506 THE NERVOUS SYSTEM. 



ance to the pressure of the blood. The blood moves accordingly under 

 the influence of two opposite forces, namely : First, the cardiac im- 

 pulse, which tends to urge it rapidly through the circulation ; and 

 secondly, the tonic arterial resistance, which tends to delay its passage 

 into the capillary vessels. The tonic arterial contraction varies with 

 the nervous influences which control it; and in this way the local 

 activity of the circulation is increased or diminished. There appears 

 to be furthermore a compensating action in this respect, between the 

 blood-vessels of different parts. When the arteries of one organ are 

 contracted, diminishing the quantity of blood which it contains, vascu- 

 lar pressure must be increased in the neighboring parts, unless a pro- 

 portionate enlargement of their blood-vessels restores the normal rela- 

 tion between them. But when the vascular tone is abolished in any 

 region by division of its sympathetic nerves, its blood-vessels yield to 

 the pressure of the rest of the arterial system, and remain in a state 

 of turgescence and relaxation. 



Dilator Nerves. Beside the nerve fibres which cause contraction 

 of the blood-vessels, there are others which cause their dilatation. The 

 latter class, which, from their mode of action, are called " dilator 

 nerves," do not all pass through the sympathetic system, but are dis- 

 tributed from the cerebro-spinal nerves to the vascular organs. 



The most striking and earliest known instance of the action of a 

 dilator nerve is that of the pneumogastric in connection with the heart 

 (page 489). This action is characterized essentially as follows : 1st. 

 Galvanization of the nerve causes relaxation of the heart, and conse- 

 quently its dilatation by blood flowing in from the large veins ; 2d. If 

 the nerve be divided and galvanization applied to its peripheral extrem- 

 ity, the same effect is produced, showing that the influence is direct in 

 its operation, following a centrifugal course through the nerve to the 

 heart. 



A similar action is exerted on the circulation in the tongue and sub- 

 maxillary gland. These observations, first made by Bernard,* have 

 been corroborated and extended by subsequent experimenters, and 

 especially by Yulpian.f 



The vascular supply of the tongue and submaxillary gland receives 

 nerve fibres from two sources, namely : 1st, sympathetic fibres coming 

 from the carotid plexus and passing with the arterial branches to their 

 distribution ; and 2d, fibres coming from the facial nerve through the 

 chorda tympani, which join the lingual branch of the fifth pair, and 

 are thence distributed to the tongue and submaxillary gland. The 

 influences exerted by these two sets of fibres on the vascularity of the 

 parts are the opposite of each other. Section of the sympathetic 

 filaments causes relaxation of the blood-vessels, increased circulation, 

 ruddy color of the venous blood, and abundant salivary secretion ; 

 while galvanization of their peripheral extremity produces contraction 



* Lepons sur les Liquides de 1'Organisme. Paris, 1859, tome i., p. 312. 

 f Lepons sur 1'Appareil Vasomoteur. Paris, 1875, tome i., p. 153. 



