this process, there are to be observed the formation of a second order of tubes within the original tube ; a peculiarly regular arrangement of discs within these second tubes ; the formation, first of rings and then of spirals, out of discs so arranged ; the interlacing of the spirals ; and the origin, in the space circumscribed by these, of spirals having a minuter size ; which in their turn surround others still more mi- nute ; and so on. The outer spirals enter for the most part into the formation of the investing membrane discovered by Schwann, but for the only complete description of which, in a formed state, we are indebted to Mr. Bowman. The inner spirals constitute what are de- nominated the fihrillce. The fibril appears to the author to be no other than a state of the object which he designates ?i flat filament and which, as he shows, is a compound structure. The fibril he finds to be, not round and beaded, as it has been supposed, but a flat and grooved filament ; the description above given of the structure of the filament being especially applicable here. This flat filament is so situated in the fasciculus of voluntary muscle, as to present its edge to the observer. It seems to have been the appearance presented by the edge of this filament, that is to say, by the curves of a spiral thread, that suggested the idea of longitudinal bead-like enlarge- ments of the fibril, as producing striae in the fasciculus of volun- tary muscle. In the author's opinion, the dark longitudinal striae are spaces (probably occupied by a lubricating fluid) between the edges of flat filaments, each filament being composed of two spiral threads, and the dark transverse striae, rows of spaces between the curves of these spiral threads. The filament now mentioned, or its edge, seems to correspond to the primitive marked thread or cylinder of Fontana — to the primitive fibre of Valentin and Schwann — to the marked filament of Skey — to the elementary fibre of Mandl — to the beaded fibril of Schwann, Miiller, Lauth, and Bowman — and to the granular fibre of Gerber. The changes known to be produced by the alternate shortening and lengthening of a single spiral are exhibited in the microscope by a fasciculus of spirals, not only in its length and thickness, but in the width of the spaces (stria) between the curves of the spirals. And a muscle being no other than a vast bundle of spirals, it is in contraction short and thick ; while in relaxation it is long and thin ; and thus there occurs no flattening of bead-like segments in contraction. The author has found no segments that could undergo this change. These observations on the form of the ultimate threads in voluntary muscle, were first made on the larva of a Batrachian reptile ; and have been confirmed by an examination of this structure in each class of vertebrated animals, as well as in the Crustacea, MoUusca, Annelida, and Insects.
He finds that the toothed fibre, discovered by Sir David Brewster in the crystalline lens, is formed out of an enlarged filament ; the projecting portions of the spiral threads in the filament, that is, the apparent segments, becoming the teeth of that fibre.
The compound filaments are seen with peculiar distinctness in the blood-vessels of the arachnoid membrane. In connexion with the spiral direction of the outer filament in these vessels, the author refers
