FOSSIL MossEs. The earliest fossil mosses, Muscites, allied to the modern genus Polytri chum, are found in the Carboniferous rocks of Commentry, France. Throughout the Mesozoic formations fossil mosses are rare, though their existence during the Liassic is inferred from the presence in rocks of that age of a beetle of the genus Birrhus, which at the present time is known to live in mosses only. Fossil mosses are tissue occurs in three different forms: ( 1 ) Striated Voluntary Muscle. This form of muscle is found in all those muscles which are under the control of the will, in fact all those which in the common use of the term are called 'muscles,' such, e.g. as the biceps. triceps. etc. (2) :Von striated Involuntary Muscle. This form of muscle occurs in the muscles which are not under the control of the will, those which carry on the auto matic functions of life. such as the muscles of the intestine, which con trol its peristaltic action. or the muscles of the arteries and veins, which govern their contractil ity. (3) Striated Inrol Vitto•y ifusrlc or Cardiac uRcle. This is a pecu liar form of muscular tissue found only in the heart. It appears to occupy a sort of interme diate position between the other two forms.
(1) _\on-striatcd Involuntary Muscle. This form of muscular tissue, while not occurring in any such large masses as does the voluntary muscle, has an extremely wide distribution. It forms the museularis mucosa. and muscular coats of the gastrointestinal canal, the muscular walls of the arteries and veins. it the trachea and bronchi of the larger duets of glands. and occurs in the urinary tract. and in the male and female generatixe organ-. etc. It consists of long, slen der. spindle shaped from .075 to .2:30 milli meter in length and from .(104 to .010 milli meter brood. Its sub-4;111(T shows tine longi tudinal markings. A delicate sheath or cell wail has been described and within this is the semi fluid granular protoplasm of the cell body. in the centre of the latter lies a long rod shaped nucleus to ward each end of which a few fine granules are found. These cells are ar ranged in bundles, the individual cells being held together by a cement sub stance. the bundles being surrounded by more or less con nective t issue. Blood vessels ramify in this connective tis sue where they break up into capillaries which form a network among the muscle cells. Nerves. mainly of the syinpa thetic system, pass to the individual muscle cells.
(2) Striated Voluntary Muscle. This form of muscular tissue constitutes the ordinary colon tary muscles of the skeletal system, and occurs in the pharynx, la rynx, diaphragm. genera tive organs, etc. It is composed of long cylindrical fibres. each one of which represents a highly speciailsod devel opment of a single cell. The muscle fibre consists of the muscle sub stance proper, the muscle nuclei, and :111 external sheath or sareolom ma. The muscle snbstance of a fibre shows a longitudinal stria tion wLieh indicates its division into ultimate NI.hile tran,VVr?elY it consists of alter nate light and dark bands or disks. The light ills!: is tossed by a delicate dark line. tie line or membrane of Krause. A light line
across the dark disk I Ilensen's line) has also been but is probably an artefaet.
The minute hit of muscular tissue separated till In Krause's line at either end and by the dividing line between the fibrillae on each side is known as the •sareous element of Bowman.' while the appearanee presented by the eros--cut ends of the tibrilke composing a single fibre is called •Cohnheim's field.' Each fibre is inclosed in a transparent. very delicate, but tough and elastic sheath, which can not always be readily seen. but often shows distin ctly. stretching be tween Ilse separated frag ments of a fibre which has been broken within it, its toughness and elas ticity allowing it to re sist a force sufficient to rupture its more brittle contents. This sheath is known as the sarcolemma or myolemma. the former term being derived from the Greek words sang (flesh) and lemma (skin or husk), the latter from the Greek words may (muscle) and lemma. Just beneath the sareolemma. in the muscle protoplasm, lie the muscle nuclei. They are oval or spindle in shape. their long axes cor responding to those of the fibre. The individual muscle fibres are held together be a delicate connective tissue called the endomysium. The muscle fibres are bound together into bundles or fascicles by a coarser connective tissue, the perimysium, while the entire muscle is sur rounded by a connective tissue sheath, the epimysium. In short muscles single fibres may extend the entire length of the muscle. In longer muscles the single fibres do not extend the en tire length of the muscle. averaging 25 to 50 mil limeters in length. Very long fibres, 100 to 120 millimeters long, are sometimes found. The fibres taper somewhat at their ends and terminate, if in the middle of the muscle, by a blending of the sareolemma with the endomysium of the neigh boring fibres; if in tendon, periosteum, or peri ehondrium, by union of the sarcolemma with the connective tissue. Striated muscle is extremely vasenlar. The larger vessels run in the perims ium, where they branch, and these smaller branches give off capillaries which form a rec tangular meshed net work among the individual fibres. The larger nerve trunks run in the perimy wIfere they subdivide and send branches to the mu sole fibres. Here they terminate in special 'end plates.' See Nutvors SYSTEM.
IRl Oardiae or Wart If uselr Rtriafed inrol untory Muscle). This is a type of muscular tissue peculiar to the heart. It resembles the smooth involuntary muscle in being cmup.ised of nucleated cells. These cells an.. however, much larger and broader than the smooth muscle cells, are rectangular in shape, and send off lateral branches hick join similar branches of with which they are connected by eement substance. The cell protoplasm shows both longitudinal and transverse stria tions, but much less marked than in striated voluntary umsc•le. A cross-section Of a cell gives some what the appearance of Cohn beim's field, but the fibrilla. show a more less radial arrangement about the circumference. The nu cleus is round or slightly oval in shape and is situated near the centre of the cell.