308 THE CIRCULATION IN THE BLOOD-VESSELS. [CH. xxi. 



produces no effect on the vessels, but stimulation of its peripheral 

 end causes great enlargement of all the arterioles, so that the sub- 

 maxillary 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 there- 

 fore arterial in colour in the veins. Another effect, free secretion 

 of saliva, we shall study in connection with that subject. 



Other examples of vaso-dilatator nerves are the nervi erigentes 

 to the erectile tissue of the penis, &c., and of the lingual nerve to 

 the vessels of the tongue. 



Fig. 289. Flethysmograph. By means of this apparatus, the alteration in volume of the 



arm, E, which is enclosed in a glass tube, A, filled with fluid, the opening through 



which it passes being firmly closed by a thick gutta-percha band, r, is communicated 



to the lever, D, and registered by a recording apparatus. The fluid in A communicates 



with that in B, the upper limit of which is above that in A. The chief alterations in 



volume are due to alteration in the blood contained in the'arm. When the volume is 



' increased, fluid passes out of the glass cylinder, and the lever, D, also is raised, and 



when a decrease takes place the fluid returns again from B to A. It will therefore be 



evident that the apparatus is capable of recording alterations of the volume of blood 



in the arm. 



It is, however, probable that all the vessels of the body receive 

 both constrictor and dilatator nerves. But the presence of the 

 latter is difficult to determine unless they are present in excess ; 

 if they are not, stimulation affects the constrictors most. The 

 effect of section is also inconclusive ; for if a mixed nerve is cut 

 the only effect observed is a dilatation due to removal of the tonic 

 constrictor influence. 



To solve this difficult problem, three methods are in use. 



i. The method of degeneration. If the sciatic nerve is cut, the 

 vessels of the limb dilate. This passes off in a day or two. If 

 the peripheral end of the nerve is then stimulated, the vessels are 

 dilated, as the constrictor fibres degenerate earliest, and so one 

 gets a result due to the stimulation of the still intact dilatator fibres. 



