1 68 A MANUAL OF PHYSIOLOGY 



cervical region causes a marked fall of pressure, this is not per- 

 manent 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 extir- 

 pated, 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 



FIG. 68. EFFECT ON BLOOD-PRESSURE IN DOG OF FREEZING SPINAL CORD 



(PlKE). 



At i the first or second dorsal segment of the 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.) 



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 main- 

 tenance 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 surgical shock (p. 175) (Henderson). In 

 addition to the direct influence of carbon dioxide, and possibly of 

 other substances, the arrival of afferent impulses at the centre 

 seems to play a part in maintaining that continual discharge 



