186 PRINCIPLES OF PHYSIOLOGY 



way muscles are not loose but firm and slightly 

 contracted, even while at rest. And when a 

 more powerful nervous impulse reaches them, 

 they are ready to contract efficiently. To use 

 a nautical phrase, the muscles are not on the 

 " slack " but always " taut," and no energy 

 is lost in " gathering them in." But certain 

 nerve fibres have the power, not of causing, 

 but of restraining activity. Such nervous 

 actions are said to be inhibitory. A striking 

 mechanism of this kind is seen in connection 

 with the innervation of the heart 



104. This organ has numerous little ganglia 

 in its own substance, and possibly these may 

 have to do with its rhythmic contractions, 

 although this is doubtful. Two great pairs of 

 nerves give off branches that can be traced into 

 the heart. These are the vagi, which come from 

 the medulla oblongata, the portion of the spinal 

 cord inside the skull, and the sympathetics, 

 that arise from a chain of ganglia running along 

 each side of the vertebral column. The fibres in 

 these ganglia are derived from the cord by the 

 anterior roots of the spinal nerves. Suppose the 

 heart to be beating rhythmically, stimulation 



