 

 along fibres but are chemical substances which are 

 produced* in one organ of the body and carried by the 

 blood to another organ, perhaps in some very remote 

 part. They throw the particular organ for which they 

 are destined into activity without aflfecting other parts of 

 the body. Such messengers are called hormones. 



12. Life and Death. — The various functions which 



