The arterial system is very completely developed in both Limulus and Scorpio, branching repeatedly until minute arterioles are formed, not to be distinguished from true capillaries; FIG.
The delicate connectivetissue fibrillae of the inner coat of the arterioles are usually first and most affected.
The change appears to begin in the fibrils which lie between the circular muscle fibres of the middle coat of the smaller arterioles and extends both backwards and forwards along the vessels.
Physostigmine, indeed, stimulates nearly all the non-striped muscles in the body, and this action upon the muscular coats of the arteries, and especially of the arterioles, causes a great rise in blood-pressure shortly after its absorption, which is very rapid.
Not only is the respiratory centre stimulated but the cardiac centre is acted upon both directly by the drug and indirectly for a time by the enormous rise in blood pressure due to the contraction of the arterioles all over the body.