156 



THE MECHANISM OF THE CIRCULATION. 



tains his thorax and abdomen in the position of forced expiration. By 

 this means the intrathoracic pressure is raised, the outlet from the veins 

 blocked, and the muscles are distended with blood. The pressure rises 

 in the capillaries and veins of the muscles almost to arterial pressure. 



The control of the blood supply to the limbs has been studied by 

 the therm ome trie and plethysmographic methods. The temperature in 



the upper limb is 

 raised by destruc- 

 tion of the first 

 thoracic ganglion, 

 and by section 

 of the brachial 

 plexus or midthor- 



FIG. 95. Limb plethysmograph. A, glass chamber tilled with ^CIC roots. 1 In the 



air; B, broad indiarubber band; C, Marey tambour. The hllld-limb, a rise 



junction between the limb and the plethysmograph is rendered of temperature 



air-tight by the free application of thick vaseline beneath the QCClirS after divid- 

 rubber-band (B). 



ing the sciatic 



nerve. The vasomotor fibres have been traced from this nerve to the 

 lower thoracic roots. They pass down the sympathetic chain in the lower 

 lumbar region. 2 On excitation of these vasomotor nerves, the greatest 

 change of temperature is found in the feet. This is so, because the pad of 

 an animal's foot is 

 free from a furry 

 coat, and thus forms 

 one of the more im- 

 portant places for 

 the regulation of 

 the temperature of 

 the body. 3 



By Bradford and 

 Bayliss 4 the vaso- 

 motor nerves of the 

 limbs have been 

 minutely studied, 

 with the aid of the 

 plethysmograph. 

 According to these 

 authors, the vaso- 

 constrictor fibres FIG. 96. Record of aortic pressure and the volume of the fore 

 which supply the a11 ^ hind limbs. Curarised dog. Effect of excitation of the 

 f u I,T Ur thirteenth dorsal root. Passive dilatation of the fore-limb, 

 Oie-limb lea\6 by contraction of the hind-limb. Bradford and Bayliss. 



the anterior roots 



of from the third to the eleventh thoracic nerves. The hind -limb is 

 supplied by fibres from the eleventh thoracic to the third lumbar nerve. 

 By each root the volume of the whole limb is affected. 



1 Schiff, " Untersuchungen, etc.," Frankfurt-am-Maine, 1855, S. 176; Bernard, Compt. 

 rend. Acad. d. sc., Paris, 1862, tome Iv. p. 305; Cyon, Her. d. k. Sachs. Gcsellsch. d. 



JVissensch., Leipzig, 1868, S. 73. 



2 Bernard, Compt. rend. Acad. d. sc., Paris, 1862, tome Iv. p. 232 ; Brown-Sequard, 

 Comp. rend. Acad. d. sc., Paris, 1854, tome xxxviii. p. 76 ; Ostroumoff, Arch. f. d. ges. 

 PhysioL, Bonn, 1876, Bd. xii. S. 261. 



3 Lewaschew, Arch. f. d. (jes. PhysioL, Bonn, 1882, Bd. xxviii. S. 397 seq. 

 4 Journ. PhysioL, Cambridge and London, 1894, vol. xvi. p. 10. 



