MOVEMENT 



The chief motor organs in the metazoa are the muscles 

 which constitute the "flesh" of the body. Muscular 

 tissue consists of contractile cells — that is to say, of 

 cells with the sole property of contraction. When the 

 muscular cell contracts, it becomes shorter and its diam- 

 eter increases. This brings nearer together the two 

 parts of the body to which its ends are attached. In 

 the lower metazoa the muscle-cells have, as a rule, no 

 particular structure; but in the higher animals the con- 

 tractile plasm undergoes a peculiar differentiation, which 

 has the appearance under the microscope of a transverse 

 streaking of the long cells. On this ground a distinction 

 is drawn between striated muscles and simple non- 

 striated or smooth muscles. The more vigorous, rapid, 

 and definite is the contraction of the muscle, the more 

 marked is the streaky character, and the more pro- 

 nounced the difference between the doubly refractive 

 muscular particles from the simple refractive. The 

 striated muscle is "the most perfect dynamo we know 

 of " (Verworn). The normal heart of a man accomplish- 

 es every day, according to Zuntz, a work of about twenty 

 thousand kilogrammetres — in other words, an energy 

 that would suffice to lift to a height of one metre a 

 weight of twenty thousand kilogrammes. In many fly- 

 ing insects (gnats, for instance) the flying muscles make 

 three hundred to four hundred contractions a second. 



In the lower and higher classes of the metazoa the 

 muscle amounts to no more than a thin layer of flesh 

 underneath the skin. This layer consists of muscular 

 cells, which come originally from the ectoderm in the 

 form of internal contractile processes of the skin-cells 

 themselves, as in the polyps. In other cases the muscle- 

 cells are developed from the connective-tissue cells of 

 the mesoderm, the middle skin-layer, as in the cteno- 

 phora. This mesenchymic muscle is less common than 

 epithelial muscle. In most of the askeletal vermalia the 



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