CHAPTER 3 

 HISTOLOGY 



Animals are constituted of ''living substance" or protoplasm together 

 with various non-living materials which are produced by protoplasm. 

 It is chemically complex and possesses a definite, elaborate and minute 

 physical structure. Its basic activities as "living" substance are nutri- 

 tion, respiration and excretion. For the adequate carrying on of these 

 processes, every particle of protoplasm must be in close relation to an 

 environment containing food and oxygen and providing for removal of 

 wastes. Therefore protoplasm cannot exist in indefinitely large con- 

 tinuous masses. The protoplasm of larger animals is subdivided into 

 minute (usually microscopic) structural and physiological units called 

 cells. Circulation of fluid in intercellular spaces provides for the meta- 

 bolic requirements of the individual cell. Animals, e.g., most of the 

 Protozoa, may be so small as to be organized as single cells. 



The body of a large animal is locally differentiated for the carrying 

 on of various functions. The specialized regions, more or less definitely 

 delimited from one another and each characterized by a configuration 

 which is consistent with its special function, we call organs. These 

 organs, in contrast to the organs of a protozoan, comprise many cells, 

 and the cells of any one organ, so far as they are concerned in carrying 

 on one common function, all exhibit intracellular differentiation of the 

 same kind. Such a group or system of cells, coordinated in one common 

 function and alike in their internal differentiation, constitutes a tissue. 



An ideally simple organ would consist of only one tissue. As a matter 

 of fact, nearly all organs are concerned with more than one function. An 

 organ's primary function usually demands certain accessory functions, 

 and a corresponding diversity of tissues enters into the constitution of the 

 organ. In a stomach the primary tissue is the lining layer or digestive 

 epithelium. Muscular, nervous, vascular and connective tissues play 

 accessory but nevertheless necessary roles. Vascular and connective 

 tissues enter into the constitution of all major organs. 



Anatomy deals with organs as such. Histology concerns itself with 

 the internal and specific structure and organization of tissues. Since 

 the tissue is constituted of cells, histology is necessarily concerned with 

 them. Cytology, narrowly defined, deals with cells as such — that is, with 

 that fundamental cell mechanism which is common to all cells and inde- 

 pendent of tissue specialization. 



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