364 PHYSIOLOGICAL DEVELOPMENT. 



of nerve applies equally in the case of muscle. A portion 

 of undifferentiated tissue containing a predominance of the 

 colloid that contracts in changing, will, during each change, 

 tend to form new molecules of its own type from the other 

 colloids diffused through it : the tendency of these entangled 

 colloids to fall into unity with those around them, will be 

 aided by every shock of isomeric transformation. Hence, re- 

 peated contractions will further the growth of the contracting 

 mass, and advance its differentiation and integration. If, 



too, we remember that the muscular colloid is made to con- 

 tract by mechanical disturbance, and that among mechanical 

 disturbances one which will most readily affect it simulta- 

 neously throughout its mass is caused by stretching, we shall 

 be considerably helped towards understanding how the con- 

 tractile tissues are developed. If extension of a muscular 

 colloid previously at rest, produces in it that molecular dis- 

 turbance which leads to isomeric change and decrease of 

 bulk, then there is no difficulty in explaining the movements 

 of cilia; the formation of a contractile layer in the vascular 

 sjrstem becomes comprehensible; each dilatation of a blood- 

 vessel caused by a gush of blood, will be followed by a con- 

 striction; the heart will pulsate violently in proportion as 

 it is violently distended; arteries will develop in power as 

 the stress upon them becomes greater; and we shall simi- 

 larly have an explanation of the increased muscularity of 

 the alimentary canal which is brought about by increased 

 distension of it. 



That the production of contractile tissue in certain locali- 

 ties, is due to the more frequent excitement in those localities 

 of the contractility possessed by undifferentiated tissue in 

 general, is a view harmonizing with traits which the diffe- 

 rentiated contractile tissue exhibits. These are the rela- 

 tions between muscular exercise, muscular power, and mus- 

 cular structure; and it is the more needful for us here to 

 notice them because of certain anomalies they present, 

 which, at first sight, seem inconsistent with the belief that 



