42 



AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



electrical, mechanical, thermal, and chemical. Evidently the muscle-proto- 

 plasm is irritable and is capable of developing a contraction independently of 

 the nerves. 



Other Proofs that the Muscle-protoplasm can be Directly Irritated. 

 Muscles with long parallel fibres, such as the sartorius of the frog, contain no 

 nerves at their extremities, the nerve-fibres joining the 

 muscle-fibres at some little distance from their ends. 

 The tip of such a muscle, where no nerve-fibres can 

 be discovered by the most careful microscopical exam- 

 ination, is found to be irritable. The fact that in some 

 of the lower animals there are simple forms of contrac- 

 tile tissue in which nerves cannot be discovered, and 

 which are irritable, is interesting as corroborative evi- 

 dence, although it is not a proof, of the independent 

 irritability of a highly differentiated tissue such as 

 striated muscle. Another similar piece of evidence is 

 to be found in the fact that the heart of the embryo 

 beats rhythmically before nerve appears to have been 

 developed. A proof can be found in the observation 

 that if a nerve be cut it begins to undergo degenera- 

 tion and loses its irritability and conductivity in four 

 or five days, and the excitation of such a nerve has 

 no effect upon the muscle although direct stimulation 

 of the muscle itself is followed by contraction. As 

 degeneration involves not only the whole course of the 

 nerve, but also the nerve end-plates, the contraction 

 must be attributed to the irritability of the muscle- 

 substance. Another point of interest in this connection 

 is the behavior of a dying muscle. If it be struck, 

 instead of contracting as a whole it contracts at the 

 place where it was irritated, the drawing together of 

 the fibres at the part forming a local swelling, or welt. 

 If such a muscle be stroked, a wave of contraction spreads over it, following 

 the' instrument, instead of extending, as under normal conditions, by means of 

 the excited nerve-fibres to other parts. Under these circumstances the circum- 

 scribed contraction would seem to show that the nerves had lost their irrita- 

 bility, or that the nerve-ends no longer transmitted the stimulus to the muscle, 

 and the response was due to the direct excitation of the dying muscle-fibres. 

 This phenomenon is known as an idiomuscular contraction. 



CONDITIONS WHICH DETERMINE THE EFFECT OF EXCITATION. 



The result of the irritation of nerve and muscle is dependent on two sets 

 of conditions namely, (1) Conditions which determine the irritability ; (2) 

 Conditions which determine the efficiency of the irritant. 



It will be necessary for us to study the second set of conditions first, for, 



FIG. 4. Curare experiment : 

 the shaded parts show the re- 

 gion of the body to which the 

 drug had access; the unshaded 

 part, the portion which was 

 protected hy the ligature 

 from the action of the drug. 

 The unbroken lines represent 

 the sensory nerves which 

 carry sensory impulses from 

 the skin to the central nerv- 

 ous system ; the broken lines 

 indicate the motor nerves, 

 which carry motor impulses 

 from the central nervous sys- 

 tem out to the muscles (after 

 Lauder Brunton: Pharmacol- 

 ogy, Therapeutics, and Materia 

 Medico). 



