140 



THE MECHANISM OF THE CIRCULATION. 



spasms, by each of winch the arterial pressure is driven up to a great height. 1 

 This phenomenon suggests a clonic fit of the centre, the clonus of in- 

 voluntary muscle naturally taking place at a far slower rate than that of 

 skeletal muscles. 



Fig. 86. — Injection of absinthe. Curarised (log. A, aortic pressure ; B, vena 

 cava pressure ; C, intracranial pressure ; D, cerebral venous pressure. — 

 Bayliss and Hill. 



The vasomotor mechanism of the veins.— The portal venous 

 system. — Tf the thoracic aorta be clamped, the pressure in the carotid 

 artery rises ; if the splanchnic nerves be then excited, the pressure in the 

 carotid artery is raised to a still higher level. This further rise is to be 

 attributed to the increased diastolic filling of the heart, which in its turn 

 is caused by the expulsion of the blood from the portal system, an 

 expulsion which is brought about by constriction of the splanchnic 

 and hepatic vessels. After ligature of the portal vein, the same result 

 is obtained, owing to constriction of the hepatic vessels. 2 



Bayliss and Starling localise the outflow of vaso-constrictor nerves 

 to the portal system in from the third to the eleventh anterior thoracic 

 roots. By excitation of the eighth to the eleventh roots, they obtained, 

 first, a rise of pressure in the portal vein, due to constriction of the 

 mesenteric arteries, forcing blood on into the portal vein ; secondly, a fall 

 due to the diminished flow of blood through the mesenteric arteries ; and, 

 lastly, a rise due to constriction in the hepatic branches of the portal vein. 3 



The portal venous system is peculiar in its structural arrangement, 

 and generally bears a close analogy to an arterial vascular area. As to 

 the innervation of the other veins we know very little. Thompson, 4 

 after tying the crural artery and on excitation of the peripheral end 

 of the divided sciatic nerve, has witnessed contraction of the superficial 

 veins of the rabbit's leg. He has also witnessed the same in abdominal 

 veins, on stimulation of the peripheral end of the divided cervical spinal 

 cord. The contraction is usually in the form of band-like constrictions, 

 between which the veins appear full of blood. Such contractions appear 



1 Bayliss and Hill, Journ. Physiol., Cambridge and London, 1895, vol. xviii. p. 350. 

 2 Mall, Arch./. Physiol., Leipzig, 1892, S. 409. 



3 Journ. Physiol., Cambridge and London, 1894-5, vol. xvii. p. 120, cf. Cavazzani 

 and Manca, Arch. ital. de biol., Turin, 1895, vol. xxiv. pp. 83, 295. 



4 Arch. f. Physiol., Leipzig, 1893, S. 102. 





