270 VASO-MOTOR NERVES OF THE LIMBS. [Book i. 



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 plethysmograph, of which we have already 

 spoken (§ 104), 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. 



So far the results are quite like those obtained by division and 

 stimulation of the cervical sympathetic, and we might infer that 

 the sciatic nerve and brachial plexus contain vaso-constrictor 

 fibres only for the vessels of the skin of the hind limb and fore 

 limb, vaso-dilator fibres being absent. But sometimes a different 

 result is obtained ; on stimulating the divided sciatic nerve the 

 vessels of the foot are not constricted but dilated, perhaps widely 

 dilated. And this vaso-dilator action is almost sure to be mani- 

 fested when the nerve is divided, and the peripheral stump stimu- 

 lated some time, two to four days, after division, by which time 

 commencing degeneration has begun to modify the irritability of 

 the nerve. For example, if the sciatic be divided, and some days 

 afterwards, by which time the flushing and increased tempera- 

 ture of the foot, following upon the section, has wholly or largely 

 passed away, the peripheral stump be stimulated with an inter- 

 rupted current a renewed flushing and rise of temperature is the 

 result. We are led to conclude that the sciatic nerve (and the 

 same holds good for the brachial plexus) contains both vaso-con- 

 strictor and vaso-dilator fibres, and to interpret the varying result 

 as due to variations in the relative irritability of the two sets of 

 fibres. The constrictor fibres appear to predominate in these 

 nerves, and hence constriction is the more common result of 

 stimulation ; the constrictor fibres also appear to be more readily 

 affected by a tetanizing current than do the dilator fibres. When 

 the nerve after division commences to degenerate the constrictor 

 fibres lose their irritability earlier than the dilator fibres, so that 

 at a certain stage a stimulus, such as the interrupted current, 

 while it fails to affect the constrictor fibres, readily throws into 

 action the dilator fibres. The latter, indeed, appear to retain their 

 irritability after section of the nerve for a much longer time than 



