1122 



PHYSIOLOGY 



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

 are generally embedded in the organs to which the nerves are dis- 

 tributed. 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 sympa- 

 thetic 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 stimulation of the cervical sympathetic of the same 



A B 



Nerve freshly divided. 

 Constriction. 



Nerve four days degenerated. 

 Dilatation. 



FIG. 469. Plethysmographic tracing of hind limbs, showing effect of stimu- 

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

 section of the nerve ; B, four days after section. The nerve was stimu- 

 lated between the two vertical lines. Curves to be read from right to 

 left. (BOWDITCH and WABEEN.) 



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 the 

 active principle of 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 constrictor 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 depending on the unequal time taken for the two sets of fibres to 



^enerate and on the varying excitability of the two sets of fibres 

 kinds of stimulation. Thus, if the sciatic nerve be cut, 

 v dilatation of the vessels of the leg and foot is produced, 



