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 

 system 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 



