332 VASO-MOTOR NERVES OF THE LIMBS. [BOOK i. 



The cervical sympathetic on the other hand is not exclusively 

 vaso-constrictor. It contains as we have seen vaso-constrictor 

 fibres for the ear. It also contains vaso-constrictor fibres for other 

 regions of the head and face. For instance the branches of the 

 cervical sympathetic going to the submaxillary gland of which we 

 just spoke (Fig. 74 n. sym. sm.), contain vaso-constrictor fibres for 

 the vessels of the gland ; stimulation of these fibres produces, on 

 the vessels of the gland, an effect exactly the opposite of that 

 produced by stimulation of the chorda tympani ; to this point we 

 shall have to return when we deal with the gland in connection 

 with digestion. And we might give other instances ; in fact the 

 dominant effect on the blood vessels of stimulating the cervical 

 sympathetic is a vaso-constrictor effect. There are however certain 

 cases in which the opposite effect, a vaso-dilator effect, in certain 

 regions has been observed as the result of stimulating the cervical 

 sympathetic. And we may now turn to other nerves in which 

 such a double effect, now a vaso-constrictor, now a vaso-dilator 

 effect, may be more readily obtained. 



In the frog as we have seen, division of the nerves of the leg 

 leads to a widening of the arteries of the web of the foot of the same 

 side, and stimulation of the peripheral end of the nerve causes a 

 constriction of the vessels, which, if the stimulation be strong, may 

 be so great that the web appears for the time being to be devoid 

 of blood. Also in a mammal division of the sciatic nerve causes a 

 similar widening of the small arteries of the skin of the 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. If more- 

 over the peripheral end of the divided nerve be stimulated, the 

 vessels of the skin become constricted, the skin grows pale, and the 

 temperature of the foot falls. And very similar results are obtained 

 in the forelimb by division and subsequent stimulation of the 

 nerves of the brachial plexus. 



The quantity of blood present in the blood vessels of a part of the 

 body or of an organ of the mammal may sometimes be observed 

 directly by means of the plethysmoyraph, of which we have already 

 spoken ( 122), but has frequently to be determined indirectly. The 

 temperature of a passive structure subject to cooling influences, 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 variations of 

 temperature ; but in dealing with more active structures such as 

 muscles 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. 



