126 Prof. Miiller and Dr. Marshall Hall 



one. Tiedemanii's interesting discovery that the arteria cen- 

 tralis retinas is accompanied by a fine twig from the ciliary 

 ganglion, cannot in this case explain anything. For all vessels 

 are accompanied by nerves; but this twig is distributed with 

 the arteria centralis, and is in no proved connexion with the 

 retina. This reflex action from the brain to the iris takes 

 place through the oculo-motor nerve, which according to 

 Mayors experiments at every stimulus excites a contraction of 

 the iris*. We know from the same author that the cerebral 

 end of the divided optic nerve, when stimulated, still induces 

 contraction. Thus, in the contraction of the iris there is pre- 

 sented a kind of "Statik" of the excitement between centri- 

 petal-sensorial and centrifugal-motor action through the me- 

 dium of the brain. Other nerves also may alter this equili- 

 brium, as the sensorial branches of the trigeminus, for cold 

 water thrown into the nose produces contraction of the iris. 

 To these more simple instances of reflected excitement be- 

 longs also the winking of the eyelids from long impression 

 of light, or from a loud sound (what has the optic nerve to 

 do with the auditory?), or from a' threatening impression on 

 the sight. 



" Further, to these belong the contraction of all the muscles 

 of the perinaeum, the sphincter ani, levator ani, bulbo-caverno- 

 sus, and ischio-cavernosus in the emission of semen, in conse- 

 quence of the irritation of the sensitive nerves of the penis ; 

 in these cases the spinal marrow is the connecting link be- 

 tween the sensations and motions. Exposed muscles, whose 

 motor nerves are themselves coincidently stimulated in the 

 stimulation of the muscles, do not require these centripetal and 

 centrifugal actions to excite contractions. But the muscles 

 which are covered by sensitive membranes, and do not them- 

 selves lie exposed to stimuli, must receive the stimulus to mo- 

 tion through the sensorial excitement of their sensitive co- 

 vering, the centripetal action of these sensorial nerves, and the 

 centrifugal motor excitement from the brain. Thus the con- 

 tractions of the glottis and air-passages induced by irrespi- 

 rable acid gases, cannot result immediately from the excite- 

 ment of these passages, but from the centripetal-sensorial and 

 centrifugal-motor excitement. Brachet has very fully proved 

 this. For if the nervus vagus on both sides of an animal be 

 divided, an exciting chemical substance introduced into the 

 trachea ceases to act as a stimulus to cough. The cough 

 from stimulus in the air-passages is induced only by sensorial 

 centripetal, and centrifugal motor excitement. It is the same 

 with the contraction of the sphincter ani and sphincter vesicae 

 * Magendie's Journal de Physiologie, iii. 348. 



