Chap, hi.] OF MOVEMENTS IN THE ANIMAL KINGDOM. 353 



in vertebrates constitute the flesh, properly so called, the muscu- 

 lar mass, obeying the orders of the will, obeying the volitions 

 formulated in the brain. It is, in effect, a constant character- 

 istic of the striated muscular fibre, and likewise of the fibro-cell, 

 that of being in relation, in contact more or less immediate, with 

 central nervous masses. On the contrary, the elementary sarcodic 

 substance contracts of itself, obeying direct exterior impressions. 

 In the same way the vibratile cilia oscillate independently of all 

 nervous incitation, even in the superior animals, for even the 

 death of the individual does not suspend their movement. In 

 certain inferior animals, in which the nervous system is not yet 

 differentiated, there already exist smooth muscular fibres con- 

 tracting as spontaneously as the sarcodical substance. 



In the vertebrates, in which the two systems of smooth fibres 

 and striated fibres are largely represented, there is between them 

 a certain division of labour. The smooth fibres, or fibre-cells, 

 appertain in a general manner to the apparatus and to the organs 

 of nutritive life, while the striated muscles are the servants of 

 the conscious will, the muscles of the life of relation. Thence 

 the classical division into muscular system of the organic life and 

 muscular system of the animal life. 



This division, however, is true only to a certain degree. Here, 

 as everywhere in biology, there is no absolutely determined dis- 

 tinction. We find striated fibres in the intestine of the tench, 

 in the gizzard of birds, in the pharynx of some gasteropods. 

 Finally, the wall of the heax-t of all vertebrates is formed of 

 striated fibres. But those cardiacal fibres have an arrangement 

 in some sort transitory. They anastomose into each other, as do 

 the smooth fibres in certain invertebrates. 



To the anatomical differences of the two orders of fibres, 

 correspond differences of functions, physiological differences. 

 Contractility, that is to say, the shortening and the widening of 

 the fibre, is effected according to different modes in the smooth 

 muscles and in the striated muscles. In the first it is slow and 

 durable : in the second it is prompt and instantaneous, and does 



A A 



