from the circumstance of their frequently exhibiting no trace of longitudinal striz, and that probably the greater portion of them do not contain other more minute primitive fibres, or at least only such as are imperfectly developed. In this respect they are not so highly developed as the voluntary muscles. Perhaps the peculiar secondary deposit upon the cell-membrane of the secondary cell is all that is essential to the contraction of muscle; and it may not be important that that substance should consist of minute longitudinal fibres.
In order briefly to recapitulate our researches into the generation of muscle, the process may be thus stated. Round cells, furnished with a flat nucleus, are first present, the primary cells of muscle. These arrange themselves close together in a linear series; the cells thus arranged in rows, coalesce with one another at their points of contact ; the septa, by which the different cell-cavities are separated, then become absorbed, and thus a hollow cylinder, closed at its extremities, the secondary cell of muscle, is formed, within which the nuclei of the original cells, from which the secondary cell has been formed, are contained, generally lying near together on its wall. This secondary cell, then, passes through all the stages of a simple one. It expands throughout its entire length, whereby the nuclei are farther removed from one another, and sometimes even become elongated in the same direction. A deposit of a peculiar substance, the proper muscular substance, takes place at the same time upon the inner surface of the cylinder, by which the cavity is at first narrowed, and at length completely filled. The cell-nuclei lie external to this substance, between it and the cell-membrane of the secondary cell.
The transverse striae in the voluntary muscles become more manifest, and the deposited substance is more distinctly seen to be composed of longitudinal fibres, as the foetus advances in age, The nuclei are gradually absorbed. The cell-membrane of the secondary muscle-cell remains persistent through- out life, so that each primitive muscular fasciculus is always to be regarded as a cell.
2. Nerves. The nervous system presents two forms of elementary structure: 1st, fibres, nervous fibres in the ex-
