CHAP, iv.] THE VASCULAR MECHANISM. 199 



Vaso-motor nerves. In warm-blooded animals, though we 

 cannot readily, as in the frog, watch the circulation under the 

 microscope, we have abundant evidence of the influence of the 

 nervous system on the calibre of the arteries. Thus in the 

 mammal, division of the cervical sympathetic on one side of the 

 neck causes a dilation of the minute arteries of the head on the 

 same side, shewn by an increased supply of blood to the parts. 

 If the experiment be performed on a rabbit, the effect on the 

 circulation in the ear is very striking. The whole ear of the side 

 operated on is much redder than normal, its arteries are obviously 

 dilated, its veins unusually full, innumerable minute vessels before 

 invisible come into view, and the temperature may be more than a 

 degree higher than on the other side. 



Division of the sciatic nerve in a mammal causes a similar 

 dilation of the small arteries of the foot and leg. Where the 

 condition of the circulation can be readily examined, as for instance 

 in the hairless balls of the toes, especially when these are not 

 pigmented, the vessels are seen to be dilated and injected ; and a 

 thermometer placed between the toes shews a rise of temperature 

 amounting, it may be, to several degrees. 



The quantity of blood present in the blood-vessels of the mammal 

 may sometimes be observed directly, but has frequently to be determined 

 indirectly. The temperature of passive structures subject to cooling in- 

 fluences, such as the skin, is largely dependent on the supply of blood, 

 the more abundant the supply the warmer the part. Hence in these 

 parts variations in the quantity of blood may be inferred from varia- 

 tions of temperature ; but in dealing with more active structures there 

 are obviously sources of error in the possibility of the treatment 

 adopted, such as the stimulation of a nerve, giving rise to an increase of 

 temperature due to increased metabolism, independent of variations in 

 blood supply. 



The quantity of blood may also be determined by the plethysmograph. 

 In this instrument, a part of the body, such as the arm is introduced into 

 a closed chamber tilled with fluid, ex. gr. a large glass tube, the opening 

 by which the arm is introduced being secured with a stout caoutchouc 

 membrane. An increase or decrease of blood sent into the arm 

 will lead to an increase or decrease of the volume of the arm, and 

 this will make itself felt by an increase or diminution of pressure in the 

 fluid of the closed chamber, which may be registered and measured iii 

 the usual way. We shall have to speak again of a modification of this 

 instrument when we are dealing with the kidney! 



Division of the brachial plexus produces a similar dilation of 

 the blood-vessels of the front limb. Division of the splanchnic 

 nerve produces a dilation of the blood-vessels of the intestines and 

 other abdominal viscera. Division in the mammal of the hypo- 

 glossal nerve on one side causes a dilation of the vessels in the 

 corresponding half of the tongue. Division of a nerve supplying 

 a muscle causes a large and sudden increase in the venous flow 



