BLOOD VESSELS 455 



2iid. Central. — The portions of the central nervous 

 system jjresiding over these and the nerves which pass 

 from them. 



1st. Peripheral Mechanism. — The muscular fibres are 

 maintained in a state of tonic semi-contraction by nerves 

 passing to them, and when these nerves are divided, the 

 muscular fibres relax. But if, after these nerves have been 

 cut, the animal be allowed to live, in a few days the 

 arterioles again pass into a state of tonic semi- contraction, 

 although no union of the divided nerve has taken place. 



Certain drugs, e.g. digitalis and the salts of barium, act 

 as direct stimulants to these muscle fibres, while nitrites 

 inhibit their activity. 



The precise part played by the nerve plexus in the walls 

 of the arterioles has not been definitely established, but 

 certain drugs appear to act specially upon it. Thus, 

 apocodeine, while it does not prevent barium salts from 

 constricting the vessels, prevents the constricting action of 

 adrenalin, even when the nerves are cut. Hence, it must 

 be concluded that apocodeine paralyses a nervous mechanism 

 in the arteriole wall which is stimulated by adrenalin. On 

 the other hand, nicotine seems to block the action of barium, 

 but not that of adrenalin. Deductions from the antagonistic 

 action of drugs are by no means satisfactory, as their action 

 varies so much with dosage and with the functional 

 condition of the tissues at the time when they are 

 administered. 



2'n.cZ. Central Mechanism. — When a nerve, going to any 

 part of the body, is cut, the arterioles of the part 

 generally dilate ; when it is stimulated, the arterioles 

 usually contract ; sometimes, however, they dilate. In no 

 case does section of a nerve cause constriction of the 

 arterioles. 



These facts prove that the central vaso-motor nervous 

 mechanism may be divided into two parts : — 



A. Vaso-constrictor mechanism. 



B. Vaso-dilator mechanism. 



A. Vaso-constrictor Mechanism- — The fact that section of 

 most nerves at once causes a dilatation of the arterioles 



