354 PEACTICAL PHYSIOLOGY 



junction. Gently press on the inferior vena cava, the pressure falls 

 as the diastolic filling is diminished, and so the output is lessened. 

 Expose the sciatic nerve without damaging the blood-vessels. Tie, cut, 

 and tetanise the central end. This will excite the vaso-motor centre 

 and cause reflex constriction and rise of arterial pressure. 



CHAPTER XXVIII. 

 VASO-MOTOR SYSTEM. 



Innervation of the Blood-vessels. Pith the cerebrum of a large 

 frog and plug the hole with a blunt-pointed match to prevent haemor- 

 rhage. Curarise the frog lightly, place it on the cork board provided 

 for studying the circulation in the web. Tie out the toes so as to 

 spread the web over the hole in the board. Observe the rate of 

 circulation. Next pass a pin through the occipito-vertebral membrane 

 and destroy the spinal bulb. The circulation will become more rapid 

 owing to dilatation of the arteries. The vaso-motor centre in the 

 rabbit has been localised in the spinal bulb on either side of the middle 

 line extending from a point 1 mm. to a point 4 mm. below the corpora 

 quadrigemina. 



After five minutes the flow through the capillaries will be less rapid. 

 The spinal cord contains subsidiary vaso-motor centres, and exerts 

 a tonic action on the arteries after destruction of the chief centre in 

 the bulb. 



Expose the cervical cord and tetanise it with a weak current. The 

 flow will be lessened. 



Now remove the frog from the board and expose the heart. Sus- 

 pend the frog in the vertical head-up position. Note that the heart 

 and large vessels are filled with blood. Pass a blanket-pin down the 

 vertebral canal and destroy the spinal cord. The heart and vessels 

 will soon become bloodless owing to the loss of vaso-motor tone. The 

 blood sinks into the dilated abdominal vessels under the influence of 

 gravity. The vaso-constrictor nerves arise from the anterior roots, 

 they have their cell-stations in the sympathetic ganglia, and from 

 thence pass to the limbs along the grey rami to the corresponding 

 spinal nerves. These facts may be demonstrated on the curarised cat 

 by plethysmographing the fore limb, exposing and exciting the anterior 

 roots in the upper thoracic region, and then injecting 10 mgrms. of 

 nicotine to paralyse the cell-stations in the stellate ganglion. 



