Royal Society, 597 



folding over each other in the middle, thus dividing the aperture 

 into two. They do not however unite ; and in some specimens, 

 being less elongated, they leave the pupil entire, but very nar- 

 row in the middle, forming two opposed ovals, broad at their 

 ends but narrow at the point of junction. Usually, however, 

 there are two distinct apertures. The crystalline, instead of 

 being spherical, is not even circular, having a small projection 

 at the lower edge, directly behind the smaller aperture- When 

 examined in the microscope, a small bundle of muscular fibres 

 is seen coming from the capsule of the vitreous humour in the 

 lower part, and entering that of the crystalline just at the disc 

 where the smaller curve joins the larger one ; the action of this 

 muscle being to bring the lower mammillary process of the lens 

 downwards and backwards into the centre of the lower aperture 

 in the iris ; thus constituting a complete organ for vision at near 

 distances, independent of the part of the lens opposed' to the 

 large aperture, which is destined for more distant objects. 



The authour regards this structure as destined to a similar 

 purpose with that of the marsupium in Birds; viz. to obviate a 

 difficulty arising from a want of motion in one direction in the 

 ball of the eye. He considers that by its means also the fish, 

 when lying with its eye-ball above the surface of the water, 

 may enjoy distinct vision both in air and water, by the motions 

 of the crystalline and eye-ball, combined with the adapting 

 power of the two apertures of the iris to a circular form. 



Nov. 22. — A Paper was read On the structure of the Knee- 

 joint in the Echidna setosa and the Ornithorhynchus paradoxus / 

 by G. Knox, M.D., &c., communicated by Sir James Mac- 

 Gregor, F.R.S. 



After a short review of the labours of comparative anatomists 

 on the animals which are the subject of this memoir, the authour 

 describes a peculiarity of structure which was discovered by his 

 brother in the knee-joint of the Echidna, consisting of an ex- 

 tension of the ligamentum adiposum, or re-duplication of the 

 synovial membrane, transversely across the whole joint, dividing 

 it into two cavities which have no distinct communication with 

 each other. The articular surfaces of the upper cavity are the 



