THE PROPERTIES OF MUSCULAR TISSUE. 143 



seems to be some connection between that arrangement of 

 the contractile substance which shows itself under the micro- 

 scope as striation and the power of rapid contraction, since 

 we find that the heart, which is not a skeletal or voluntary 

 muscle but yet one that contracts rapidly, agrees with these 

 in having its fibres striated. This connection is further illus- 

 trated by facts of comparative anatomy: insects are, as a 

 rule, rapidly moving animals, and they are characterized by 

 very marked striation of nearly all their muscular tissue; 

 while in the slow-moving molluscs nearly all the muscular 

 tissue is unstriped except in a few, as Pecten, which make 

 rapid movements, and in that genus the muscles concerned 

 in producing these movements are striated.. 



