540 



PRINCIPLES OF GENERAL PHYSlOLO(,Y 



and not of a discontinuous nature, in fact it seems like that of the mechanical 

 state of the muscle. One would be inclined to associate it with the persistence of 

 the state of tensile stress, which, owing to the existence of the mechanism pre- 

 venting its disappearance, remains in the state to which it was brought by the 

 stimulus. 



If the electrical response is due to the disappearance of the polarised state 

 of some membrane, owing to its becoming completely permeable, it might be 

 supposed that the mechanism consists in some method of maintaining the state of 

 permeability. 



THE AURICLE OF THE TORTOISE 



The auricle of the tortoise, as shown by Fano (1887), undergoes slow rhythmic 

 changes in tonus, on which the ordinary beats are superposed. Rosenzweig (1903) 



showed that these tonic contractions 

 are due to the presence of non-striated 

 muscle fibres. Fano and Fayod (1888) 

 had shown that the vagus nerve 

 increases these tonic changes, an 

 action opposite to that which it 

 exerts on the auricular muscle proper. 

 Oinuma (1910) confirmed this result, 

 and showed further that the sympa- 

 thetic nerve inhibits the tonic 

 changes. 



The interests of these facts is that, as 

 Gaskell points out, it seems that some 

 extraneous muscle, which has similar rela- 

 tions to the vagus and sympathetic as that 

 of the alimentary canal, must have been 

 incited in the heart muscle during the 

 course of evolution. 



FIG. 173. RELATIVE DURATION OF THE MECHAN- 

 ICAL AND ELECTRICAL CHANGES IN THE 

 AURICLE OF THE TORTOISE. 



Upper curve electrogram. 

 Lower curve mechanical response. 



(Mines, 1913, 2, p. 333.) 



TONUS IN SKELETAL MUSCLE 



The tonic state into which certain skeletal muscles, most markedly the 

 extensors of the limbs, fall after separation of the centres below the corpora 

 quadrigemina from the higher parts of the brain, has been described above 

 (page 417). Sherrington (1909, 3) has discovered that this condition shows 

 several remarkable properties, which remind one in many ways of those of the 

 smooth muscles just described, the "difference being that, in the skeletal nniM-le, 

 the phenomena are due to the intervention of the central nervous system. These 

 phenomena are of such a nature as to have led their discoverer to call the state 

 " Plastic Tonus." 



Suppose that we start with a limb in the rigid state, with extensors contracted, 

 grasp the fore limb and attempt to flex it. After a certain latent time, the 

 resistance to. movement will be felt to relax and the limb can be flexed to any 

 point desired. The striking thing about this degree of flexion is that the limb 

 remains fixed in the position to which it was moved. Again, suppose that the 

 tonic rigidity has relaxed spontaneously, as usually happens, and that we place 

 the limb in various degrees of extension, again it remains where it was placed. 

 These phenomena are due to reflexes from the proprio-eeptors in the muscles 

 themselves, since they disappear after section of the dorsal roots containing the 

 afferent fibres from these receptors. 



The existence of this tonic reflex from the muscle itself can be shown by 

 stimulating the uncut nerve to it at a rate of about four to five stimuli per second. 

 A continuous steady contraction curve is traced; whereas, if the nerve be cut and 

 its peripheral end stimulated in the same way, a series of separated twitches is 

 traced. Similar results can be obtained by causing reflex contractions in the 

 normal and in the " de-afferented " muscle. The former are of a tonic nature even 



