40 PRACTICAL MICROSCOPY. 



brane inclosing a clear gelatinous material, traversed by a reticulum 

 of fibrillae. 



The nucleolus consists of a spherical granular enlargement upon 

 the fibrillae of the nucleus. 



Deviations from the type are most frequent, and vary greatly as to 

 form, number of elements, and chemical composition. 



FIG. 24. A CELL NUCLEUS, WITH NETWORK AND NUCLEOLUS. DIAGRAMMATIC. 



The typically perfect cell is rarely seen in human tissue on account 

 of the length of time which commonly elapses between death and 

 observation of the structure, the delicate fibrillae of the nuclei usu- 

 ally appearing as a mass of granules. 



CELL DISTRIBUTION. 



The complex mechanism of the body had its origin in a single cell. 

 This preliminary structure, endowed with the power of proliferation, 

 became two cells. Two having been produced, they became four ; 

 the four, eight; and thus progression advanced until they became 

 countless. Some of these ceils remained as such ; others altered in 

 form and composition gave birth to muscle, bone, etc., etc. The 

 study of these processes belongs to physiology. 



The adult body is composed largely of cells of various forms. The 

 different physiological processes, as secretion, absorption, respiration, 

 etc., are effected through the intervention of these anatomical 

 elements. 



All free surfaces, within or without the body, are covered ivith 

 cells. The entire skin, the outside of organs, as lung, liver, stomach, 

 intestine, brain, etc., etc.; all cavities, as alimentary tract, heart, 

 ventricles of the brain, blood-vessels, ducts, all present a superficial 

 layer of cells. 



VARIATION" IK FORM OF CELLS. 



Alteration from the typical or spherical form is effected mainly 

 through pressure consequent upon active proliferation of contiguous 

 cells, or growth of surrounding fibrous tissues. 



