THE SPINAL CORD. 453 



If the cell activity is continuous, though variable in degree from time 

 to time, it gives rise to what is termed tonus, e. g., trophic tonus, 

 vascular, muscle tonus, etc. If the cell activity is intermittent, 

 it imparts to muscles a certain rhythmic activity, e. g., the respiratory 

 movements, tf 



As no effect arises without a sufficient cause, the term automatic 

 has been objected to and the term autochthonic has been suggested 

 (Gad), expressive of the idea that the energy originates in the nerve- 

 cell as a result of a reaction between the cell and its ever-changing 

 environment. A center so acting could not be regarded as primarily 

 a center for reflex action, however much it might be influenced or 

 conditioned secondarily by afferent impulses. Though automatic 

 activity of the spinal cord centers is advocated by some physiologists, 

 the fact must be recognized that with increasing knowledge of reflex 

 activities some of the phenomena hitherto regarded as automatic 

 have been found to be reflex in origin. Whether this will eventually 

 be found true for all forms of so-called automatic or autochthonic 

 activity remains to be seen. 



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

 connective tissue which underlies the assimilation of food, the storing 

 of energy, 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 undergo degeneration in their peripheral portions, but the 

 muscles, glands, and connective tissues to which they are distributed 

 will also undergo similar changes. This is to be attributed not 

 merely to inactivity, but rather to a loss of nerve influence, inasmuch 

 as inactivity leads merely to atrophy and not to degeneration. 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 cus- 

 tomary activities.^The trophic centers and the motor centers are 

 identical, though the two modes of their activity are separate and 

 distinct. 



Vascular Tonus. The state of moderate contraction of the 

 arterioles throughout the body, in consequence of which the average 

 arterial pressure is maintained, is attributed to constant activity of 

 the vaso-motor centers, this activity being conditioned by variations 

 in the composition of blood, either an increase in the quantity of 

 carbon dioxid or a decrease in the quantity of oxygen. The vaso- 

 motor centers are regarded as primarily automatic, though capable 

 of being influenced secondarily by reflected excitations from the 

 periphery or direct excitations from the cerebrum. 



Muscle Tonus. It is well known that if a muscle be divided in 



