306 THE CIRCULATION IN THE BLOOD-VESSELS [CH. XXI. 



the ear of the rabbit. This is a classical experiment associated with 

 the name of Claude Bernard. Division of the cervical sympathetic 

 produces an increased redness of the side of the head, and looking at 

 the ear, the transparency of which enables one to follow the phenomena 

 easily, the central artery with its branches is seen to become larger, 

 and many small branches not previously visible come into view. The 

 ear feels hotter, though this effect soon passes off as the exposure of a 

 large quantity of blood to the air causes a rapid loss of heat. On 

 stimulating the peripheral end of the cut nerve, the ear resumes its 

 normal condition, and then becomes paler than usual owing to exces- 

 sive constriction of the vessels. 



The first part of the experiment, the dilatation following section, 

 can be demonstrated in a very simple way, by pressing the thumb- 



FIG. 300. Tracing showing the effect on blood-pressure of stimulating the central end of the Depressor 

 nerve in the rabbit. To be read from right, to left. T, indicates the rate at which the recording- 

 surface was travelling, the intervals correspond to seconds ; C, the commencement of faradisation 

 of the nerve ; O, moment at which excitation was discontinued. The effect is some time in develop- 

 ing, and lasts after the current has been taken off. The larger undulations are the respiratory 

 curves ; the pulse oscillations are very small. (Foster.) 



nail forcibly on the nerve where it lies by the side of the central 

 artery of the ear. 



Section of a vaso-dilator nerve, such as the chorda tympani, pro- 

 duces no effect on the vessels, but stimulation of its peripheral end 

 causes great enlargement of all the arterioles, so that the submaxillary 

 gland and the neighbouring parts supplied by the nerve become red 

 and gorged with blood, and the pulse is propagated through to the 

 veins ; the circulation through the capillaries is so rapid that the blood 

 loses very little of its oxygen, and is therefore arterial in colour in 

 the veins. Another effect, free secretion of saliva, we shall study in 

 connection with that subject. 



Other examples of vaso-dilator nerves are the nervi erigentes to 

 the erectile tissue of the penis, etc., and of the lingual nerve to the 

 vessels of the tongue. 



