. A manual of zoology. Zoology. GENERAL HISTOLOGY 81 Smooth and Cross-striated Muscle Fibres.—The smooth muscle- substance represents a lower stage than the cross-striated, since it chietiy occurs in the less highly organized and more inactive animals. Interest- ing in this respect is the fact that in the two stages of development of the same animal the simple and inert polyp has smooth muscles, while the more highly organized and actively motile medusa has cross-striated muscles (fig. 49). The difference in their action has led in the verte- brates to a pecuHar distribution of the muscle-subs
. A manual of zoology. Zoology. GENERAL HISTOLOGY 81 Smooth and Cross-striated Muscle Fibres.—The smooth muscle- substance represents a lower stage than the cross-striated, since it chietiy occurs in the less highly organized and more inactive animals. Interest- ing in this respect is the fact that in the two stages of development of the same animal the simple and inert polyp has smooth muscles, while the more highly organized and actively motile medusa has cross-striated muscles (fig. 49). The difference in their action has led in the verte- brates to a pecuHar distribution of the muscle-substance, the smooth musculature being chietiy in internal organs, which are not under control of the will {involuntary mtisdes), while the musculature of the body, subject. Fig. 49.—Epithelial muscle-cells, a, of a medusa; h, of an actinian. to the will and hence demanding more rapid action, is cross-striated {voluntary muscles). We must not conclude that the difference between smooth and cross-striated musculature coincides with the distinction be- tween visceral and body musculature; it should be noticed that the body musculature of all molluscs is smooth, the visceral as well as the body muscles of many insects and Crustacea, and the muscles of the heart of vertebrates are cross-striated. It was pointed out above, in connection with epithelia and connective tissue, that these tissues differed fundamentally. This contrast has its bearing in dealing with the muscles, for both epithelial and mesenchy- matous cells may form contractile substances and therefore there are two genetically different kinds of muscles, the epithelial and the mesenchy- matous (contractile fibre-cell). Both kinds of muscle-cells can a priori form smooth as well as cross-striated muscle-substance; but the collection of connective (mesenchymatous) tissue around inner organs has caused most contractile fibre-cells to be smooth, while most of the epithelial muscle-cells are cross-striated. Epithelial musc
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