OF DI US CLE.
Ix treating on the subject of the muscles, we shall ar range our remarks under four heads : we shall, first, de scribe the structure and composition of muscles; se condly, their properties and uses ; thirdly, their mechanism ; and lastly, we shall make some remarks upon the hypo theses that have been formed to explain their action.
Muscles constitute what is usually termed the flesh of animals, and, in their most usual form, they are composed of masses of fibres, lying parallel to each other, intermix ed with a quantity of membranous matter. \Ve may ob tain an accurate idea of the structure of a muscle by cut ting it transversely, and boiling it for some time. \Ve shall, in this way, perceive, that the whole muscle is in closed in a sheath of membrane, which covers it in every part, except where its ends arp attached to the base. The fibres, we shall find, are disposed into small bundles, each of which is enclosed in a sheath of membrane, while these bundles are divisible into still smaller bundles, appa rently without any limit except the imperfection of our in struments.
By the aid of microscopes, the ultimate fibre, as it is called, or that which is no longer capable of farther subdi vision, has been observed and described ; but as is too often the case in microscopical observations, the account of the different observers differ, both with respect to its size and its structure. Leeuwenhoek, who is so celebrated for the use which be made of the microscope in anatomical researches, describes the ultimate filaments as almost inconceivably minute, some thousands of them uniting to form one vi sible fibre. \Ve learn from him, that the ultimate fibres are serpentine and cylindrical bodies, lying parallel to each other, that they are of the same figure in all animals, but differ considerably in their size ; and he says that their size bears no relation to that of the animal to which they belong. In some instances, we are informed, that the smallest animals have the largest fibres, that of the frog, for example, being larger than that of the ox.
Nuys paid particular attention to the minute structure of muscles, and his descriptions, in many respects, agree with those of Leeuwenhoek, except that he regards the ul timate filament as being always of the same size. He sup poses that the fibres are divided into regular gradations or series ; and estimates that the smallest fibrils of which the last series is composed, arc some hundred times less than the finest hair, a proportion larger indeed than that assign ed by Leeuwenhoek, yet still too minute to enable us to form any distinct conception of it.
Among the more modern anatomists, the labours of Prochaska seem to be entitled to our particular notice, as there is every reason to suppose that he examined the subject with great care, and has given us an accurate de tail of what he observed. To the smallest division of the muscle, which can be separated by mechanical means, he gives the specific name of fibre, while the still more mi nute divisions, which are only to be detected by the use of glasses, he 'styles threads or filaments. He informs us, that each of the fibres, as well as the larger portions of the muscles, or lacerti, is furnished with a distinct mem branous sheath, but it does not appear that this is the case with the filaments. The fibre, when carefully sepa rated from all extraneous matter, he conceives to be of a uniform thickness, through its whole extent, and continu ous from one end to the other. He differs from Leeu wenhock, Nuys, and other preceding anatomists, respect ing the form of the fibre, which they have described as being cylindrical, whereas Prochaska says that it is ob viously of a polyhedral form, and is generally flattened, or thicker in one direction than the other. He farther observes, that they are not always of the same diameter, being thicker in some parts than others ; also, that they are smaller in young subjects, and increase in size as the body increases in bulk generally. With respect to the filaments, their shape and extent are said to be similar to the larger fibres, being flattened polyhedrons, but they differ from the proper fibre in being always of the same mngnitude ; and this he conceives to be about 1-50th part the size of the red globules of the blood. The filaments are solid and homogeneous; when prepared for examina tion, a number of depressions or wrinkles are observable on their surface, which give them a waved appearance ; and, when viewed in certain directions, cause them to as sume a serpentine or zig-zag form ; these he conceives to be produced by the blood-vessels, nerves, and membranous bands which cross them.