x VASCULAR MUSCLE AND NEKVES 347 



completely separated from the central nervous system by the 

 plethysmograph, that artificial depression of blood pressure 

 caused by compression of the abdominal aorta, or by stimulating 

 the peripheral trunk of the vagus, or depressor nerve produced 

 an initial shrinkage of volume. Even during the fall of blood 

 pressure, however, there was a gradual return to the earlier 

 volume, while after normal pressure was restored there was a 

 marked increase in volume, a proof that the vessels reacted to 

 the fall of blood pressure by a definite dilatation. Artificial 

 rise of blood pressure (e.g. from stimulation of the splanchnic) 

 produced constriction of the vessels of the limb. 



The muscular coat of the vessels (like the muscular wall of 

 the heart) is automatically active both in constriction and in 

 dilatation the former caused by the shortening, the latter by 

 the lengthening or expansion of the sarcoplasm of the spindle- 

 shaped muscle cells. This hypothesis, which found little favour 

 when we propounded it in 1871-73, as the logical deduction 

 fro ni Weber's theory of muscular elasticity, is now, on the 

 strength of recent work on the automatic rhythmicity of the 

 cardiac muscle cells (that of Gaskell and Engelmann in particular), 

 not only unopposed, but even included, in many modern text- 

 books. " We have repeatedly insisted " (says Foster l ) " that the 

 relaxation of a muscular fibre is as much a complex vital 

 process, is as truly the result of the metabolism of the muscular 

 substance, as the contraction itself ; and there is a priori no reason 

 why a nervous impulse should not govern the former as it does 

 the latter." 



Vascular tonicity and its slow rhythmical oscillations (Fig. 

 158) are probably inherent properties of the smooth muscle 

 cells and the vascular nerves that direct and regulate them, 

 since they exert a double and opposite influence, katabolic and 

 anabolic, upon the metabolism of muscle. The vaso-constrictors 

 function by promoting the dissimilatory processes ; the vaso- 

 dilators, on the contrary, by favouring the assimilatory processes 

 of cellular sarcoplasm. The first are therefore katabolic, the 

 second anabolic nerves to the vessels. The active movements of 

 the vessels are thus regulated by mechanisms perfectly analogous 

 to those which govern the active movements of the heart. 



III. It is known, on the strength of numerous experiments, 

 that every vascular region is supplied with vaso -constrictor nerves. 

 This is plain on recapitulating the most important heads of the 

 copious literature of the subject. 



The great splanchnic nerve contains numerous constrictor 

 fibres which supply the most extended area, since it controls the 

 blood-vessels of the greater portion of the abdominal viscera. 

 Ludwig and Cyon (1866), v. Bezold and Bever (1867), found 



1 Text-Book of Physiology, Part I. p. 313, 5th ed., 1888. 



