BLOOD VESSELS — PEKIPHERAL ACTIO^T. 469 



long diastolic pauses tlie pressure does not sink as it ordinarily 

 does, but sometimes remains as liigb as 165 millimetres of 

 mercury. When the arterioles are in their normal condition as 

 regards dilatation, the blood flows readily out of the arteries 

 into the veins, and the pressure rapidly falls in the arterial 

 system during the cardiac diastole. When the arterioles are 

 much contracted, however, as after the administration of casca, 

 the flow of the blood out of the arterial into the venous system 

 is impeded, the arteries remain full, and the tension of the blood 

 within them high. 



Digitalis also contracts the arterioles and causes the fall of: 

 pressure during the cardiac diastole to be slow. The mode in^- 

 which casca and digitalis produce contraction of the blood- 

 vessels, however, seems to be different. Digitalis causes it by 

 stimulatino: the vaso-motor centre in the medulla obloncjata,. 

 and this centre acts through the vaso-motor nerves upon the 

 vessels. These nerves pass down from the medulla, through 

 the cervical part of the spinal cord, along the splanchnics, &c.,. 

 to the vessels. Consequently, when the communication between 

 the vaso-motor centre in the medulla oblongata and the vessels - 

 is destroyed by dividing the cervical and spinal cord, the 

 vessels dilate, and no stimulation of the vaso-motor centre has. 

 any power to cause them to contract. The contraction usually 

 produced by digitalis, therefore, does not occur if the cord be- 

 divided before its injection, and is removed if the cord be 

 divided after contraction has already taken place. This is not 

 the case with casca, however, for \vc found that after the 

 spinal cord had been completely divided in a cat opposite the 

 second cervical vertebra, the blood-j^ressure after the injection 

 of casca rose higher than in any other experiment. The casca 

 must therefore act either on the blood-vessels themselves, the 

 vaso-motor nerves, or some vaso-motor centre not contained in 

 the medulla. 



A further proof that casca acts on peripheral ^^iso-motor 

 ganglia or nerves is afforded by an experiment in which the 

 sympathetic nerve was divided in the neck of a rabbit, so as 

 to sever the connection between the vaso-motor centre and 

 the vessels of the ear on one side. On the other the nerves 



