504 TEXT-BOOK OF PHYSIOLOGY. 



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 contin- 

 uous 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 417); (e) the acceleration of the heart-beat (page 313). 



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. 



Trophic Tonus. The normal metabolism of muscle, gland, and 

 connective tissue which underlies the assimilation of food, the production 

 and storage of energy-holding compounds, and the production of new 

 compounds, is dependent, in the higher animals at least, on the connection 

 of these tissues with the central nerve system; for if the efferent nerves be 

 divided, not only will they themselves undergo degeneration in their per-, 

 ipheral portions, but the muscles, glands, and connective tissues to which 

 they are distributed will also undergo similar changes. This is to be attrib- 

 uted not merely to inactivity, but rather to a loss of nerve influence. It 

 would appear from facts of this character that the normal metabolism is 

 dependent for its continuance on nerve influences. There is no evidence, 

 however, as to the existence of special trophic nerves, separate from 

 those which impart to glands and muscles their customary activities. The 

 trophic centers and the motor centers are identical, though the two 

 modes of their activity are separate and distinct. The activity of the so- 

 called trophic centers which was at one time believed to be automatic is 

 now regarded as due to reflex influences. 



Vascular Tonus. The arteriole muscles throughout the vascular 

 apparatus are also constantly in a state of slight but continuous contraction 

 which assists in the maintenance of an average arterial pressure and is due 

 to the continuous discharge of nerve energy from the general or dominating 

 vasomotor (constrictor) center in the medulla oblongata. The automaticity 

 of this center has also largely been discredited; but whether it is automatic 

 or not it is capable of being influenced in its activity not only by variations 

 in the composition of the blood but by nerve impulses reflected to it from all 

 regions of the body (see page 372.) 



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 



