370 TEXT-BOOK OF PHYSIOLOGY. 



gestive dilatation of the blood-vessels of the upper and lower lips, gums, 

 cheeks, nasal mucous membrane, and corresponding cutaneous regions. 

 Stimulation of peripheral nerve trunks, providing the stimulation is not too 

 rapid will be followed by a dilatation of the blood-vessels and an increase in the 

 volume of the limb. Stimulation of the peripheral end of the divided 

 chorda tympani nerve is at once followed by an active dilatation of the 

 blood-vessels of the submaxillary gland. The inflow of blood is so great that 

 the gland becomes bright red in color. Its tissues being unable to appropriate 

 all the oxygen, the blood emerges in the veins almost arterial in character. 

 Stimulation of the peripheral ends of the divided nervi erigentes is followed 

 by similar effects in the blood-vessels of the corpora cavernosa. Slow stimu- 

 lation, once per second, of the peripheral end of a divided sciatic nerve is 

 followed by dilatation of the blood-vessels of the leg. 



From these and many other facts of a similar character it is probable 

 that the blood-vessels of each organ are under the control of two antagonistic 

 classes of nerve-fibers, one augmenting the degree of their contraction, 

 the vaso-constrictors, the other diminishing it through inhibition, the 

 vaso-inhibitors. Through the cooperative antagonism of these two classes 

 of nerves the caliber of the blood-vessels and thereby the volume of the 

 blood is accurately adapted to the needs of each organ both during rest 

 and during activity. It is also to the alternate activity of these nerves that 

 the variations occurring from time to time in the volume of organs are to be 

 attributed. 



A general vaso-dilatator center has never been located and there are 

 many reasons for thinking that such a center has no anatomic existence. 

 There are, however, special or local vaso-dilatator centers in the medulla 

 oblongata and in various regions of the spinal cord especially in the sacral 

 region. 



Antidromic Vaso-dilatator Nerve-fibers. Though it has been generally be- 

 lieved that the vaso-dilatator nerves for the blood vessels of the limbs and trunk 

 arise from nerve cells in the ventral horns of the grey matter; that they pass out- 

 ward through the ventral roots of the thoracic and lumbar nerves, that they 

 belong to the efferent system of nerves, yet these facts have never been positively 

 determined. While this may be the correct interpretation doubt has been thrown 

 upon it by the investigations of Bayliss. From the results of a long series of ex- 

 periments this investigator concludes that special vaso-dilatator nerves for the 

 regions of the body just mentioned, do not leave the spinal cord in the ventral 

 roots; that the vase-dilatation observed on stimulation of the mixed spinal nerve 

 is due to the presence of nerve fibers that do not differ from the ordinary 

 afferent or sensor, posterior or dorsal root fibers; that these nerve fibers moreover 

 have their origin in the nerve cells of the ganglia of the dorsal roots. From the 

 fact that they transmit nerve impulses to blood vessels in a direction contrary to 

 that of other afferent nerve fibers, the term antidromic has been given to them. 

 The centers from which they arise are capable apparently of being aroused to 

 activity by impulses transmitted to them from other regions of the body. These 

 statements are based on the following facts: Stimulation of the peripheral ends 

 of the divided dorsal roots of the upper thoracic and lumbo-sacral nerves gives 

 rise to vascular dilatation in the upper and lower limbs; separation from the cord 

 is not followed by their degeneration, hence they are not efferent nerves; extirpa- 

 tion of the ganglia of the dorsal roots is, however, followed by their degeneration, 

 hence their trophic centers are in these ganglia. Whether the blood-vessels of 



