54 TEXT-BOOK OF PHYSIOLOGY. 



tenance of physiologic conditions. If the nutrition is impaired by fatigue, 

 deficient blood-supply, or any pathologic condition, the elasticity is at once 

 impaired. 



Tonicity. Muscle tonus may be defined as a state of tension of a 

 muscle due to a slight but continuous contraction of its individual fibers in 

 consequence of which it tends to become shorter, and would do so, were it 

 not restrained by its attachments. As a result of this tension its efficiency 

 as a quickly responsive motor organ is increased. Though the skeletal 

 musculature of the body as a whole is in a state of tonus, individual muscles 

 vary in the degree of their tonus from time to time in consequence of varia- 

 tions in the causes that give rise to it. That such a tonus or tension exists 

 is apparently shown by the fact that when a muscle in a living animal is 

 divided the two portions will retract and separate a certain distance. This 

 would indicate tfyat the muscle even in a state of relative rest is in a slight 

 degree of contraction. 



This condition of tonus is attributed to the continuous arrival of nerve 

 impulses through efferent nerves discharged by nerve-cells in the spinal cord. 

 The tonus was therefore at one time attributed to an automatic activity of 

 the spinal cord. Brondgeest, however, showed that this was not the case, 

 but that the activity of the spinal cord and hence the tonus of the muscles is 

 partly reflex in origin inasmuch as it largely disappears on division of the 

 posterior or dorsal roots of the spinal nerves. The afferent impulses excit- 

 ing the cord reflexly may come from the skin in which they are developed 

 by the impressions made by external stimuli, or from the tendons and 

 muscles themselves in which they are developed by the slight degree of 

 extension and variations in extension to which these are subjected from 

 moment to moment. That this latter is a considerable factor in the pro- 

 duction of the tonus is shown by the effects which follow division of the 

 afferent nerves coming from any given muscle group; with the division of 

 the nerves the muscles relax and lose their usual tone. It is also probable 

 that the activity of the cord is partly the result of impulses descending the 

 cord in consequence of cerebral and sense organ activities. 



Muscle tonus or elastic tension plays an important role in muscle con- 

 traction; being always on the stretch the muscle loses no time in acquiring 

 that degree of tension necessary to immediate action on the bone to which 

 it is attached. The working power of a muscle is also considerably increased 

 by the presence within limits of some resistance to the act of contraction. 

 According to Marey, the amount of work is considerably increased when the 

 muscle energy is transmitted by an elastic body to the mass to be moved, 

 while at the same time the shock of the contraction is lessened. The posi- 

 tion of a passive limb is the resultant also of the elastic tension of antago- 

 nistic groups of muscles. Again as a result of the slight but continuous 

 stimulation from the spinal cord the metabolic changes in the muscle material 

 are maintained at a certain level, with a corresponding production of heat. 

 A function of the tonicity would thus be the production of heat, other functions 

 which the tone subserves being more or less secondary. 



Irritability, Contractility. These are terms employed to denote 

 that property of muscle-tissue by virtue of which it responds by a change of 

 form, becoming shorter and thicker on the application of a stimulus. On 



