NERVES OF THE BLOOD-VESSELS. 253 



The Nerves of the Blood- Vessels. The arteries, as already 

 pointed out, possess a muscular coat composed of fibres 

 arranged across them, so that their contraction will narrow 

 the vessels. This coat is most prominent in the smaller 

 vessels, those of the size which go to supply separate organs, 

 but disappears again in the smallest branches which are about 

 to divide into capillaries for the individual tissue elements 

 of an organ. These vascular muscles are under the control 

 of certain nerves called vaso-motor (p. 186) and these latter 

 can thus govern the amount of blood reaching any organ at 

 a given time. The vaso-motor nerves of the arteries are, 

 like those of the heart, intrinsic and extrinsic. The intrin- 

 sic fibres originate from ganglion-cells in the coats of the 

 arteries or lying alongside them, while the extrinsic origi- 

 nate from cells in the cerebro-spinal centre, from which 

 they commonly pass into the sympathetic system before 

 they reach the vessels. The intrinsic ganglia, like those of 

 the heart, are automatic and tend to keep the muscular 

 coats of the arteries in a constant state of feeble contrac- 

 tion so that, apart from their physical elasticity, the arteries 

 always hold a certain grip on the blood. The contraction, 

 however, is as a rule persistent and steady, or tonic, instead 

 of rhythmic like that of the heart, although slow rhythmic 

 contractions have been seen to occur in some arteries. The 

 difference probably depends rather on the kind of muscle 

 concerned in each case than on the ganglion-cells, since 

 plain muscular tissue, such as is found in the arteries, con- 

 tracts so slowly and remains contracted so long when excited, 

 that stimuli reaching it at intervals which would give a 

 rhythmic beat in cardiac muscle, would keep the arterial per- 

 manently contracted or tetanized. As in the heart, the 

 activity of the arterial intrinsic nervous mechanism is 

 under the control of extrinsic nerves, certain of which, the 

 vaso-constrictors, answer to the accelerator nerves of the 

 heart and increase the activity v of the intrinsic ganglia, 

 while others, corresponding to the cardio-inhibitory fibres, 

 check the activity of the intrinsic vascular nerves. 



/The VaSO-MotOr CentrQ. Tl^ Vfl.gQ^nr>gfrinf.m- gyf.^^^ 



arterial nerves are nearly always in a state of slight activity, 



