TRANSACTIONS OF SECTION D. — DEPT. ANATOMY AND PHTSIOLOGT. 405 



sciously executed by the eyeballs attempting to follow the real movements, and 

 continuing their unconscious slipping alterwards, so causing apparent motion in 

 objects that are really stationary. The theory of Dr. Javal, however, appears to 

 be untenable in the presence of two facts of capital importance : first, that the 

 illusion of apparent motion is confined to objects occupying that portion of the 

 visual field occupied before by the moving body ; secondly, that if two or more 

 simultaneous motions occur in different directions, in different parts of the visual field, 

 each produces its own illusive motion in a complementary direction, and all these 

 different motions appear to be going on at the same time. Thus, if a turning 

 spiral pattern be steadily regarded for some minutes, the gaze being directed at 

 the centre, the sense of rotation being such, that from all directions there appears 

 to be a movement of convergence to the centre, on turning the gaze to some other 

 object, that object appears to be enlarging and approaching. 



The muscular- slipping theory is therefore out of the question, since the muscles 

 of the eye cannot slip in all directions at once : and if they slipped in any one 

 direction, this would affect objects over all the visual field, not over one region 

 only. 



The theory which the author has, however, to propound is virtually a new law 

 of retinal activity. It is as follows : the retina ceases to perceive as a motion a Heady 

 motion of images that jjass for some time over a particular region ; and to a portion of 

 the. retina so affected, a body not in motion appears to be moving in a complementary 

 sense. It is a law analogous to that of the subjective complementary colours seen 

 after looking at a coloured body. It is analogous to other laws of nerve perception, 

 where we lose consciousness of steady phenomena, and become conscious only of 

 changes. Thus a steady sound of one pitch and intensity ceases to be heard. A 

 steady light of one colour, as gas-light, ceases to be noticed as yellow. A steady 

 taste — as that of garlic pervading every kind of food in some countries, ceases to 

 be perceived until it is perceived by its absence. The same is true of our percep- 

 tions of change of temperature. All these laws are probably only different aspects 

 of a much more general law of nervous perception. It is quite consonant with 

 these laws, that when any portion of the retina is affected by an image of objects 

 moving steadily in any direction over it that portion of the retina gradually loses 

 consciousness of the motion, and perceives it only as if at rest. When, however, 

 an object really at rest is looked at, to that portion of the retina thus affected the 

 fixed object appears in motion, but in an opposite direction. To the law expressing 

 this fact the author proposes to give the name of the Law of Subjective Comple- 

 mentary Motion. 



4. On the Comparative Osteology of the Arm. By Dr. T. P. Durand. 



The author, after referring to Martin's theory of the torsion of the humerus, 

 states his belief that in tracing the variations of the forelimb from the amphibia 

 upwards, two groups of animals may be distinguished. In one, including • the 

 Cetacea and Aves, the torsion is outwards ; in the other, including the Ernydae, 

 most other reptiles, and all terrestrial mammals, the torsion is inwards. The 

 exceptional forms of humerus observed in Monotremata, the Sirenia, Proboscidea, 

 and Pinnepedia are then treated at length. Due weight is given to the action of 

 muscles as a modifying agent in the form of the humerus. 



SATURDA Y, A UG VST 23. 



The Department did not meet. 



