596 AUTOMATIC ACTION. [BOOK in. 



Of the various regular automatic centres, both the numerous 

 ones in the medulla oblongata, such as the vaso-motor, respiratory, 

 &c., and the more sparse ones in other regions of the cord, such as 

 those connected with micturition, defsecation, erection, parturition, 

 and so on, we have treated or shall have to treat so fully in 

 reference to their respective mechanisms, and discussed how far 

 they are purely automatic, or in reality merely reflex in nature, 

 that nothing more need be said here. 



It has been much disputed whether the spinal cord exercises 

 over the skeletal muscles a tonic action comparable to that of the 

 vaso-motor centres over the smooth muscles of the arteries. The 

 arguments which were once brought forward as proving the exist- 

 ence of such a tone are invalid. It is true that when a muscle is cut 

 across in the living body, the section gapes ; but this is because all 

 the muscles of the body are slightly stretched beyond their normal 

 length. Again, when one side of the face is paralysed the mouth 

 is drawn to the opposite side, not because the paralysed muscles 

 have lost tone, but because there are on the paralysed side no con- 

 tractions to antagonise the effect of the continually repeated con- 

 tractions of the sound side. And indeed the existence of such a 

 tone seems distinctly disproved by the fact that, according to most 

 observers, when in the living body the nerve going to a muscle is 

 cut no permanent lengthening of the muscle is caused. On the 

 other hand, when the sciatic plexus of one leg of a brainless frog is 

 cut, and the animal is suspended, that leg hangs down more help- 

 lessly than the other ; that is to say the sound leg is rather more 

 flexed than the other. The difference which is sometimes marked, 

 but sometimes hardly visible, disappears entirely when the whole 

 cord is destroyed. But the same flaccidity is observed in a leg 

 in which the posterior roots only of the sciatic plexus have been 

 divided. Hence it is to be regarded as an instance not so much of 

 automatic tone, as of feeble reflex action occasioned by afferent 

 impulses. 



Though however the view of a real tone lacks adequate support, 

 several considerations favour the idea that the condition of a 

 muscle, apart from its being in a state of contraction or at rest, is 

 closely dependent on influences proceeding from the spinal cord. 

 We saw, in treating of muscle and nerve (p. 92), that the irritability 

 of a muscle is markedly affected by the section of its nerve, i.e. 

 by severance from the central nervous system ; and more recently, 

 (p. 471) in speaking of the so-called trophic action of the nervous 

 system, we referred to changes in the nutrition of muscles oc- 

 casioned by diseases of the nervous system. An instance of a 

 similar action is afforded by the so-called ' tendon-phenomena.' It 

 is well known that when the leg is placed in an easy position, as 

 when resting on the other leg, a sharp blow on the patellar tendon 

 will cause a sudden jerk forward of the leg, brought about by a 

 contraction of the quadriceps femoris. Similarly the muscles of the 



