THE NERVOUS REGULATION OF THE BLOODVESSELS 177 



mechanism being thus constituted. There is some evidence that carbon 

 dioxide is not the most potent of these substances. 



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. And changes in the systemic arterial pressure may favour or 

 hinder the discharge of the left ventricle, and therefore affect the pres- 

 sure in the left auricle and the pulmonary veins. Nevertheless, evidence 

 has been obtained from a number of sources that the lungs are supplied 

 with vaso-constrictor fibres. Plumier, perfusing isolated, ' surviving ' 

 lungs with blood under constant pressure and measuring the outflow, 

 showed that adrenalin and also stimulation of the annulus of Vieussens 

 caused great diminution in the flow that is, constriction of the vessels. 

 Wiggers also obtained constriction with adrenalin. Fuhner and 

 Starling, working with a preparation including the heart as well as the 

 lungs, found that adrenalin caused a rise of pressure in the pulmonary 

 artery coupled with a fall of pressure in the left auricle, which could 

 only be due to constriction of the vessels of the lungs. It is assumed 

 that adrenalin produces vaso-constriction only in vessels supplied with 

 vaso-constrictor nerves (p. 638), and that in organs where this substance 

 does not cause vaso-constriction no such fibres are present. In mam- 

 mals the vaso-constrictor fibres seem to pass out from the upper half 

 of the dorsal spinal cord, and some of them can be detected nearer their 

 destination in the annulus of Vieussens. The vago-sympathetic of the 

 tortoise contains vaso-constrictors for the lung of the same side (Krogh). 



Vaso-Dilator Fibres. In most of the peripheral nerves these 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 or pelvic nerves; 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 sub- 

 maxillary 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 VI. But a 

 most marked vascular change is produced by stimulation of the 

 peripheral end of the divided chorda tympani nerve. The glands 

 flush red; more blood is evidently passing through their vessels. 

 Allowed to escape from a divided vein, the blood is seen to be of a 

 bright arterial colour and shows a distinct pulse. The small arteries 

 have been dilated by the action of the vaso-motor fibres in the nerve. 

 The resistance being thus reduced, the blood passes in a fuller and 

 more rapid stream through the capillaries into the veins, and on the 

 way there is not time for it to become completely venous. These 

 vaso-dilator fibres are not in constant action, for section of the 



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