It has already been mentioned that the blood-pressure rises during muscular exertion, as, for example, during the struggles of an animal. The cause of this has not been definitely ascertained, but it is probably, to a great extent, due to the flow of blood through the muscles being mechanically obstructed by the contraction of the muscular fibres and to a more rapid action of the heart.

The flow of blood through those organs which consist of involuntary muscles, e.g. the intestine, may be also obstructed.

When physostigmine is given to an animal, the blood-pressure is sometimes noticed to rise considerably, and this rise of pressure was at first attributed to contraction of the arterioles. According to Von Bezold and Gotz, however, this is due, to a great extent, not to the contraction of the arterioles themselves, but to mechanical obstruction of the intestinal vessels by the tetanic contraction of the muscular walls of the intestine.1