CHAPTER 3 

 HISTOLOGY 



The material, source and basis of everything that an animal is or does 

 is the chemically specific and complex "living substance," protoplasm. 

 In such very small animals as Protozoa the protoplasm of the minute body 

 exhibits an organization similar to that of a single one of the structural 

 subdivisions or "cells" of the body of a large animal. Within the body 

 of one of these so-called "unicellular" animals there is more or less local- 

 izing of functions and a corresponding structural differentiation of regions. 

 There may be contractile fibrils, digestive vacuoles, excretory vacuoles 

 and other localized intracellular organs. Similarly, the body of a large 

 animal is locally differentiated for the carrying on of various functions. 

 The specialized regions, more or less definitely dehmited from one another 

 and each characterized by a configuration which is consistent with its 

 special function, we call organs. But these organs, in contrast to the 

 intracellular 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. The primary function of a liver is secretion of bile. But secretion 

 calls for blood supply, and a complex of hepatic tubules and blood vessels 

 requires mechanical support. Therefore vascular tissue and connective 

 tissue make up a large part of the substance of the liver. In a stomach, 

 the primary tissue is the 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. A certain tissue may be the primary and essential tissue 

 in one organ and occur as an accessory tissue in another organ. Nervous 

 tissue is the essential tissue in a brain or spinal cord but it appears as an 

 accessory tissue in nearly all other organs. 



Anatomy deals with organs as such. It considers their form, relations 

 and all those grosser features of structure which are external to tissue 

 structure or cell structure. 



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