THE SPINAL CORD 535 



objected to and the term autochthonic has been suggested, as more nearly ex- 

 pressing the facts stated. A center so acting could not be regarded as prima- 

 rily a center for reflex activity, however much it might be influenced second- 

 arily by afferent nerve impulses. If the cell excitation is continuous though 

 variable from time to time, it is said to possess tonus and the organ or tissue 

 thus excited is also said to possess tonus or to be in a state of tonic activity. 

 If the cell discharge is intermittent in character it imparts to certain muscles, 

 e.g., the respiratory muscles, a rhythmic activity. It must, however, be kept 

 in mind that the tonus of nerve-centers as well as of peripheral organs can also 

 be developed and maintained by the inflow of nerve impulses transmitted 

 from the periphery. The reason for the belief that the cord and its upper 

 prolongation, the medulla oblongata, are endowed with autochthonic activity 

 is based on the fact that certain peripheral organs are in a state of continu- 

 ous activity and apparently uninfluenced to any marked extent except tem- 

 porarily by nerve impulses transmitted to the cord through afferent nerves. 

 As illustrations of such continuous activity may be mentioned: (a) the 

 contraction of the abductor muscle of the larynx (the posterior crico-arytenoid) 

 whereby the vocal membranes are separated and the- glottis kept open under 

 all circumstances except during the emission of a vocal sound; (b) the con- 

 traction of the dilatator muscle of the iris; (c) the contraction of the anal 

 and vesic sphincters; (d) the periodic contraction of the respiratory muscles 

 (see page 425); (e) the acceleration of the heart-beat (page 423). 



Though automatic activity of the spinal cord is yet upheld by some 

 physiologists, the fact must be recognized that with increasing knowledge 

 of reflex activities many phenomena previously regarded as automatic have 

 been found to be dependent on peripheral stimulation and therefore reflex 

 in origin. Whether this will eventually be found true for all instances of so- 

 called automatic or autochthonic activity will depend on the results of future 

 investigations. Among the phenomena removed from the sphere of auto- 

 matic, to the sphere of reflex activity may be mentioned muscle tonus, vascular 

 tonus and, trophic tonus. 



Muscle Tonus. All the skeletal muscles of the body are at all times^in 

 a state of slight but continuous contraction, termed tonus, by virtue of which 

 their efficiency as quickly responsive organs is increased. That such a 

 slight contraction is present even in a state of rest is shown by the fact that 

 if a muscle be divided in the living animal the two portions will contract 

 and separate to a certain distance. The condition of the muscle was 

 formerly attributed to an automatic and continuous discharge of energy from 

 the nerve-cells. Brondgeest, however, showed that this tonus is entirely 

 reflex in origin and immediately disappears on division of the posterior 

 roots of the spinal nerves, which would not be the case if the cells in the cord 

 were acting automatically. The afferent nerves in this reflex arise in the 

 muscle or its tendons, and the stimulus is the slight degree of extension to 

 which the muscle is subjected in virtue of its attachments and the ever-varyin 

 position of the limbs and trunk (see page 57). 



Vascular Tonus. The arteriole muscles throughout the vascular appara- 

 tus are also constantly in a state of slight but continuous contraction which 

 assists in the maintenance of an average arterial pressure and i 

 continuous discharge of nerve energy from the general or dominating vaso- 

 tonic (constrictor) center in the medulla oblongata. This center it will 



