GENERAL STRUCTURE AND COMPOSITION 



are indicated by numbers; n, 

 n, nerves and vessels. 



liquids during life, either blood or a watery-looking fluid known as 



lymph. These tubes, the blood and lymph-vessels respectively, are 



not, however, characteristic of the limbs, 



for they are present in abundance in the 



dorsal and ventral cavities and in their 



walls. 



Microscopic Structure of the Body. R > 

 For the detailed study of objects too 

 small to be examined with the unaided 

 eye the compound microscope is em- 



. . , , . t . f FIG. 4. A section across 



ployed. Important Optical conditions for the forearm a short distance 



the successful use of this instrument are ^ V % tr^pporting 

 adequate illumination and sharpness of bones, the radius and ulna; 



e, the epidermis, and d, the 

 focUS. To Secure these in the Study of dermis of the skin; the latter 



tissues the materials are cut in very 

 thin slices and observed by transmitted 



light. Viewed thus tissues in their nat- 



, , , . 



ural state are so nearly transparent that 

 relatively little of their detailed structure can be made out. The 

 practice of histologists, therefore, is first to subject the tissues to 

 the action of preservatives, and then to stain them with suitable 

 dyes. By applying the principle that the different structures of 



the tissues are likely to differ chem- 

 ically as well as in other respects, 

 dyes can be selected which have 

 greater affinity for some of the 

 chemical components of the tissues 

 than for others. Thus certain of 

 the tissue components will stain 

 with one sort of dye; other com- 

 FIG. 5. Diagram of a cell (Schafer). ponents are unaffected by this 



P. protoplasm; n, nucleus; c, centrosome. ^ but can be gtamed with an _ 



other. This method of differential staining enables the various 

 features of tissues to be made clearly visible. 

 . Cells. Examination of the different tissues with the aid of the 

 microscope reveals that they are made up of minute structures, 

 the cells. These vary in form and size in different tissues. They 

 are all constructed on a common plan, although in the more 

 highly organized tissues, such as nerves and muscles, this plan is 



