144 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



several sympathetic cells. If such a fiber were stimulated at x, 

 then the impulse passing back along the axon in a direction the 

 reverse of normal would stimulate cells b and c, giving effects that 

 are apparently reflex, but which differ from true reflexes in that 

 the stimulating axon belongs to a motor neuron. Under normal 

 circumstances it is not probable that an effect of this kind can be 

 produced. 



The Tonic Activity of the Spinal Cord. In addition to the 

 definite reflex activities of the cord, each traceable to a distinct 

 sensory stimulus, there is evidence to show that many of its motor 

 neurons are in that state of more or less continuous activity which 

 we designate as tonic activity or tonus. There is abundant reason 

 for this belief in regard to many of the special centers of the cord 

 and brain, such as the vasomotor center, the center for the sphinc- 

 ter muscle of the iris, the centers for the sphincter muscles of the 

 bladder, the anus, etc. But the evidence includes the motor 

 neurons to the voluntary as well as the involuntary musculature. 

 In a decapitated frog the muscles take a definite position, and 

 Brondgeest showed that if such an animal is suspended, after cut- 

 ting the sciatic plexus in one leg, the leg on the uninjured side 

 takes a more flexed position. The explanation offered for this 

 result is that the muscles on the sound side are being innervated 

 by the motor neurons of the cord. Inasmuch as a result of this 

 kind cannot be obtained from a frog whose skin has been removed, 

 or in one in which the posterior roots have been severed it seems 

 evident that this tonic discharge from the motor neurons is due 

 to a constant inflow of impulses along the sensory paths. The 

 muscle tonus, in other words, is really a reflex tonus, which differs 

 from ordinary reflex movements only in the absence of a sudden, 

 visible contraction and in the more or less continuous character 

 of the innervation. In the section on animal heat the importance 

 of this constant innervation of the muscles as a source of heat is 

 further emphasized. The idea of a more or less continuous but 

 varying activity of the centers in the brain and cord in consequence 

 of the continuous inflow of impulses along the sensory paths fits 

 in very well with many facts observed in the peripheral organs, 

 facts that will be referred to from time to time as the physiology 

 of these organs is considered. 



Effects of Removal of the Spinal Cord. Numerous investi- 

 gators have sectioned the cord partly or completely at various 

 levels. The general results of these experiments as regards loss 

 of sensation or voluntary movement are described in the next 

 section treating of the cord as a path of conduction to and from 

 the brain. But attention may be called here to some of the gen- 



