TONUS 581 



7. TONUS 



By tonus we mean in general a state of continuous excitation observable 

 in many organs, the intensity of which may vary a great deal according to 

 circumstances. Eecent contributions to the subject of internal secretion (cf. 

 page 356) have resulted in showing that tonus is very often caused by a 

 direct stimulating influence of substances formed in the body upon the 

 peripheral organs or upon peripheral or central nerve cells. 



A very interesting example of tonus, which is not dependent upon the central 

 nervous system, is furnished by the observation of Goltz and Ewald that a dog 

 gradually recovers his vascular tonus after extirpation of a large part of the 

 spinal cord (cf. page 583). 



The importance of direct excitation of peripheral organs or nerve cells 

 for the tonus of the different organs cannot be justly estimated at present, 

 for the simple reason that our information on the subject is quite too limited. 

 Nevertheless we know that many organs are kept in a state of tonic excitation 

 through their efferent nerves, and this is evidence that the corresponding 

 centers in the brain and spinal cord are themselves tonically stimulated. The 

 cardiac vagus and the vasoconstrictor centers are notably of this class. Know- 

 ing that both these centers may be stimulated either directly as in asphyxiation, 

 or reflexly by afferent nerves, we are driven to suppose that their tonus is of 

 mixed origin. Whether the automatic or the reflex factor is the more im- 

 portant we cannot decide at present. 



Cross-striated muscles, particularly the sphincters (sphincters ani and 

 vesicas), are usually in a state of tonic contraction (cf. pages 299, 393). 



It has been no simple matter to demonstrate tonus in cross-striated muscles. 

 The observation made in amputations that on cutting through a muscle the 

 cut ends draw asunder leaving a gaping wound has no bearing on the ques- 

 tion; for this merely means that the distance between the points of origin 

 and insertion of a limb muscle is greater than the natural length of the muscle 

 when it is not loaded that is to say, a muscle completely at rest is stretched 

 somewhat and when it is cut, must, of course, gape open. 



The following observation, however, makes it clear that skeletal muscles 

 are in a state of tonus. If a decapitated frog be vertically suspended with the 

 hind legs downward and one, say the right, sciatic nerve be cut, the leg of 

 the same side will hang down more limply than the other. This difference 

 can only be due to the fact that the left leg is still under the influence of the 

 central nervous system (Brondgeest). 



This form of tonus appears to be of reflex origin, for when the afferent 

 spinal roots of the frog are cut the gastrocnemius of the same side relaxes 

 somewhat (Cyon and Steinmann). Some muscles, however, do not elongate 

 when their efferent nerves are cut ; which means that some muscles are not 

 always tonically stimulated to the same extent as some others. 



Muscular tonus may be, to a certain extent, of peripheral origin also. This 

 conclusion is drawn from the experiments by Meade-Smith cited on page 402, 

 showing that heat is formed in a resting mammalian muscle even when physio- 

 logical connection with the nervous system has been interrupted by ligating 

 the nerve. 



