. A manual of zoology. Zoology. 1)2 OENERAL PRTNCIPLES OF ZOOLOGY. ti) :i peculiur distribution of tlie muscle-substance, the smooth nnisculaturc being chiefly distributed to the internal organs, which are not under control of the will (involuntary muscles), while the musculature of the body, subject to the will and hence demanding more rapid action, is cross-striated (voluntary muscles). Wc must not conclude that the difference between smooth and cross-striated musculature coincides with the distinction between visceral and body musculature; it should be noticed that the body musculature of a
. A manual of zoology. Zoology. 1)2 OENERAL PRTNCIPLES OF ZOOLOGY. ti) :i peculiur distribution of tlie muscle-substance, the smooth nnisculaturc being chiefly distributed to the internal organs, which are not under control of the will (involuntary muscles), while the musculature of the body, subject to the will and hence demanding more rapid action, is cross-striated (voluntary muscles). Wc must not conclude that the difference between smooth and cross-striated musculature coincides with the distinction between visceral and body musculature; it should be noticed that the body musculature of all molluscs is smooth, the visceral as well as the. Fig. 48.—EpitheUal muscle-cells, a, of a medusa; h, of an actinian. body muscles of many insects and Crustacea, and the muscles of the heart of vertebrates are cross-striated. It was pointed out above, in connexion with epithelia and connective tissue, that these tissues differed fundamentally. This contrast has its bearing in dealing with the muscles, for both epithelial and mesenchymatous cells may form contractile sub- stances and therefore there are two genetically different kinds of muscles, the epithelial and the mesenchymatous (contractile fibre- cell). Both kinds of muscle-cells can a priori form smooth as well as cross-striated muscle-substance; but the collection of con- nective (mesenchymatous) tissue around inner organs has caused most contractile fibre-cells to be smooth, while most of the ej)ithelial muscle-cells are cross-striated. E'pitlieliul Miiscie-ccUs are cells of which one end extends to the surface of the body or the surface of an internal cavitv (body cavity, lumen of the gut, etc.), and may here have a cuticle, cilia, or flagella, while at the opposite end it has secreted contractile substance in the form of muscle-fibrils (fig. 48). They combine the double function of epithelial and muscle cells. Con/riir/ile fibre-cells, on the other hand, are connective-tissue cells, which usually have surrounded
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