8 ELEMENTARY PHYSIOLOGY [less. 



after they have formed part of the living substance) are 

 leaving the body, combined with more oxygen. And the 

 incessant breaking down and oxidation of the complex 

 compounds which enter the body are definitely propor- 

 tioned to the amount of energy the body gives out, whether 

 in the shape of heat or otherwise ; just in the same way as 

 the amount of work to be got out of a steam-engine, and 

 the amount of lieat it and its furnace give off, bear a strict 

 proportion to its consumption of fuel. 



From these general considerations regarding the nature 

 of life, considered as physiological work, we may turn for 

 the purpose of taking a like broad survey of the apparatus 

 which does the work. We have seen the general per- 

 formance of the engine, we may now look at its build. 



2. The General Build of the Body. —The human body 

 is obviously separable into head, trunk, and limbs. 

 In the head, the brain-case or skull is distinguishable 

 from the face. The trunk is naturally divided into the 

 chest or thorax, and the belly or abdomen. Of the 

 limbs there are two pairs — the upper, or arms, and the 

 lower, or leg's ; and legs and arms again are subdivided 

 by theii- jt»ints into parts which obviously exhibit a rough 

 correspondence — thigh and upper arm, leg and fore- 

 arm, ankle and wrist, fingers and toes, plainly 

 answering to one another. And the two last, in fact, are 

 so similar that they receive the .same name of digits ; 

 while the several joints of tlie fingers and toes have the 

 common denomination of phalanges. 



The whole body thus composed (without the viscera or 

 organs which fill the cavities of the trunk) is seen to be 

 bilaterally symmetrical ; that is to .say, if it were split 

 lengthways by a great knife, which should be made to 

 pass along the middle line of both the dorsal and ventral 

 (or back and front) aspects, the two halves would almost 

 exactly resemble one another. 



One-half of the body, divided in the manner described 

 (Fig. 1, A), would exhibit in thti trunk, the cut faces of 



