514 PHYSIOLOGY CHAP. 



opposite pupil, followed shortly by that of the pupil on the same 

 side, with pronounced opening of the lids and raising of the 

 eyebrows. The eyes turn up and towards the opposite side ; the 

 head moves in the direction of the eyes ; and the ears are lowered. 

 If the excitation is prolonged the tail is raised, the lips spread 

 out, the jaws close, the angles of the mouth are drawn back as 

 far as possible ; the upper limbs are flexed at the elbow-joint, 

 adducted and drawn back. If the excitation is continued, 

 complete opistothonus results. . 



Excitation of the posterior quadrigeminal bodies in monkeys 

 produces the same effects, but there is further emission of sounds 

 of a character varying with the duration of the stimulus. The 

 motor effects which are at first confined to the opposite side 

 subsequently extend to both sides. 



It is not easy to ascertain the value or physiological significance 

 of these experiments on the corpora quadrigemina with the 

 excitation method. The motor effects of electrical excitation may 

 depend on the transmission of the stimulus to the motor tracts 

 or to subjacent centres. At the same time they are evoked by 

 very weak currents, which are hardly perceptible at the tip of the 

 tongue. Other forms of excitation which are incapable of spread- 

 ing may also produce the same effects under certain conditions. 



The phenomena produced by excitation of the corpora quadri- 

 gemina are undoubtedly reflex in character, that is, they depend 

 on the transmission of an active state from the sensory centres 

 to the motor centres or tracts. The effects of momentary stimula- 

 tion of the mesencephalon strongly resemble the movements of 

 repulsion that take place when an object is suddenly brought near 

 the eyes, which makes it probable that the excitation gives rise 

 to subjective luminous sensations, and this reflexly discharges the 

 reaction movements. 



Trismus, contraction of the facial muscles, and opistothonus, 

 which ensue on strong and protracted stimulation of the quadri- 

 geminal bodies, may be looked on as symptoms or manifestations 

 of pain. The dilatation of the pupil is a phenomenon of the same 

 character, since we know that it occurs with every sudden excita- 

 tion of the sensory nerves. So, too, the cries of distress due to 

 excitation of the posterior quadrigeminal bodies. 



Danilewsky demonstrated that electrical stimulation of the 

 deep layers of the corpora quadrigemina produces a marked 

 increase in arterial pressure, which is associated with retardation 

 and reinforcement of the heart-beat. Kespiration is disturbed 

 too, expiration in particular being exaggerated. Probably these 

 effects are due, at least in part, to transmission of the electrical 

 stimulus to the subjacent cerebral peduncles. 



Valentin and Budge found that electrical excitation of the 

 corpora quadrigemina also affected the viscera, producing con- 



