.HE SEASON WHY. 



221 



"And I will lay the sinews upon you, and will bring up flesh upon you, and 



cover you with skin, and put breath in you, and ye shall live ; and ye shall know 



that I am the Lord." EZEKIEL xxxvii. 



932. Why are the heart, lungs, fyc., placed within the chest ? 

 Because the functions of those organs require considerable 



space, while their importance in the system of life, renders it 

 essential that they should be securely protected from the 

 probabilities of accident. 



933. Why are the heart and lungs enclosed for protection 

 in a series of ribs, and not in a close case, like the brain ? 



Because, by the inflation and contraction of the lungs, their 

 capacity is constantly changing. When man takes a moderate 

 inspiration, he inhales about thirty cubic inches of air, and the 

 lungs increase in size one-eighteenth of their whole capacity. 

 Consequently, were they enclosed in a frame of fixed dimensions, it 

 must needs be, to that extent at least, larger than is necessary, 

 when the frame is made to dilate and contract with the capacity of 

 the lungs. 



So perfect is the Almighty contrivance, that not only are the ribs 

 made to protect the lungs, but, by their elasticity, and the contrac- 

 tions and dilations of the muscles which lie between them, they assist 

 the lungs in their labours, and work with them in perfect harmony. 



934. Why are the bones of the arms, 

 legs, fyc., made hollow ? 



Because lightness is thereby combined with 

 strength. There is a provision by which, in 

 the extremities of bones, where an enlarged 

 surface is required, lightness is still com- 

 bined with the necessary degree of strength. 



The bones are made up of a cellular forma- 

 tion ; and this generally occurs in parts 

 which are much called into action, in the 

 various movements of the body. 



A. Lower part of the bone of the thigh. 



B. Head of the bone of the leg. 



C. The knee cap, showing its relation to the other 



THE CELLULAR STEUC- bones, and the manner in which it is enclosed by the 

 TURE OP BONE, BY tendons seen at Fig. 58. 



D. A pad of fat, lessening the frictipn of the bones, 

 and modifying the shocks produced by jumping, Ac. 



WHICH LIGHTNESS AND 



