THE SPINAL CORD 539 



psychic states of a volitional, or an affective or emotional character; the 

 gland discharges, the vascular and visceral muscle movements are due to 

 psychic states of an affective or emotional character only. 



Automatic Excitation. By this expression is meant a discharge of 

 energy from the spinal nerve-cells occasioned by (a) a change in the chemic 

 composition of the blood or lymph by which they are surrounded or prob- 

 ably a reaction between the constituents of the lymph and the constituents 

 of the nerve-cell or (b) the development within the cell of a stimulus, the 

 so-called "inner stimulus," the outcome of metabolic activity. 



As no effect arises without a sufficient cause the term automatic has been 

 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 arriving 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 larynged sound; (b) the 

 contraction 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 430); (e) the acceleration of the heart-beat (page 323). 



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, 

 and to some extent vascular tonus, and trophic tonus. 



Muscle Tonus. All the skeletal muscles of the body are at all tunes 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 



