58 MUSCULAR IRRITABILITY. [BOOK i. 



the other hand, if the muscle be allowed to remain in the body, 

 and so be exposed to the action of the poison, but the nerve be 

 divided high up and the part connected with the muscle gently 

 lifted up before the urari is introduced into the system, so that no 

 blood flows to it arid so that it is protected from the influence of 

 the poison, stimulation of the nerve will be found to produce no 

 contractions in the muscle, though stimuli applied directly to the 

 muscle at once cause 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 recognisable effect in the microscopic appearance 

 of these structures) which are affected. The phenomena of urari 

 poisoning therefore go far to prove that muscles are capable of 

 being made to contract by stimuli applied 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 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 con- 

 traction. 



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

 have recourse to the ' graphic method ' as it is called, and obtain 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 irritable 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 

 irritability of muscles and nerves after removal from the body. 



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



