CIRCULATION IN SPECIAL PARTS 245 



and thus forms one of the more important places for the regulation of 

 the temperature of the body. 



With the aid of the plethysmograph, the vaso-motor nerves of the 

 limbs have been minutely studied. The vaso -constrictor fibres which 

 supply the fore-limb leave by the anterior roots 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 effected. 



The chief outflow of constrictor fibres occurs from the sixth dorsal 

 to the first lumbar nerve. Thus the limbs and the abdominal viscera 

 get their supply approximately from the same part of the spinal cord. 

 The vaso-motor mechanism of the limbs is not powerfully developed. 



By stimulation of a sensory nerve of a limb the vessels of that 

 limb expand, and elsewhere the vessels constrict. Usually, the other 

 limbs expand owing to the passive dilatation produced by rise of 

 aortic pressure which follows the reflex constriction of the splanchnic 

 vessels. The reflex constriction of these limbs can be obtained after 

 section of the splanchnic nerves. . ~. . ' 



The sympathetic cell station for the fibres of the upper limb is the 

 stellate ganglion; of the hind-limb, the sixth, seventh lumbar, and 

 first sacral ganglia. The vaso-motor fibres to the trunk follow the 

 same distribution as the pilomotor fibres. 



The general changes which take place on exciting the central end 

 of an afferent nerve are shown exceedingly well in Fig. 128. In this 

 there is shown a simultaneous record of 



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 (innervated). 



On exciting the central end of the left sciatic nerve, the arterial 

 pressure rises, and the spleen, jejunum, and kidney, constrict. The 

 right foot is passively expanded by the rise of arterial pressure. There 

 next occurs a compensatory slowing of the heart, accompanied by 

 active dilatation of the left limb. The arterial pressure then falls to 

 a lower height. 



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

 vena cava pressure. If the arterial pressure and the venous pressure 

 rise, as on performing a Valsalva experiment, then the volume of 

 the limb increases greatly. Here the venous pressure rises towards 

 the mean arterial pressure, for the outlet of the veins is blocked by 

 the rise of intrathoracic pressure. If the vena cava pressure rises 

 while the arterial pressure falls, the two effects may balance each 

 other, and the volume of the limb remain constant. 



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

 oscillations. The limb may expand most either with expiration or 



