MUSCULAR CONTRACTILITY. 133 



The striped and unstriped muscular fibres present certain peculiari- 

 ties in their action, and so do the muscles of the heart. Thus, when 

 the heart is artificially excited at any one point, a rapid and powerful 

 contraction of a large part of its walls ensues, quickly followed by 

 relaxation, and then by a succession of contractions and relaxations. 

 When similarly excited, the unstriped muscular tissue of the intestines 

 contracts more slowly, but more permanently. In non-striated mus- 

 cles generally, the contractions induced by electrical discharges are 

 partial or local, slowly induced, sometimes interrupted, and last after 

 the stimulus is removed; but, in the striated muscles, rigid, general 

 contractions quickly occur, continue so long as the stimulus is applied, 

 and cease suddenly on its withdrawal. Contractions, induced indi- 

 rectly through the nerve-trunks, are more sudden, general, and ener- 

 getic than those occasioned by direct stimulation of the muscle. Some 

 involuntary muscular fibres are more easily and powerfully excited 

 than others, as, for example, those of the intestines, as compared with 

 those of the gall-bladder and the ureters, even cold air being sufficient 

 to induce contractions in them. The peculiarities now described are 

 manifested also in the ordinary actions of the different kinds of mus- 

 cular tissue during life. Thus the voluntary striped muscular tissues 

 act suddenly, powerfully, and in effective combination, whilst the plain 

 involuntary muscles contract much more slowly, partially, and feebly; 

 as witness the quick, general, and energetic movements of the limbs, 

 fingers, tongue, and eyelids, as compared with the slower, more local, 

 and weaker movements of the muscular coat of the stomach and intes- 

 tines. In the latter organs the degree of contraction is likewise about 

 one-third. The contraction also presents a great peculiarity, viz., that 

 of being propagated onwards, or travelling along successive portions 

 of the alimentary canal, by what is called vermicular or peristaltic 

 action. It is, moreover, in these and other cases, excited by the con- 

 tents of the muscular canals. In the heart of man and the higher 

 animals, the imperfectly striated muscle, probably from some peculiar- 

 ity of its nerves, contracts repeatedly, in regular and continuous order, 

 alternating with certain periods of repose : this is called rhythmic con- 

 traction. It is observed also in the lymphatic hearts of the frog, and 

 in the hearts or dorsal vessels of the lower animals. 



The force with which a voluntary muscle contracts, is very great, 

 much greater than the mechanical resistance offered by a dead muscle. 

 It is equal to the lifting, through a minute distance, of a mass six- 

 teen to seventeen thousand times its own weight ; but as the distance 

 is increased, the weight lifted is diminished. This force is exerted 

 more favorably at the commencement of contraction, and gradually 

 diminishes to zero, as the muscle shortens to its extreme degree. The 

 amount of force depends on the number of the contracting fibres, 

 whilst the degree of shortening depends on the length of those fibres. 



In order that a muscle should act properly, its temperature must be 

 at a due elevation, its supply of blood must be sufficient in quantity 

 and of proper quality, and its nutrition amply provided for in the 

 intervals of contraction. Arterial blood is essential to the healthy 

 maintenance of muscular contractility. If the muscles be subjected to 



