672 POSITION OF THE VASO-MOTOR CENTRE. 



tory or depressing effect, but that it ultimately improves the condition of the heart as regards 

 force, rate, or regularity-one or all of these. He regards it as a true anabolic nerve (f 342, d).] 



371 VASO-MOTOR CENTRE AND VASO-MOTOR NERVES. Vaso-motor 

 Centre. The chief dominating or general centre, which supplies all the non- 

 striped muscles of the arterial system with motor nerves (vaso-motor, vasocon- 

 strictor vaso hypertonic nerves), lies in the medulla oblongata, at a point which 

 contains many ganglionic cells (Ludwig and Thiry). Those nerves which pass to 

 the blood-vessels are known as vaso-motor nerves. The centre (which is 3 

 millimetres long and H millimetre broad in the rabbit) reaches from the region of 

 the upper part of the floor of the medulla oblongata to within 4 to 5 mm. of the 

 calamus scriptorius. Each half of the body has its own centre, placed 2J milli- 

 metres from the middle line on its own side, in that part of the medulla oblongata 

 which represents the upward continuation of the lateral columns of the spinal cord \ 

 according to Ludwig, Owsjannikow, and Dittmar, in the lower part of the superior 

 olives. Stimulation of this central area causes contraction of all the arteries, and, 

 in consequence, there is great increase of the arterial blood-pressure, resulting in 

 swelling of the veins and heart. Paralysis of this centre causes relaxation and 

 dilatation of all the arteries, and consequently there is an enormous fall of the 

 blood-pressure. Under ordinary circumstances, the vaso-motor centre is in a condi- 

 tion of moderate tonic excitement ( 366). Just as in the case of the cardiac and 

 respiratory centres, the vaso-motor centre may be excited directly and reflexly. 



[Position How ascertained. As stimulation of the central end of a sensory 



nerve, ?.#., the sciatic, in an animal under the influence of cnrara, causes a rise in 

 the blood-pressure, even after removal of the cerebrum, it is evident that the centre 

 is not in the cerebrum itself. For the effect of chloral, under the same conditions, 

 see p. 674. By making a series of sections from above downwards, it is found 

 that this reflex effect is not affected until a short distance above the medulla 

 oblongata is reached. If more and more of the medulla oblongata be removed from 

 above downwards, then the reflex rise of the blood-pressure becomes less and less, 

 until, when the section is made 4 to 5 mm. above the calamus scriptorius, the 

 effect ceases altogether. This is taken to be the lower limit of the general vaso- 

 motor centre. The bilateral centre corresponds to some large multipolar nerve- 

 cells, described by Clarke as the antero-lateral nucleus.] 



I. Direct Stimulation of the Centre. The amount and quality of the gases 

 contained in the blood flowing through the medulla are of primary importance. In 

 the condition of apncea (% 368, 1), the centre seems to be very slightly excited, as 

 the blood-pressure undergoes a considerable decrease. When the mixture of blood- 

 gases is such as exists under normal circumstances, the centre is in a state of 

 moderate excitement, and running parallel with the respiratory movements are 

 variations in the excitement of the centre (Traube-Hering curves 85), these 

 variations being indicated by the rise of the blood-pressure. When the blood is 

 highly venous, produced either by asphyxia or by the inspiration of air containing 

 a large amount of CO.,, the ceutre is strongly excited, so that all the arteries of the 

 body contract, while the venous system and the heart become distended with blood 

 (Thiry). At the same time, the velocity of the blood-stream is increased 

 (Heidenhtiin). The same result is produced by ligature of both the carotid and 

 subclavian arteries, thus causing sudden anaemia of the medulla oblongata ; and, 

 no doubt, also by the sudden stagnation of the blood in venous hyperemia. 



Emptiness of the Arteries after Death. The venosity of the blood which occurs after death 

 always produces an energetic stimulation of the vaso-motor centre, in consequence of which the 

 arteries are firmly contracted. The blood is thereby forced towards the capillaries and veins, 

 and thus is explained tin- "emptiness of the arteries after death." 



Effect on Haemorrhage. Blood flows much more freely from large wounds, when the vaso- 

 motor centre is intact, than if it be destroyed (frog). As psychical excitement undoubtedly 



