THE VASOMOTOR NERVES. 609 



Vasodilator Center and Vasodilator Reflexes. — Since the 

 vasodilator fibers form a system similar to that of the vasocon- 

 strictors, it might be supposed that, like the latter, their activity 

 is controlled from a general center, forming a vasodilator center in 

 the brain similar to the vasoconstrictor center. What evidence 

 we have, however, is against this view. In the dog with his spinal 

 cord severed in the lower thoracic region the penis may show normal 

 erection when the glans is stimulated, — a fact that indicates a 

 reflex center for these dilator fibers in the lumbar cord. For the 

 other clear cases of vasodilator fibers we have no reason at present 

 to believe that they are all normally connected with a single group 

 of nerve cells located in a definite part of the nervous system. The 

 dilator fibers in the facial, glossopharyngeal, and cervical sympa- 

 thetic (distributed through the trigeminal) all arise probably in the 

 medulla, but not, so far as is known, from a common nucleus. 

 Intimately connected with the question of the existence of a general 

 vasodilator center is the possibility of definite reflex stimulation 

 of the vasocUlator fibers. As stated above, reflex dilatation of the 

 blood-vessels may be produced by stimulating various nerve trunks 

 containing afferent fibers. The depressor nerve fibers of the heart 

 give only this effect, and the sensory fibers from certain other 

 regions, notably the middle ear and the testis, cause mainly, if not 

 exclusively, a fall of arterial pressure due presumably to vascular 

 dilatation. The sensoiy nerves of the trunk and limbs, when 

 stimulated by the gross methods of the laboratory, give either 

 reflex vasoconstriction or reflex vasodilatation, and, as was stated 

 above, there is reason to believe that these trunks contain two kinds 

 of afferent fibers, — the pressor and the depressor. The action of the 

 former predominates usually, but in deep anesthesia, and particu- 

 larly in those conditions of exposure and exhaustion that precede 

 the appearance of "shock," the depressor effect is more marked or, 

 indeed, may be the only one obtained. To explain such depressor 

 effects we have two possible theories. They may be due to reflex 

 excitation of the centers giving origin to the vasodilator fibers or to 

 reflex inhibition of the tonic activity of the vasoconstrictor centers. 

 The latter explanation is the one usually given, especially for the 

 typical and perhaps special effect of the depressor nerve of the heart. 

 This explanation seems justified by the general consideration that 

 in the two great vascular areas through whose variations in capacity 

 the blood-flow is chiefly regulated, — namely, the abdominal viscera 

 and the skin, — the vasoconstrictor fibers are chiefly in evidence 

 and are, moreover, in constant tonic activity. On the other hand, 

 the fact that vasodilator fibers exist is presumptive evidence that 

 they are stimulated reflexly, since it is by this means only that they 

 oan normally affect the blood-vessels. Some of the many depressor 



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