340 THE HUMAN BODY AND ITS FOOD 



set apart to do special kinds of work. A group of cells in which all 

 the cells do the same kind of work is called a tissue. Out of certain 

 tissues, or mixtures of tissues, the organs of the body are made. The 

 muscles contain muscle tissue, or groups of muscle cells, and nerves 

 contain nerve tissue, which consists of nerve cells. 



One form of tissue is called connective tissue, because it connects 

 other cells. Its cells have slender, fine branches which extend between 

 the other cells, and hold all together. So muscles really consist of 

 muscle tissue held together by connective tissue. 



352. Structure of Bones. Bones are made up of 

 branching, interlaced cells of connective tissue, stiffened 

 with limestone and calcium phosphate (cf. 302). The 

 limestone is about % of the weight of the bone. Among 

 the bone cells there are tubes, or " canals," containing 

 blood vessels and nerves. The outside layer of a bone is 

 very compact, and the whole bone has a tough covering 

 of connective tissue. This is called the periosteum, 

 meaning "around the bone." The animal matter (that 

 is, the connective and other tissues) of a bone may be 

 destroyed by burning. The ashes will contain the 

 mineral part. The limestone we can remove by soaking 

 the bone in some dilute acid, such as dilute hydrochloric 

 acid (cf. 128). The tough connective tissue will reihain, 

 but the stiffening will be gone. Long bones have cavities. 

 They thus give the greatest possible strength, with the 

 least possible weight. 



Cartilage is pliable, immature bone. It contains very little lime- 

 stone, and forms the skeletons of young animals. As limestone is 

 added, bones become more hard, but also more brittle. However, all 

 the cartilage in the body does not become hardened. Some remains 

 at the end of almost every bone, to diminish friction, and to prevent 

 the jarring of one bone against another. 



