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TEXT-BOOK OF PHYSIOLOGY 



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- varying 

 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 is due to the 

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

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

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

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

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



,sp.c. 



FIG. 225. 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. 



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

 from all regions of the body (see page 387). 



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 

 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 



