Fontana's account of his observations on the ultimate fibre, is not very different from that of Prochaska. The smallest filaments into which he could divide the muscular fibre by means of a needle, he conRiders to be the prinii the fibres, and a great number of these collected together, he conceives to form the primitive fasciculus ; but it seems somewhat doubtful whether by this he means the same di vision which Prochaska names a fibre. Fontana describes his primitive filament as a solid cylinder, marked exter nally with transverse lines or bands. The filaments lie parallel to each other, and are not twisted together, as he saws is always the case with membrane, and therefore affords a means of easily distinguishing between them. The smallest vessel capable of containing red blood, is about three times larger than the muscular filament, and the smallest nerve about four times larger than the smallest blood-vessel.
The observations of Sir A. Carlisle differ, in many re spects, from those of preceding writers, especially of Pro chaska and Fontana. He describes the ultimate fibre as a solid cylinder, the covering of which is a reticular mem brane, and the contained part a pulpy substance irregu larly granulated; and, when dead, of little cohesive power. He does not specify its exact size, but speaks of it as not being so inconceivably minute as has been hitherto sup posed. The extreme branches of the blood-vessels and nerves may be seen ramifying on its surface, but they do not appear to enter into its substance. Upon the whole, we feel disposed to place confidence in the statements of Sir A. Carlisle ; but it is desirable that the observations should be repeated, as they differ so materially from those of former anatomists, whose authority is too respectable to be hastily abandoned.
Besides the descriptions of the muscular fibre, which profess to be the result of actual observation, many ac counts have been published of their nature and structure, derived from mere speculative opinions. Some writers have spoken of them as being hollow tubes, some as being jointed, and others as composed of a number of parts con nected together like a string of beads. Borelli announced that the muscular fibre consists of a series of hollow vesicles, and deduced from this structure a theory of mus cular contraction, which he supported by a long train of mathematical problems ; and, while mathematical reason ing was admitted into physiology, his demonstrations were conceived to be incontrovertible.
Another opinion entertained respecting the nature of the muscular fibre was, that it is entirely composed of vessels, either possessing some peculiar structure, or con sisting of the small branches of arteries. This hypothesis,
which appears to have been first broached by Hooke, was afterwards adopted by many learned physiologists, espe cially those of the mathematical sect, and was made the foundation of some of their speculations concerning mus cular contraction. The celebrated names of Willis and Baglivi are attached to an erroneous opinion, that besides the longitudinal fibres, muscles possess transverse fibres, crossing the others at right angles, and that these are im portant agents in muscular action. The sagacity of Haller perceived the futility of these fanciful opinions, and his authority greatly contributed to effect their downfall.
Among the more noted hypotheses that have been formed respecting the nature of muscles, independent of their visible appearance, we must not omit to mention one which prevailed very generally about fifty years ago, and was supported by Cullen, that muscles are, to use his own expression, the moving extremities of nerves. The nerves are supposed to be continuous with the fibres of the muscles, and to be absolutely the same substance, but that they experience a change in their structure, so that, when the nerve is col•ierted into muscle, it loses the power of communicating feeling, and acquires that of producing motion. In answer to this hypothesis, it may be sufficient to observe, that which differ in their appear ance and structure, as well as in their physical and chemi cal properties, can have no clan to be regarded as iden tical. The same remark will apply to a similar kind of hypothesis, that muscles and tendons are the same sub stance, differing only in the more condenmed state of the latter ; an opinion which was transmitted from the ancients, embraced by Boerhaave and his disciples, and waz so gene rally adopted, even in the last century, that Haller scarce ly ventures to give a decided opposition to it.
The figure of muscles is infinitely varied, according to their situation and uses, but they may be described as ap proaching to an oval form, swelling out in the centre, and tapering towards the extremities, and commonly having a tendon attached to one at least of their ends. There are considerable spaces between many of the muscles, which are occupied either by fat or membrane; and a safe lodg ment is afforded in these intervals for the trunks of blood vessels and nerves. Most of the larger muscles are si tuated near the surface, covering the bones, and filling up the interstices between them, so as to produce the gene ral form or outline of the body.