50 STUDENTS HISTOLOGY 



brane inclosing a clear gelatinous material, traversed by a reticulum 

 of fibrillae. 



The nucleolus consists of a spherical, highly refracting body 

 lying inside of the nucleus, sometimes appearing to be a granular 

 enlargement upon the fibrillas of the nucleus. 



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

 to form, number of elements, and chemical composition. 



FIG. 27. 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 usually 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 pro- 

 liferation, became two cells. Two having been produced, they 

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

 they became countless. Some of these cells remained as such; 

 others, altered in form and composition, gave birth to muscle, bone, 

 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, res- 

 piration, etc., are effected through the intervention of these 

 anatomical elements. 



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

 cells. The entire skin, the outside of organs, as the lungs, 

 liver, stomach, intestines, brain, etc.; all cavities, as the alimen- 

 tary tract, heart, ventricles of the brain, blood-vessels and ducts, 

 present a superficial layer of cells. 



The cells are held together by an intercellular substance, which 

 may be so abundant that the cells form a comparatively small part 

 of the tissue. The intercellular material is to be regarded as hav- 



