Histology 



Animals are constituted of "living substance" or protoplasm, 

 together with various nonliving materials which are produced by 

 protoplasm. It is chemically complex and possesses a definite, elabo- 

 rate, and minute physical structure. Its basic activities as "living" 

 substance are nutrition, 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 pro- 

 viding for removal of wastes. Therefore protoplasm cannot exist in 

 indefinitely large continuous masses. The protoplasm of larger animals 

 is subdivided into minute (usually microscopic) structural and physio- 

 logic units, cells. Circulation of fluid in intercellular spaces provides 

 for the metabolic requirements of the individual cell. 



Animals — e.g., most of the Protozoa — may be so small as to be 

 organized as single cells. Attainment of relatively large size along with 

 capacity for powerful movement and diversified activities, compels the 

 differentiation of a high degree of structural complexity. To say that 

 an animal is large because it has many organs and is complex, reverses 

 the truth. Protoplasm is so constituted that a large animal must be 

 multicellular, and, if it is also powerful and active, it must be complex. 



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 



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