154 A MANUAL OF PHYSIOLOGY 



has been suggested that the muscular vessels are widened 

 in contraction, not through vaso-motor nerves, but by 

 the direct action of the acid products of the active muscle 

 itself, since it has been found that very dilute acids 

 (lactic acid, e.g.) cause general dilatation of the small 

 vessels. A similar explanation has been extended to the 

 dilatation of the vessels of the brain during cerebral activity 

 by some of those who deny the existence of vaso-motor 

 nerves for that organ. But this ingenious speculation rests 

 upon a very narrow basis of fact. 



Vaso-motor Nerves of the Lungs. There has been much 

 discussion as to the course, and even as to the existence, of 

 vaso-motor fibres for the lungs. The problem is perhaps the 

 most difficult in the whole range of vaso-motor topography, 

 for the pulmonary circulation is so related to other vascular 

 tracts, that changes produced in the vessels of distant 

 organs by the stimulation or section of nerves may affect 

 the quantity of blood received by the right side of the heart, 

 and therefore the quantity propelled through the lungs and 

 the pressure in the pulmonary artery. All that we really 

 know is that the lungs are supplied with vaso-constrictor 

 fibres, although in all probability less richly than most other 

 organs. Some of these fibres appear to pass out from the 

 upper half of the dorsal spinal cord (Bradford and Dean), 

 but perhaps others reach their destination by the vagus. 



In most of the peripheral nerves vaso-dilator fibres are 

 mingled with vaso-constrictors ; but in certain situations, 

 for an anatomical reason that will be mentioned presently, 

 nerves exist in which the only vaso-motor fibres are of the 

 dilator type. Of these, the most conspicuous examples are 

 the chorda tympani and the nervi erigentes ; and, indeed, it 

 was in the chorda that vaso-dilators were first discovered by 

 Bernard. The chorda tympani contains vaso-dilator and 

 secretory fibres for the submaxillary and sublingual salivary 

 glands. With the secretory fibres we have at present 

 nothing to do; and the whole subject will have to be 

 returned to, and more fully discussed in Chapter IV. But 

 a most marked vascular change is produced by stimulation 

 of the peripheral end of the divided chorda tympani nerve. 



