INNERVATION OF THE BLOOD-VESSELS OF DIFFERENT ORGANS 417 



exclusively by the quantities of blood which are permitted to escape 

 through this gate. 



The Nature of the Reaction. Two views are held regarding the 

 manner in which vasomotor changes are brought about. Thus, it 

 may be assumed that the blood-vessels are constantly kept in a state of 

 tonicity and that vasoconstriction is had in consequence of an extra 

 discharge of impulses by the center, while vasodilatation is the result 

 of a loss of tonus which is immediately followed by a passive enlarge- 

 ment of the blood-vessels. For this reason, the former condition may 

 be regarded as an augmentor and the latter as an inhibitor phenome- 

 non. A condition comparable to this one exists in the heart, where 

 accelerator and inhibitor impulses are played against one another. 

 The second theory proposes that vasoconstriction and vasodilatation 

 are two distinct processes resulting in consequence of the activity of 

 two separate mechanisms. 



If the first theory is accepted, the effector need not possess special 

 structural characteristics, because vasoconstriction could then be 

 assigned to the contraction, and vasodilatation to the extreme relaxation 

 of the circular musculature. But, if the second view is adhered to, 

 two distinct effectors would have to be present, namely, one for vaso- 

 constriction and one for vasodilatation. Regarding the former, no 

 difficulty need arise, because it could justly be ascribed to the con- 

 traction of the circular layer of muscle cells. Less manifest is the 

 vasodilator mechanism, because the only other available element is 

 the layer of smooth muscle cells which is arranged longitudinally to 

 the lumen of the blood-vessel. In the absence of a structurally more 

 definite effector, we are practically forced to assume that these cells 

 accomplish the dilatation either alone, or through an interaction with 

 the circular coat. 



It is quite impossible at the present time to decide with certainty 

 whether the first or the second theory is the correct one. The evi- 

 dence favoring the second view, namely, that the vasoconstrictor and 

 vasodilator reactions are effected by separate mechanisms, is as 

 follows : 



(a) Certain nerves are in existence which possess solely a dilator function. 

 First among these is the chorda tympani, a branch of the facial nerve, which 

 embraces dilator fibers for the submaxillary and sublingual glands, as well as the 

 tympanic branch of the glossopharyngeal nerve which contains dilator fibers for the 

 posterior third of the tongue, the tonsils, pharynx, and parotid gland. In this 

 group should also be placed the cervical sympathetic nerve, by way of which the 

 dilators gain access to the mucous membrane of the lips, gums, palate and the skin 

 of the cheeks and nostrils. 1 Some direct evidence is also at hand to prove that the 

 abdominal sympathetic system contains nerves of this kind. 2 It is also possible 

 to incite dilator effects in the domain of the nervi erigentes, by way of which the 

 erectile tissues of the reproductive organs are reached. It must be remembered, 

 however, that the tenseness of these organs is not caused by vasodilatation alone, 



1 Dastre and Morat, Red. exper. sur le systeme nerv. vasomoteur, 1884. 



2 Burton-Opitz, Pfltiger's Archiv, cxxiii, 1908, 553. 



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