1 5 8 THE ME C NANISM OF THE CIR C ULA TION. 



1. The pressure in the femoral artery. 



2. The volume of the spleen. 



3. The volume of the jejunum. 



4. The volume of the kidney. 



5. The volume of the left hind-limb. 



6. The volume of the right hind-limb. 



On exciting the central end of the sciatic nerve, the arterial pressure rose, the 

 spleen, jejunum, and kidney contracted. The feet 1 expanded (perhaps passively 

 by the rise of arterial pressure). The arterial pressure then fell somewhat 

 (owing to cutaneous vaso-dilatation and pulmonary vase-constriction). 2 



The volume of the limbs depends both upon the arterial and the 

 vena cava pressures. If the arterial pressure remain constant, and the 

 vena cava pressure rise, as on performing a Valsalva experiment, then 

 the volume of the limb increases greatly. Here the venous tension 

 rises towards the mean arterial tension, for the outlet of the veins is 

 blocked by the rise of intrathoracic pressure. If the vena cava pressure 

 rise while the arterial pressure fall, the two effects may balance each 

 other, and tbe volume of the limb remain constant. 



The tracing of the limb volume sbows all the respiratory and cardiac 

 oscillations. The limb may expand more, either with expiration or witli 

 inspiration, according as the expiratory rise of vena cava pressure, or 

 the inspiratory rise of arterial pressure, has the greater effect upon the 

 capillary pressure. 



During Traube-Hering oscillations of arterial pressure the volume of 

 a limb follows the rise and fall of aortic pressure. 



It has been said that an antagonism exists between the vasomotor 

 mechanisms of the splanchnic and locomotor organs. Thus, while 

 during asphyxia the splanchnic vascular area contracts, the vessels of 

 the skin and muscles dilate. 3 The dilatation of the latter, however, is 

 in all probability not occasioned by active dilatation, but is due to the 

 over-mastering power of the splanchnic constrictors. By the rise of 

 aortic tension the vessels in the remaining parts of the body are passively- 

 dilated, and the blood flow is thus increased through the skin and 

 muscles. That this is so is suggested by the fact that after the 

 circulation has, by ligature of the thoracic aorta and vena cava inferior, 

 been limited to the fore-part of the body, either asphyxia or excitation 

 of the vasomotor centre produces a slight rise of arterial tension, owing 

 to the constriction of the vascular areas of the face and fore-limbs. 

 Previous to the double ligature the same excitation produces splanchnic 

 contraction, a great rise of aortic tension, and dilatation in the face and 

 fore-limbs. Bayliss 4 has obtained plethysmographic evidence of con- 

 traction of the leg during asphyxia. 



By driving normal saline through the vessels of a recently dead animal, 

 and measuring the outflow, Stefani 5 determined that a given head of pressure 

 dilates the vessels of the skin and muscles to a far greater extent than the 

 splanchnic vessels. The cerebral vessels enclosed in the rigid cranium dilate 



1 By means of the foot the circulation in the skin is studied. 



- Hallion and Francois-Franck, Arch, de physiol. norm, etpath., Paris, 1896, p. 502. 



3 Heidenhain, Arch. f. d. ges. Physiol., Bonn, 1870, Bd. iii. S. 78 ; 1872, Bd. v. S. 504 : 

 Dastre and Morat, "Recherches exp. sur le syst. nerv. vaso-moteur," p. 287 : Hallion and 

 Fran9ois-Franck, loc. cit. 



4 Journ. Physiol., Cambridge and London, 1893, vol. xiv. p. 307. 



5 Arch. tial. de Uol., Turin, 1894, vol. xx. pp. 91-109. 



