CHAPTER Y. 

 TISSUES OF SPECIAL FUNCTION. 



THE elementary tissues included in this group are highly differ- 

 entiated in structure so as to adapt them for the performance of 

 some special function of a high order. The constituent of the 

 tissues which appears most highly specialized is the cell, which is 

 often so greatly modified in structure as to have lost many of the 

 general characters of the cells hitherto studied. Thus, for example, 

 the cells of striated muscle are multinucleated, and the cytoplasm 

 has become transformed into a substance known as contractile sub- 

 stance, which occupies nearly the whole bulk of the cell, leaving 

 only a small amount of relatively undifferentiated cytoplasm imme- 

 diately surrounding the nuclei. 



In like manner the intercellular substances of some of these 

 tissues show a complexity of structure in great contrast to those 

 with which we have become familiar in the preceding tissues. In 

 fact, it is stretching a point to regard the tissues lying between the 

 cells of striated muscle as forming an intercellular substance 

 belonging to that tissue. In this case those tissues are identical 

 in structure with the loose areolar tissue that was described in the 

 preceding chapter. We may, therefore, with propriety, regard the 

 striated muscles as organs in which the muscle-cells constitute the 

 parenchyma and this areolar tissue the interstitium (see Chapter 

 VII.). But in other tissues of the group there is either an inter- 

 cellular substance resembling those of the preceding tissues, or 

 some special form of sustentacular tissue e. g., the neuroglia of 

 the central nervous system. 



The tissues of special function are arranged in two groups : the 

 muscular tissues and the nervous tissues. As is implied in the 

 title, these tissues are grouped together because of their functional 

 powers, and not with regard to peculiarities of structure, so that it 

 is impossible to give concise statements of any common general 

 structural characters possessed by all the members of each of these 



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