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Literature

limulus, muscle, disks, intercalated and vertebrate

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LITERATURE.

The only work known to me on the microscopic structure of

Limulus muscle is that by Meek (1909) on the cardiac muscle. He describes its structure as a double syncytium composed of trabeculte "individ ualized by connective-tissue sheaths"; the peripheral nuclei of the trabeculEe he interprets as connective-tissue nuclei. He records also the absence of sarcolemma and of intercalated disks, and notes the close similarity between the heart-musculature of Limulus and that of lower vertebrates. Carlson (1904) demonstrated the applicability of the neurogenic theory of heart-beat to Limulus. Meek concludes that a syncytial heart-musculature, accordingly, does not necessarily imply the verity of the myogenic theory of conduction. But this functional difference between Limulus and vertebrate hearts may in fact inhere fundamentally in the absence of an analogue of the vertebrate atrio ventricular conducting bundle in Limulus.

The gross structural condition of the Limulus heart is very signifi cant in this connection. The Limulus heart consists essentially of a metameric series of 9 syncytial muscular rings, for the physiologic coordination of which the very scattered peripheral and central longi tudinal fibers seem quite inadequate. In the absence of a direct muscular coordinating mechanism the nervous impulse to heart-beat must be conducted by the longitudinal nerve-cord.

Patten (1912) only states that the striped muscle of Limulus arises very early from the somites. The histogenetic process is not described, but a detailed description is given of the origin and history of so-called "fiber cells," some of which give rise to definite muscles, others to "semi-amceboid cells resembling blood corpuscles." The original fiber cells, derivatives of the germ-wall, are said to lie in the first 5 thoracic segments in an intermediate zone median to the germ-wall.

Other problems here considered are touched upon in the following recent works: (i) Baldwin's (1912) on the heart-muscle of the mouse, in which he describes "muscle cells" separated from the myofibrillar substance by a "cell membrane"; this interpretation was shown to be untenable in (2) my paper (1914) dealing with cat and mouse tissue in macerated condition, and by the findings of (3) Asai (1914) in his study of striped-muscle histogenesis in the mouse; (4) Thulin's (1915) work on the wing-muscles of certain insects (Coleoptera), birds, and bats, from which he records observations which he interprets as indi cating the absence of both the meso- and the telo-phragmata, the so called Z-stripe being the only striation discernible and to be interpreted as a contraction band; (5) Heidenhain's work on the histogenesis of striped muscle in the trout, on the basis of which he further supports and extends to striped-muscle tissue his general histologic principle of protomeric analysis, namely, the conception that muscle-tissue is built up by the association into successively larger combinations of ultimate fission elements, the metafibrillie or protomeres; (6) Jordan and Steele's (1912) comparative study of the intercalated disks in vertebrate heart muscle, from which it appears that intercalated disks are present in progressively simpler forms in all the vertebrate groups to and includ ing teleost fishes; and (7) my description (1912) of the intercalated disks of hypertrophied human heart-muscle.

Discussion.

The value of the data derived from the study of the Limulus muscle depends in degree upon the extent to which they may legitimately serve as a basis for generalization with respect to vertebrate muscle. In the relative simplicity of its striations the Limulus muscle seems more like that of vertebrates than like that of arthropods. Moreover, the fundamental close similarity between the cardiac and the skeletal type in Limulus is significant, especially as indicating that a main difference between cardiac and skeletal muscle generally is essentially a degree of differentiation, minute morphologic differences following functional differences probably largely inherent in the syncytial arrange ment of the fibers in the heart-musculature. In both cases, as in verte brate heart-muscle generally, the conspicuous stripe is the Z-membrane. Accordingly the skeletal muscle of Limulus is apparently less highly differentiated than vertebrate skeletal muscle, where the Q-disk gives the most conspicuous stripe, a conclusion further supported by the manner in which the nuclei are distributed. In its syncytial character and the presence of intercalated disks the Limulus cardiac muscle resembles very closely vertebrate cardiac muscle, a point already emphasized by Carlson and by Meek (8). The infrequency of the intercalated disks precludes an interpretation of these structures in terms of cell boundaries or intercellular substances, or as regions of growth (Heidenhain (12)). The disks consist of rows of modified foci in the fibrillae. The modification consists of a change characterized tinctorially by an increase in staining intensity for a short distance on one or both sides of the telophragma. Structurally the disk is com posed of a series of rod-like areas of the fibrillae in transverse alinement. The most probable explanation of their formation is that of a change of position of the Q (anisotropic) substance from its usual location in relaxed fibers midway between two successive telophragmata, to a location on either side of these membranes. This explanation answers to the description of the formation of a contraction band. In that a disk appears permanent, it seems appropriately described as an "irre versible contraction band." In view of what was known of the com parative morphology of intercalated disks—coupled with our knowledge of the minute structure of Limulus heart-muscle--the simple comb type was to be expected in Limulus. This type of disk is actually present. But disks were expected in greater number. Possibly hearts of older limuli would show more abundant disks.

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