THE MUSCULAR TISSUES. In man, as in the other vertebrate., we distinguish three kinds of muscu lar tissue: ( 1 ) the cross-striped or voluntary muscle which forms the musculature of the body by means of which our voluntary movements are made: (2.1 the heart-musele, composing the walls of the heart: and (3) the plain or unstriped muscle which is found in the walls of the hollow viscera, such as the alhnentary canal, the blood vessels, the uterus. the bladder. etc. The com mon characteristic of these three tissues is the property of contracting. While this property may occur in other tissues. it reaches its great est development in the muscles. In cross-striped muscles the contraction is always started by a stimulus received through the nerves and orig inated usually in the brain by an act of the will. The contraction is very rapid. .\ single stimulus gives a single o• simple contraction that occupies less than one-tenth of a second. All of our or dinary voluntary contractions are. however. com pound or tetanie. that is. they are composed of a rapid series of simple contractions fused together more or less completely. The rapid series of con tractions is due to a series of stimuli or nerve ha pulses received from the brain through the nerve fibres it with the muscle. III every voluntary movement. therefore, whether short or prolonged, we IllUst imagine to ourselves a series of changes in the nerve cells of the brain. which are propagated along the connecting nerve fibres to the muscle or muscles and there set up a series of contractions so rapid that they become fused into a long-lasting contraction or movement.
Plain muscle is characterized by the slowness of its contractions. The tissue as found in dif ferent parts of the body varies somewhat in this respect. hut in all eases its contractions are very much -lower than in the cross striped muscle. The phin muscle, moreover, is not under volun tary control; we are not conscious of the n ents of the stomach, the blood-vessels. etc.
contractions of these organs are brought about by conveyed to the muscle through the so called sympathetic nervous system. The slow and prolonged contractions of plain muscle ex plain the gentle, long-continued movements of the viscera such as occur in the action of the stomach and intestines in propelling food along their length. Heart-muscle is somewhat intermediate in structure between the plain and the cross striped varieties. Its contractions are also inter mediate in duration, but approach closer to the quick MONT Wen t at the striped muscle. The pre dominant characteristic of heart-muscle, how ever, is its property of making spontaneous rhythmic contractions which are due not to ex ternal stimuli received through the nervous sys tem, but to processes arising within itself. To this property is due the rhythmic heat of the heart.