THE CIRCULATION OF THE BLOOD AND LYMPH 163 



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 pressure in the left auricle and the 

 pulmonary veins. All that we can really say is that the lungs are 

 probably supplied with vaso-constrictor fibres, although less richly 

 than most other organs. In mammals these 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 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. 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 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 com- 

 pletely venous. These vaso-dilator fibres are not in constant 

 action, for section of the nerve, as a rule, produces little or no 

 change. Vaso-constrictor fibres pass to the salivary glands from 



* Brodie and Dixon, perfusing isolated ' surviving ' lungs with blood 

 under constant pressure, and measuring the outflow, came to the conclu- 

 sion that no pulmonary vaso-motor fibres exist, since adrenalin causes no 

 vaso-constriction. They assume that adrenalin acts upon vaso-motor 

 nerve-endings (but see p. 564), and that in organs in which this drug does 

 not produce vaso-constriction no vaso-constrictor fibres are present. But 

 Plumier, working independently with the same method, saw strong con- 

 striction of the pulmonary vessels under the influence of adrenalin, and 

 also on stimulation of the annulus of Vieussens. Wiggers also obtained 

 constriction with adrenalin. 



II 2 



