BY DR. BURDON-SANDERSON. 245 



mediately below the cerebellum, all the arteries are relaxed, 

 and that a similar effect is produced if certain afferent nerve 

 fibres, which lead to the intracranial part of the cord, are ex- 

 cited. Its position has been lately determined with great pre- 

 cision in the rabbit by Ludwig and Owsjannikow, who have 

 found by experiments, to which further reference will be made, 

 that it is limited towards the spinal cord by a line four or five 

 millimetres above the calamus scriptorius, and extends 

 towards the brain to within a millimetre of the corpora 

 qaadrigemina. 



That the vasomotor centre is in constant automatic action, 

 is shown b}' the paralyzing effect of section, whether "of the 

 spinal cord, or of any nerve known to contain vascular fibres. 

 If the action of the centre were not constant, division could 

 not produce arterial relaxation. In relation to this constancy 

 of action, the word tonus is used. Arterial tonus means that 

 degree of contraction of an artery which is constant and nor- 

 mal. It is maintained only so long as the artery is in com- 

 munication witli the vaso-motor centre. 



47. Experiments relating to the Influence of the 

 Cerebro-Spinal Nervous Centres of the Vascular Sys- 

 tem. (1.) Destruction of the Nervous Centres. Two 

 frogs are slightly curarized, and placed side by side on the 

 same board, in the supine position. In both, the heart and 

 great vessels are exposed, as in the preceding section. It 

 having been ascertained that the circulation is normal in each 

 animal, and the frequency of the contractions having been 

 noted, the brain and spinal cord are destroyed in one of the 

 frogs, by inserting a strong needle into the spinal canal imme- 

 diately below the occipital bone, and then passing it upwards 

 and downwards. This may usually be accomplished without 

 much loss of blood. If now the frog which has been deprived 

 of its nervous centres is compared with the other, it is seen 

 that in the former, although the heart is beating with perfect 

 regularity and unaltered frequency, it is empty, and in conse- 

 quence, instead of projecting from the opening in the anterior 

 wall of the chest, it is withdrawn upwards and backwards 

 towards the oesophagus. 



The emptiness of the heart is not limited to the ventricle and 

 bulb. The auricles are alike deprived of blood ; and if the 

 heart is drawn forwards by the apex, it is seen that the sinus 

 venosus and vena cava inferior are in the same condition. 

 The state of the heart is therefore not dependent on any cause 

 inherent in itself, but on the fact that no blood is conveyed to 

 it by the veins. To make this still more evident, the rest of 

 the visceral cavity may be opened, when it is seen that, although 

 the vena cava is collapsed, the intestinal veins are distended. 

 The second frog, which is no longer required for comparison, 



