TISSUES OF SPECIAL FUNCTION. 87 



two groups. Thus, the individual muscular tissues differ consider- 

 ably from each other in structure, but are closely related in function, 

 each variety being specialized so as to execute a particular kind of 

 contraction when functionally active. We must also assume that 

 the variations in structure met with in the nervous system have 

 reference to the translation of various impressions into nervous 

 impulses, or the liberation of such impulses under different condi- 

 tions, as well as to their transmission and application to the func- 

 tional activities of other tissues. 



The complex functions exercised by the nervous system appear 

 to necessitate a great variety of nervous structures, and it would 

 be a matter for surprise to find the visible structure of the nervous 

 system as simple as it is, were it not for the fact, already learned, 

 that cells apparently similar in structure may have widely different, 

 though related, functional powers. 



I. THE MUSCULAR TISSUES. 



There are three varieties of muscular tissue, which differ from 

 each other both in structure and in the character of their functional 

 activities. One variety is that found in the walls of the hollow 

 viscera and larger bloodvessels. Its activities are not under the 

 control of the will, and the cells are devoid of marked cross-stri- 

 ation of the contractile substance. It has, therefore, received the 

 names, " involuntary " or " smooth " muscular tissue. The other two 

 varieties present distinct and rather coarse cross-striation of the 

 contractile substance, but differ in other structural details. One of 

 these is called "voluntary" or " striated" muscle; the other is found 

 only in the heart, is not under the control of the will except in 

 rare instances, and is known as "cardiac" muscular tissue. 



1. Smooth Muscular Tissue. — This elementary tissue is composed 

 of elongated or fusiform cells, which gradually taper to a sharp 

 point. The body of the cell, except close to the ends of the nucleus, 

 consists of a modified cytoplasm, called "contractile substance," 

 which stains a coppery red with eosin, and presents fine, indistinct, 

 longitudinal and transverse markings, possibly the optical expression 

 of certain ridges that are in contact with similar ridges on neigh- 

 boring cells. Each cell has a single, greatly elongated, rod-shaped 

 nucleus situated in its centre, with the long axis coincident with 

 that of the cell (Fig. 70). The nuclei are vesicular and possess 



