Text-book of comparative anatomy . e fibres, smooth as well as striated, arise, even in many higherMetazoa, out of epithelium; in many of these, however, the muscle-forming cells (Myoblasts) are descendants of cells which at an earlystage of their development sank below the level of the epithelium towhich they belonged. Both kinds of formation may occur in the sameanimal. VOL. I E 50 COMPARATIVE ANATOMY CHAP. IV. Nerve Tissue. As muscle elements pure and simple may be imagined to arise outof neuro-muscular cells by the gradual differentiation of most of theprotoplasm of one part of the cell in
Text-book of comparative anatomy . e fibres, smooth as well as striated, arise, even in many higherMetazoa, out of epithelium; in many of these, however, the muscle-forming cells (Myoblasts) are descendants of cells which at an earlystage of their development sank below the level of the epithelium towhich they belonged. Both kinds of formation may occur in the sameanimal. VOL. I E 50 COMPARATIVE ANATOMY CHAP. IV. Nerve Tissue. As muscle elements pure and simple may be imagined to arise outof neuro-muscular cells by the gradual differentiation of most of theprotoplasm of one part of the cell into contractile substance, themuscular function thus being brought to the front at the expense ofthe other possible functions, so nerve elements may be produced bythe suppression of the contractile part of the cell and the furtherdifferentiation of the neural portion. We can perhaps imagine thatthe simple sensory cells of the body epithelium of the lower Metazoaarose in this way, always presupposing that they remained in connection. FIG. 50.—Piece of a muscle lamella of the septum of an Actinian (Anthea cereus), with nerveplexus, after 0. and R. Hertwig. m, Muscle fibres; sz, sensory cell, with sensory hair ; gz, ganglioncell. with the neighbouring contractile elements, either by simple contactor by processes. In this way a stimulus received by the sensory cellscould be transmitted to the muscle cells. The sensory cells areepithelial cells, which are generally distinguished by a delicate, usuallyimmobile, sensory hair, projecting outwards. Division of labour couldthen go further. Single sensory or nerve cells of the most undifter-entiated sort, found in contact with other sensory cells and muscle cells,could give up their connection with the surface of the body and sinkinto the lower part of the epithelium, thus playing the part of inter-mediaries between sensory and muscle cells (Fig. 50), and transmit thestimulus received by the former to the latter. Such cells are found inm
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectanatomycomparative
