52 MANUAL OF PHYSIOLOGY. 



been mentioned (vide p. 47), they form but one structure in some 

 of the more simple and less differentiated animals. The neuro- 

 muscular tissue which is formed from the outer layer of the 

 embryo, is the forerunner of the muscles as well as of the nerves 

 of the embryo of the higher animals. 



In the higher animals and man, muscle tissue consists of two 

 distinct kinds of textures, commonly spoken of as 



a. Smooth or non-striated muscle. 



b. Striated muscle. 



In the smooth muscle the individual elements present all the 

 characters of a cell, but very much elongated and flattened, and 

 contain a single long nucleus. They contract very slowly and 

 persistently, and require a comparatively long time for the nerve 

 influence to affect them, so that an obvious interval exists between 

 the moment of their stimulation and their contraction. They are 

 therefore commonly found in the internal organs and in situa- 

 tions where gradual and lasting contractions are required. They 

 receive their nervous supply generally from the sympathetic 

 system, and perform their duty without our being conscious of 

 their activity or being able to control it by our will. 



Striated muscle tissue is made up of cylindrical fibres of such 

 length that both extremities cannot be brought into the field of 

 the microscope at the same time. Their exact relation to cells is 

 not so easily made out as in smooth muscle, and doubtless varies 

 in different muscles. Sometimes the fibres are made up of single 

 cells, and in other cases they are formed by the permanent fusion 

 of several cell elements which never differentiate into separate ele- 

 ments, owing to the imperfect division of the cells, but make up 

 one mass, the multiple nuclei of which alone make its mode of ori- 

 gin apparent. The contractile substance is made up of two kinds 

 of material, one of which refracts light singly, while the other is 

 doubly refracting. These are ranged alternately across the fibre, 

 making the transverse markings or striae from which it gets its 

 name. This striated material is quite soft, and is incased in a 

 thin homogeneous elastic sheath called sarcolemma, which keeps 

 the fibre in its normal shape. 



This form of muscle may be considered the widest departure 



