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TEXT-BOOK OF PHYSIOLOGY 



recalled has been shown by Porter to consist of two portions, a vaso-tonic 

 and a vase-reflex. The former is in a state of continuous tonus of activity ; 

 the latter is capable of being influenced in its activity not only by variations 

 in the composition of the blood but by nerve impulses transmitted to it 

 from all regions of the body (see page 383). 



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

 nective 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 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 inac- 

 tivity, but rather to a loss of nerve influence. It would appear from facts of 



,sp.c. 



FIG. 230. DIAGRAM SHOWING THE STRUCTURES INVOLVED IN THE PRODUCTION OF REFLEX 

 ACTIONS, G. Bachman. r.s. Receptive surface; af.n. afferent nerve; e.c. emissive or motor cells in 

 the anterior horn of the gray matter of the spinal cord, sp.c.; ef.n. efferent nerves distributed to 

 responsive organs, e.g., directly to skeletal muscles, sk.m., and indirectly through the interme- 

 diation of sympathetic ganglia, sym.g., to blood-vessels, b.v., and to glands, g. The nerves 

 distributed to viscera are not represented. 



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 mus- 

 cles 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. 



The tonic contraction of the visceral muscles e.g., the pyloric, the 

 vesical, the anal sphincters though regarded as automatic by some, is 

 probably reflex in origin, dependent on the arrival of afferent impulses from 

 the periphery. It is probable that future investigation will disclose the 

 existence and pathway of these afferent fibers. \; -;^j] 



Reflex Excitation. It has already been stated that the nerve-cells in'the 

 spinal cord are capable of receiving and transforming afferent nerve impulses, 



