CIRCULATION OF BLOOD 77 



(-) Pressor nerves, which produce a strong stimulation 

 of the centre and a constriction of the vessels, hence an 

 increase in blood pressure. 



(/?) Depressor nerves, i.e. centripetal nerves which re- 

 flexively inhibit the action of the centre and thereby cause 

 decreased blood pressure. 



Pressor nerves are found, e.g. in the trigeminus, superior 

 and inferior laryngeal. An example of a depressor nerve is 

 the depressor nerve going from the heart to the vagus and 

 thence to the medulla; stimulation of this nerve causes a 

 lowering of the blood pressure, and decreases the rate of 

 heart-beat. 



(fr) The vaso-dilator nerves produce a widening of the 

 vessels by decreasing the tonus of their muscles. 



Examples of vaso-dilator nerves : 



1. Through the chorda tympani there pass fibres to the 

 submaxillary salivary glands ; the stimulation of this nerve 

 causes dilation of the blood vessels of the gland. 



2. Stimulation of the nervi erigentes (fibres passing from 

 the sacral plexus to the hypogastric plexus) causes accumula- 

 tion of blood in the penis and thus produces erection. 



In general the vaso-dilators accompany the constrictor 

 nerves. In these cases the existence of the two kinds of 

 nerves can be demonstrated, as they possess different 

 irritability. The dilators are stimulated by a weak, slowly 

 interrupted electrical current, the constrictors need a stronger 

 and faster interrupted current. 



After section, the dilators retain their irritability for a much 

 longer time than the constrictors, before degeneration sets in. 



How the dilators diminish the tonus of the muscles is not 

 known. 



It is supposed that the centre for the vaso-dilators is situated in 

 the medulla. 



The innervation of the blood vessels serves to regulate the 

 distribution of the blood to the different parts of the body. 

 This distribution, in so far as it is not dependent upon pure 

 mechanical conditions, is regulated by the tonus of the 



