228 ESSENTIALS OF PHYSIOLOGY. 



The Vaso-Dilator Nerves. In many parts of the body the arterioles 

 are supplied not only with vaso-constrictor but also with vaso-dilator 

 nerves, stimulation of which produces dilatation of these vessels owing 

 to relaxation of their muscular walls. The chorda tympani nerve, for 

 example, sends vaso-dilator fibres to the vessels in the sub-maxillary 

 gland, and when it is stimulated the blood flow through the gland is 

 increased, and may become four or five times as large as that taking 

 place before stimulation of the nerve. Since the general blood pressure 

 remains unaltered, this increase in the blood flow through the gland 

 must be due to dilatation of its arterioles. Vaso-dilator fibres are also 

 found in the nerves supplying the other salivary glands, the tongue, and 

 other structures in the head. Similar fibres leave the spinal cord by 

 the anterior roots of the second and third sacral nerves, and stimulation 

 of these nerves, which are called the nervi erigentes, causes dilatation 

 of the blood-vessels of the generative organs and the rectum. 



The vaso-dilator nerves show two important points of difference 

 from the vaso-constrictor nerves. In the first place, mere section of 

 the nerves produces no obvious effect upon the calibre of the blood- 

 vessels, so that, unlike the vaso-constrictors, the vaso-dilator fibres do 

 not appear to exercise a continuous influence upon the tone of the 

 arterioles. Secondly, the cell stations for these nerves lie, not in the 

 sympathetic ganglia, but close to or even within the organ whose 

 arterioles they supply. 



In the instances just given the nerves contain only vaso-dilator 

 fibres, but in the nerves supplying the limbs both vaso-dilator and 

 vaso-constrictor fibres are present. Stimulation of the peripheral end 

 of a nerve, such as the sciatic, usually causes vaso-constriction, though 

 the existence of vaso-dilator fibres can be demonstrated in one of the 

 following ways : 



(1) If the sciatic nerve is divided and its peripheral end stimulated 

 immediately, the arterioles constrict, but when the nerve is stimulated 

 two or three days after section, the arterioles dilate. This result is due 

 to the fact that the constrictor fibres degenerate and cease to carry 

 impulses earlier than do the dilator fibres. 



(2) If the sciatic nerve is stimulated with single induction shocks 

 repeated at intervals of one to two seconds, these shocks stimulate 

 only the dilator fibres, and the arterioles dilate. 



(3) The dilator nerves are excited more readily than the con- 

 strictor nerves by mechanical stimuli, such as pinching the nerve. 



The constriction or dilatation of the arterioles, thus produced, de- 

 creases or increases the amount of blood flowing through the vessels of 

 the limb, and alters the volume of the limb. These changes in volume 



