VASCULAR NERVOUS SYSTEM. 89 



diately before. 4. Excitation of the nerves distributed to 

 the abdominal viscera produces, under all circumstances, 

 vascular constriction. Section of the same nerves pro- 

 duces as invariably vascular dilatation. 5- I n some 

 instances excitation of a cutaneous sensory nerve leads, 

 by reflex action, to vascular changes, limited to the area 

 of its distribution. These reflex effects vary according 

 to the mode of excitation. 6. Any nerve of which the 

 excitation (peripheral end after section) leads to vascular 

 dilatation and hyperaemia in the parts to which it is dis- 

 tributed, is called a vaso-inhibitory nerve. Fibres of this 

 kind are contained in the. lingual nerve, some of which 

 are distributed to the submaxillary gland, others to the 

 mucous membrane of the tongue. All erectile organs are 

 provided with vaso-inhibitory nerves, which are distributed 

 to their arterioles. On excitation of these nerves, whether 

 reflex or direct, the arterioles expand, in consequence of 

 which the venous system of the tissue becomes distended 

 with blood. 7. The normal state of contraction of the 

 arteries of a healthy part is called Tonus. The arterial 

 tonus is maintained by the constant activity of the vaso- 

 constrictor centres ; it is also influenced by conditions 

 which act independently of the vascular nervous system, 

 particularly by the temperature of the part (Mosso), by 

 the pressure under which blood flows into it (Heiden- 

 hain), by changes in the structure of the blood vessels 

 (Cohnheim), &c. It is subject to fluctuations which recur 

 at irregular intervals, and may either be limited to par- 

 ticular arteries or may affect so large a number simulta- 

 neously as to produce variations of the volume of the 

 organs supplied by them (Mosso), or fluctuations of arterial 

 pressure (Traube, Hering). 



The regulation of the Circulation of the blood, i.e., the 

 maintenance of such a relation between the activity of 

 the heart and the resistance of the blood vessels as is most 

 advantageous, is effected by the combined action of the 



