GENERAL STRUCTURE. 





FIG. 9. CELLS OF 

 CARTILAGE. 

 150. 



classes of tissues: Epithelial, which covers all free sur- 

 faces; connective, forming the bones, binding together 

 the muscular fibers and elements of the nervous system 

 and making up cartilage and ligaments (Fig. 1 1 ) ; mus- 

 cular, composing the greater part of the 

 muscular system; and nervous, consti- 

 tuting the nervous system (Fig. 85). 



The naked eye is unable to distin- 

 guish the elements of the tissues, but 

 the microscope reveals the fact that 

 each kind of tissue is formed of either 

 cells or fibers or, as is most frequently 

 the case, a combination of both. 



The different relative arrangements of these anatomic 

 elements, together with their morphology, permit one 

 to. know from what organ any particular section of 

 tissue under consideration has been taken. 



These ultimate units of structure are still further 



resolved into parts by the chemist, who 

 has shown that they are composed 

 largely of carbon, hydrogen, oxygen, 

 and nitrogen. Since there can be no 

 energy developed in the body without 

 the disintegration and consequent death 

 of some of the particles composing the 

 millions of tiny cells, it is plain that 

 they must have their losses replaced in 

 order to continue their existence. There- 

 fore a method of preparing the food for 

 the use of the cells, and a way of transporting it to each 

 of them are necessary. 



The former is accomplished by the digestive system, 

 which, through the agency of the salivary, gastric, 

 pancreatic, and other glands, transforms the food into 



FIG. 10. - -FIBERS 

 OF VOLUNTARY 

 MUSCLE. : 100. 



n, Fiber; /, tendon. 



