i84 THE CIRCULATION OF THE BLOOD AND LYMPH 



same region. When the brain, the bulb, and the upper portion of 

 the cord have been eliminated by ligation of all the arteries from 

 which blood can possibly reach them, a sufficient vascular pressure 

 persists to permit the circulation to go on in the lo\ver portion of 

 the body for hours. And while section or freezing (Fig. 80) of 

 the cord in the lower cervical region causes a marked fall of pressure, 

 this r, not permanent if the animal is allowed to survive. Forty-one 

 days after total section of the cord at the seventh cervical segment 

 in a dog an arterial pressure of 130 mm. of mercury was found. A 

 mechanism for the maintenance of vascular tone exists even beyond 

 the limits of the central nervous system. For when the lower 

 portion of the cord is completely destroyed, the dilatation of the 

 vessels of the hind-limbs, which is at first so conspicuous, passes 

 away after a time, the functions of vaso-motor centres having 

 perhaps been assumed by the sympathetic ganglia (Goltz and 

 Ewald). When the lumbo-sacral sympathetic chain is extirpated, 



Fig. 80. Effect on Blood- Pressure of Freezing Spinal Cord (Pike). At i the first 

 or second dorsal segment of a dog's cord was frozen with liquid air; at 2 and 3 

 central end of sciatic stimulated without effect on pressure (respectively one and 

 a half and three minutes after freezing of cord). (Four-fifths of original size.) 



there is a further loss of vascular tone in the affected region. But 

 even this is not irremediable. After a time recovery again occurs, 

 although it may be more partial and tardy than before. This may 

 take place either through the intervention of still more peripheral 

 ganglia, or through the development of a certain tonus by the 

 muscular fibres of the vessels when abandoned to themselves. 



As to the nature of the tone of the general vaso-motor centre, the 

 same question may be asked which has been already discussed for 

 the cardio-inhibitory centre. Is it reflex, or does it depend upon 

 direct excitation of the centre by some constituent of the blood or 

 lymph, or some substance produced in the centre itself ? The best 

 answer which can at present be made is that a constant central 

 excitation by the carbon dioxide formed in the centre or circulating 

 in the blood is a not unimportant factor in the maintenance of the 

 vaso-motor tone. A marked diminution in the carbon dioxide 

 tension of the blood, a condition which is termed ' acapnia,' may 

 indeed contribute to the severe fall of blood-pressure associated with 



