HYPOPHYSIS CEREBRI 329 



intercranial circulation and the conditions governing blood- 

 pressure within the cranium. The regulation of pressure is to a 

 certain extent automatic, for every rise in pressure within the 

 cranium mechanically stimulates the hypophysis ; this stimulation 

 is conveyed reflexly to the vagus centre and a slowing and 

 strengthening of the cardiac contractions results ; while, at the 

 same time, the vaso-dilator fibres contained in the vagus nerve 

 produce an enlargement of the vessels in the thyroid, and by 

 this means the superfluous blood is carried away from the brain. 

 The intracranial rise in pressure also causes the hypophysis to 

 part with a larger amount of its active substance, which produces 

 a chemical stimulation of the vagus and simultaneous rise in 

 blood-pressure. Thus, the auto-regulation by the hypophysis of 

 intracranial blood-pressure is effected by stimulation of the vagi 

 upon the one hand, and by increased arterial tension resulting 

 from contraction of the peripheral vessels upon the other. 



With regard to the latter point, v. Cyon believed that the 

 contraction of the peripheral vessels resulted in a reduction in 

 the amount of blood circulating within the cranium. But it must 

 be remembered against this that, as all evidence up to the present 

 goes to show, processes which give rise to a contraction of the 

 vessels of the systemic circulation, thus producing an increase 

 in arterial tension, also augment the blood contents of the cranial 

 cavity. The higher the arterial blood-pressure, the larger the 

 quantity of blood circulating through the brain, which, under 

 these conditions, may amount to several times the normal. Thus 

 it is evident that a temporary anaemia of the brain cannot be 

 effected by this means, and that the effect produced upon the 

 cranial circulation by the active vaso-tensor constituent of the 

 hypophysis would be, not the result anticipated by v. Cyon, but 

 one diametrically opposed to it (Biedl and Reiner). 



As far as the stimulation of the vagus centre is concerned, 

 the conditions are as follows : The dilation of the thyroid vessels 

 as the result of stimulation of the vagus, \vhich v. Cyon described 

 and which forms the groundwork of his theory, is not confirmed 

 by Lusenna, who found that the vaso-dilators of the thyroid are 

 contained, not in the vagus, but in the sympathetic. It is un- 

 deniable, however, that increased pressure within the cranial 

 cavity produces stimulation of the vagus centres, and that by 

 this means a slowing and strengthening of the cardiac beats is 

 effected. But the question arises as to whether this so-called 

 vagus phenomenon occurs as the result of high blood-pressure, 

 brought about by the agency of the hypophysis. In support of 

 this assumption v. Cyon states : (i) That mechanical or electric 

 stimulation of the hypophysis produces changes in the Wood- 

 pressure curve, such as would follow a sudden compression of 

 the aorta, namely, hypertension, slowing and strengthening of 

 the heart-beats, and the so-called vagus pulse ; (2) that the vagus 



