136 THE MECHANISM OF THE CIRCULATION. 



It seems highly probable that the vascular muscles are maintained in 

 a state of tone by the tension of the blood within the vessels. This would 

 be in close analogy with the fact that the heart is excited to contract 

 by a rise of internal tension. Accepting this hypothesis, we may then 

 suppose that the vaso-constrictor nerves increase the excitability of the 

 vascular muscle to the stimulus of blood tension, while the vaso-dilator 

 nerves diminish the excitability. The vaso-dilator fibres would thus be 

 brought into analogy with the cardio-inhibitory nerves. These lessen 

 the irritability of the heart towards the intracardiac tension, 1 and 

 relax the cardiac muscle. 



On this hypothesis, section of the vaso-constrictor nerves abolishes 

 that tonic condition of exalted excitability to internal tension which is 

 maintained by the vaso-motor centre. It does not, however, entirely 

 destroy the excitability of the vascular muscle to tension, and with time 

 the tone is fully restored. On the other hand, the vaso-dilator influence 

 completely relaxes the muscle by altogether inhibiting its excitability 

 to tension. It is conceivable that the quality as well as the tension of 

 the blood may be the exciting cause of vascular tone. An increase in 

 the alkalinity of the blood favours the development of tone. Certain 

 glands, such as the suprarenals, appear to secrete a material into the 

 blood which produces vascular contraction. 



Vasomotor centres. — Ludwig and Dittmar were the first to localise 

 a vasomotor centre. By excitation of the central end of the sciatic 

 nerve, after the great brain had been separated from the medulla 

 oblongata, they obtained a reflex rise of arterial pressure. This centre 

 was minutely localised by Ludwig and Owsjannikow. While a section 

 of the brain carried out below the corpora quadrigemina did not produce 

 any noteworthy change in the arterial pressure, they found, on 

 slicing away the spinal bulb by successive cuts, that the pressure fell 

 with each cut, until a maximal depressor effect was reached. By such 

 experiments the centre was located in the rabbit to a spot 3 to 4 mm. 

 in length, commencing 1 to 2 mm. below the corpora quadrigemina, and 

 ending 4 mm. above the tip of the calamus scriptorius. 2 The centre is 

 bilateral, and in the area which it covers there is situated a group of 

 nerve cells known as the antero-lateral nucleus of Clarke. 3 



The bulbar vasomotor centre is constantly in action, maintaining 

 the tone of the small arteries. 



Thus Bernard noticed, after section of the cervical cord, that the 

 arterial pressure fell markedly. By excitation of the peripheral end of 

 the cervical cord, v. Bezold 4 drove the pressure up to no less than seven 

 times the height it reached after section of the cord. Ludwig and 

 Thiry 6 showed that the same result can be obtained after section of all 

 the cardiac nerves ; and, moreover, that while tying of the aorta below 

 the renal arteries causes but a slight rise of tension, ligation of the 

 thoracic aorta causes a very great rise. These experiments indicate 

 the overwhelming importance of the splanchnic vaso-constrictor nerves. 



The vasomotor centre is not only excited reflexly, but responds to 

 every change in the circulation through the spinal bulb. A rise of 



1 Hill and Barnard, Journ. Physiol., Cambridge and London, 1897, p. 338. 



- Ikr. d. k. Si'tchs. Gesellsch. d. JFisscnsch., math.-phys. CI., Leipzig, 1871, S. 135. 



3 Dittmar, Ibid., 1873, S. 460. 



4 " Untersuch. neber d. Innervation des Herzens," 1863, Abth. ii. S. 221. 



5 Sitzungsb. d. k. Akad. d. JFissensch., math.-naturw. CI., Wien, 1864, Bd. xlix. 

 Abth. 2, S. 421. 



