[Verse: "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, &c., 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 contractions 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, &c., 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 combined with the necessary degree of strength. The bones are made up of a _cellular formation_; and this generally occurs in parts which are much called into action, in the various movements of the body. [Illustration: Fig. 55.--SECTION OF THE KNEE JOINT, SHOWING THE CELLULAR STRUCTURE OF BONE, BY WHICH LIGHTNESS AND STRENGTH ARE OBTAINED.] 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 bones, and the manner in which it is enclosed by the tendons seen at Fig. 58. D. A pad of fat, lessening the friction of the bones, and modifying the shocks produced by jumping, &c. [Verse: "Again he said unto me, Prophesy upon these bones, and say unto them, O ye dry bones, hear the word of the Lord."--EZEKIEL XXXVII.] 935. _Why are the bones of the arms and legs formed in long shafts?_ Because a considerable _leverage_ is gained, by which the advantages of _quickness of motion_, and _increase of mechanical power_, are secured. 936. _Why are the bones of the hands and feet numerous and small?_