186 



[April 14, 



April 14, 1864. 



Major-General SABINE, President, in the Chair. 



The Croonian Lecture was delivered b}^ Prof. Hermann Helm- 

 HOLTZ, For. Memb. U.S., ''On the Normal Motions of the Human 

 Eye in relation to Binocular Vision." 



The Motions of the Human Eye are of considerable interest, as well for 

 the physiology of voluntary muscular motion in general, as for the 

 physiology of vision. Therefore I may be allowed to bring before this 

 Society the results of some investigations relating to them, which I have 

 made myself ; and I may venture perhaps to hope that they are such as to 

 interest not only physiologists and medical men, but every scientific man 

 who desires to understand the mechanism of the perceptions of our senses. 



The eyeball may be considered as a sphere, which can be turned round 

 its centre as a fixed point. Although this description is not absolutely 

 accurate, it is sufficiently so for our present purpose. The eyeball, indeed, 

 is not fixed during its motion by the solid walls of an articular excavation, 

 like the bone of the thigh ; but, although it is surrounded at its posterior 

 surface only by soft cellular tissue and fat, it cannot be moved in a per- 

 ceptible degree forward and backward, because the volume of the cellular 

 tissue, included between the eyeball and the osseous walls of the orbit, 

 cannot be diminished or augmented by forces so feeble as the muscles of 

 the eye are able to exert. 



In the interior of the orbit, around the eyeball six muscles are situated, 

 which can be employed to turn the eye round its centre. Four of them, 

 the so-called recti muscles, are fastened at the hindmost point of the orbit, 

 and go forward to fix themselves to the front part of the eyeball, passing 

 over its widest circumference — or its equator, as we may call it, if we con- 

 sider the foremost and the hindmost points of the eyeball as its poles. These 

 four recti muscles are from their position severally named superior, inferior, 

 internal, and external. Besides these, there are two oblique muscles, the 

 ends of which come from the anterior margin of the orbit on the side next 

 the nose, and, passing outwards, are attached at that side of the eyeball 

 which is towards the temple — one of them, the superior oblique muscle, 

 being stretched over the upper side of the eyeball, the other, or inferior, 

 going along its under side. 



These six muscles can be combined as three pairs of antagonists. The 

 internal and external recti turn the eye round a perpendicular axis, so that 

 its visual line is directed either to the right side or to the left. The supe- 

 rior and inferior recti turn it round a horizontal axis, directed from the 

 upper end of the nose to the temple ; so that the superior rectus elevates 

 the visual line, the inferior depresses it. Lastly, the oblique muscles turn 

 the eye round an axis which is directed from its centre to the occiput, so 



