Besides these, which are more properly styled muscles, muscular fibres are found under a different and less ob vious form, but arc not less important in the animal econo my. Many of the minute, although most important, ope rations of the body, are performed by muscular fibres, not, collected together into bundles or masses, but placed in a less regular and connected manner on the surface of mem branes. This arrangement is generally found in the tubes and pouches with which the body is furnished, and it is by this structure that these organs are enabled to propel and discharge their contents ; these are called muscular coats, and will be described more minutely when we treat of the different organs, of which they compose an impor tant part.
The uses of these two classes of bodies are as different as their structure, and hear an obvious relation to it. The proper muscles are always intended for the motion of some part, by altering its relative position with respect to some other part, while the motions that are caused by the fibres of the muscular coats are destined to operate solely upon the contents of the organs to which they belong, and consist in a number of small contractions, in each of which a few fibres only act at the same time. These two kinds of organs differ in the relation which they bear to the other parts of the system, and particularly to the nerves ; for while the nerves of the proper muscles are derived either from the brain itself, or from the spine, which may be considered as an immediate appendage of the brain, and are, for the most part; more or less dependent upon the will, the muscular coats arc generally supplied from the ganglia, and their action is altogether involuntary., In man, and the more perfect animals, muscles general ly possess a red colour, but this seems not to be essential to them, as, by maceration in water and alcohol, this co lour may be removed. By this means a quantity of albu men and jelly is carried off, as well as some saline bodies, which are always present in the animal fluids ; and if the fibre be then cleared as much as possible from the mem brane and fat, it may be considered as brought to a state of purity. In consequence of the abstraction of these he terogeneous substances, the fibre is rendered much less susceptible of putrefaction, and if it be secured from mois ture, it may be preserved for a long time without any sen sible alteration.
It has been doubted whether the peculiar colour of mus cles depends upon something which is attached to the fibre itself, or whether it merely arises from the blood, which is so plentifully sent to them. Bichat labours to prove that it is not owing to the blood, but to some pro per colouring matter, independent of any fluid that is con tained in the vessels ; and, upon the whole, there seems reason to acquiesce in his opinion. It may be observed that the voluntary muscles are, for the most part, those that possess the deepest shade of colour, and that the co lour is frequently found to be increased the more the mus cles are exercised.
Besides the membranous matter which incloses the whole muscle, and forms the coverings or sheaths of its in dividual parts, there is, in the interstices of the lacerti, a portion of the same kind of cellular texture, which is dis persed through the body generally, and which here, as in other p rts, serves the purpose of containing the oily and adipose secretions. Some writers have contended that, besides this kind of fatty deposit, muscles are furnished with a quantity of oil, which is diffused through their sub stance, and tends to render their motions more easy ; but this we apprehend to be an idea unsupported by any well authenticated facts or analogies.
By digesting muscles in water, a quantity of albumen, jelly, and saline matter, may be extracted from them ; but it dues not appear whether these substances actually en tered into the composition of the fibre itself. It is proba ble that the saline matter is rather contained in the fluids which circulate through the vessels, than the proper fibres; and, as far as it has been examined, it appears to be the same with the saline matter in the blood. It is principally composed of the phosphates of soda, ammonia, and lime, the carbonate of lime, and the inuriate and lac tate of soda. If the water in which muscles have been ma cerated be evaporated, and afterwards digested in alcohol, the extract alone is dissolved, and by removing the alcohol, may be procured in a separate state. This substance was first obtained by Thouvenel. It has a brown colour, an acrid taste, an aromatic odour, is soluble both in water and in alcohol, and is thought to be the ingredient which gives its specific flavour to the flesh of different animals.