i ORGANISATION AND STRUCTURE OF MUSCLE 9 



excited locally, at one end only, contraction of the whole body 

 ensues without any perceptible latent period ; the difference of 

 time, which doubtless exists in contraction of the anterior and 

 posterior ends of the animal, being quite unnoticeable (Yerworn). 



THE MUSCLES OF METAZOA 



In Metazoa, as in unicellular animals, the typical muscle 

 makes its first appearance in the form of fibrils, or bundles of 

 fibrils, in the protoplasm of certain cells. The animal kingdom 

 exhibits an amazing variety in regard to the mass-disposition 

 and relative arrangement of these contractile fibrils, and the 

 formative plasma (" sarcoplasm "), of which they are a differentia- 

 tion product. 



In order to understand the structure and function of highly 

 differentiated muscles, it is as important to consider their racial 

 as their individual development, and we have next to study 

 in detail some instructive examples of the first of these. 



The simplest form of muscle-cell (myoplast x ) occurs in the epi- 

 thelial muscles (" neuro-muscular cells ") of the lower Ccelenterata. 



In Hydra, e.g., the ectoderm consists mainly of large, blunt, 

 cone-shaped, epithelial cells, the apices of which are directed 

 inwards, and prolonged into one or more processes which form 

 dichotomously branched fibrils bending at right angles, and run- 

 ning parallel to the body axis, to form collectively a sub-epithelial, 

 contractile layer (" muscle lamella "). Accordingly in transverse 

 section there is a small zone between the ectoderm and endoderm, 

 in which the bisected fibrils stand out as a series of strongly 

 refracting points. 



In this case, therefore, the cell-bodies help to bound the 

 body-surface, and, like the protoplasm of Ciliata, serve to establish 

 relations with the external world, since they are able to receive 

 impressions from without, i.e. are excitable. In both cases the 

 excitation is conveyed through the protoplasm of the cell (sarco- 

 plasm) to the contractile fibrils, and must be able to spread over 

 large tracts of the body by conduction from cell to cell (if nerves 

 are really wanting). The large, vacuolated, endoderm cells of 



1 In the ciliated Infusoria described above, which must be regarded as inde- 

 pendent cell-individuals, it is, in the same connection, legitimate to speak of "cell- 

 muscles." 



