THE PHENOMENA OF MUSCLE AND NERVE. 57 



the muscle, though stimuli applied directly to the muscle at once causes it 

 to contract. From these facts it is clear that urari poisons the ends of the 

 nerve within the muscle long before it affects the trunk ; and it is exceedingly 

 probable that it is the very extreme ends of the nerves (possibly the end- 

 plates or peculiar structures in which the nerve fibres end in the muscular 

 fibres, for urari poisoning, at least when profound, causes a slight, but yet 

 distinctly recognizable effect in the microscopic appearance of these struc- 

 tures) which are affected. The phenomena of urari poisoning go far to 

 prove that muscles are capable of being made to contract by stimuli ap- 

 plied directly to the muscular fibres themselves ; and there are other facts 

 which support this view. 



42. When, in a recently killed frog, we stimulate by various means 

 and in various ways the muscles and nerves, it will be observed that the 

 movements thus produced, though very various, may be distinguished to be 

 of two kinds. On the one hand, the result may be a mere twitch, as it 

 were, of this or that muscle ; on the other hand, one or more muscles may 

 remain shortened or contracted for a considerable time a limb, for instance, 

 being raised up or stretched out, and kept raised up or stretched out for 

 many seconds. And we find, upon examination, that a stimulus may be 

 applied either in such a way as to produce a mere twitch, a passing rapid 

 contraction which is over and gone in a fraction of a second, or in such a 

 way as to keep the muscle shortened or contracted for as long a time as, up 

 to certain limits, we may choose. The mere twitch is called a single or 

 simple muscular contraction; the sustained contraction, which, as we shall 

 see, is really the result of rapidly repeated simple contractions, is called a 

 tetanic contraction. 



43. In order to study these contractions adequately, we must have 

 recourse to the " graphic method," as it is called, and obtains a tracing or 

 other record of the change of form of the muscle. To do this conveniently, 

 it is best to operate with a muscle isolated from the rest of the body of a 

 recently killed animal, and carefully prepared in such a way as to remain 

 irritable for some time. The muscles of cold-blooded animals remain irri- 

 table after removal from the body far longer than those of warm-blooded 

 animals, and hence those of the frog are generally made use of. We shall 

 study presently the conditions which determine this maintenance of the irri- 

 tability of muscles arid nerves after removed from the body. 



A muscle thus isolated, with its nerve left attached to it, is called a 

 muscle-nerve preparation. The most convenient muscle for this purpose in 

 the frog is, perhaps, the gastrocnemius, which should be dissected out so as 

 to leave carefully preserved the attachment to the femur above, some portion 

 of the tendon (tendo Achillis) below, and a considerable length of the sciatic 

 nerve with its branches going to the muscle. (Fig. 11.) 



44. We may apply to such a muscle-nerve preparation the various 

 kinds of stimuli (mechanical, such as pricking or pinching ; thermal, such 

 as sudden heating ; chemical, such as acids or other active chemical sub- 

 stances ; or electrical) and these we may apply either to the muscle directly 

 or to the nerve, thus affecting the muscle indirectly. Of all these stimuli 

 by far the most convenient for general purposes are electrical stimuli of 

 various kinds ; and these, except for special purposes, are best applied to the 

 nerve, and not directly to the muscle. 



Of electrical stimuli, again, the currents, as they are called, generated by 

 a voltaic cell, are most convenient, though the electricity generated by a 

 rotating magnet, or that produced by friction may be employed. Making 

 use of a cell or battery of cells Daniell's, Grove's, Leclanche, or any other 

 we must distinguish between the current produced by the cell itself, the 



