NERVOUS CONTROL OF THE BLOOD-VESSELS 



997 



medius between the seventh and eighth cranial nerves. The nervi erigentes 

 leave the lower end of the cord by the anterior roots of the second and third 

 sacral nerves. All of them, like the vaso- constrictors and probably all 

 visceral nerve fibres, are interrupted by ganglion- cells before reaching to 

 their destination. These cells, however, lie, not in the lateral chain of the 

 sympathetic, with which the nerves have no connection at all, but peripherally, 

 and are generally embedded in the organs to which the nerves are distributed. 

 Thus the chorda tympani fibres to the submaxillary glands are interrupted 

 by cells embedded in the gland itself. The nervi erigentes pass to ganglion- 

 cells in the hypogastric plexus lying on the neck of the bladder. 



Whether any large numbers of the fibres making up the sympathetic 

 system of nerves are vaso-dilator in function is still uncertain. In the dog 

 dilatation of the vessels of the soft palate and gums can be produced by 



A B 



Nerve freshly divided. 

 Constriction. 



Nerve four days degenerated. 

 Dilatation. 



FIG. 464. Plethysmographic tracing of hind limbs, showing effect of stimulating 

 the sciatic nerve on the volume of the limb, A, immediately after section of the 

 nerve ; B, four days after section. The nerve was stimulated between the two 

 vertical lines. Curves to be read from right to left. (BOWDITCH and WAEEEN.) 



stimulation of the cervical sympathetic of the same side, or of the stellate 

 ganglion or its rami communicantes. The effect has not yet been observed 

 in any other animals. It is probable that the splanchnic nerves convey vaso- 

 dilator fibres to the vessels of the abdomen, since stimulation of these nerves 

 may cause a fall of blood-pressure, provided that the constrictor fibres, which 

 predominate, have been paralysed by the previous administration of large 

 doses of ergotoxin, derived from ergot. 



The presence of vaso-dilator fibres in the nerves going to the limbs has 

 been the subject of much debate. Since these nerves contain also con- 

 strictor fibres, the effect of the constriction overpowers any effects due to 

 simultaneous stimulation of possible dilator fibres. Moreover the dilators 

 apparently do not conduct any tonic influences to the blood-vessels, so that 

 the only effect of section of a mixed nerve is that due to the removal of the 

 tonic constrictor influences, and the vessels in the area of distribution of the 

 nerves are dilated. 



Various methods have been employed to show the presence of dilator 

 fibres in such a mixed nerve-trunk. Of these the chief two are those depend- 

 ing on the unequal time taken for the two sets of fibres to degenerate and 

 on the varying excitability of the two sets of fibres to different kinds of 

 stimulation. Thus, if the sciatic nerve be cut, a primary dilatation of the 

 vessels of the leg and foot is produced, which, however, passes off after two 



