Not all muscles are, however, extended between bones. The tendons of some pass into soft structures, such as the muscles of the face. In this case, also, the different muscles exercise a mutual power of extension, though it is but slight, and they thus effect a definite balanced position of the soft parts, as may be observed in the position of the mouth opening in the face.--ROSENTHAL, _Muscles and Nerves_.
MUSCULAR FIBERS (p. 3l).--The anatomical composition of flesh is very similar in every kind of creature, whether it be the muscle of the ox or of the fly; that is to say, there are certain tubes which are filled with minute parts or elements, and the adhesion of the tubes together makes up the substance of the flesh. These tubes may be represented grossly by imagining the finger of a glove, to be called the sarcolemma, or muscle- fiber pouch, and this to be so small as not to be apparent to the naked eye, but filled with nuclei and the juices peculiar to each animal. Hundreds of such fingers attached together would represent a bundle of muscular fibers. The tubes are of fine tissue, but are tolerably permanent; whilst the contents are in direct communication with the circulating blood and pursue an incessant course of chemical change and physical renewal.--EDWARD SMITH, _Foods_.
[Illustration: _Smooth Muscle Fibers (300 times enlarged)._]
THE SMOOTH MUSCLE FIBERS consist of long, spindle-shaped cells, the ends of which are frequently spirally twisted, and in the center of which exists a long, rod-shaped kernel or nucleus. Unlike striated muscle, they do not form separate muscular masses, but occur scattered, or arranged in more or less dense layers or strata, in almost all organs. [Footnote: An instance of a considerable accumulation of smooth, muscle fibers is afforded by the muscle pouch of birds, which, with the exception of the outer and inner skin coverings, consists solely of these fibers collected in extensive layers.] Arranged in regular order, they very frequently form widely extending membranes, especially in such tube-shaped structures as the blood vessels, the intestine, etc., the walls of which are composed of these smooth muscle fibers. In such cases they are usually arranged in two layers, one of which consists of ring-shaped fibers surrounding the tube, while the other consists of fibers arranged parallel to the tube. When, therefore, these muscle fibers contract, they are able both to reduce the circumference and to shorten the length of the walls of the tube in which they occur. This is of great importance in the case of the smaller arteries, in which the smooth muscle fibers, arranged in the form of a ring, are able greatly to contract, or even entirely to close the vessels, thus regulating the current of blood through the capillaries. In other cases, as in the intestine, they serve to set the contents of the tubes in motion. In the latter cases the contraction does not take place simultaneously throughout the length of the tube; but, commencing at one point, it continually propagates itself along fresh lengths of the tube, so that the contents are slowly driven forward.