414 Veterinary-Captain F. Smith. 



is, of course, the possibility that the other fibres, which have been 

 followed all along the cord to the last abdominal ganglion, also 

 branches through the nerve roots of this ganglion and serve a similar 

 purpose. The presence of collateral branches upon them seems to 

 me to be opposed to this view. The problems, however, suggest 

 the foregoing remarks can only be finally solved by means of physio- 

 logical research. 



My observations were made in the laboratory of the Marine Bio- 

 logical Association at Plymouth, with the assistance of a grant made 

 by the Government Grant Committee. I hope shortly to publish a 

 more detailed account with fuller illustrations. 



V. " The Refractive Character of the Eyes of Horses." 

 Veterinary-Captain F. SMITH, F.R.C.V.S., F.I.C., 

 Veterinary Department. Communicated by Profi 

 McKENDRiCK, F.R.S. Received March 7, 1894. 



The eyes of the horse are of great physiological interest, for there 

 are certain features in connection with them not found in other 

 animals. 



While equine vision is principally monocular, it is quite undoubted 

 that the horse can see objects situated to the right and left of his 

 body at one and the same time ; it is also equally certain that by 

 directing both eyes forward, and producing a powerful internal 

 squint, vision may be rendered binocular for objects situated directly 

 to the front. 



The corresponding points in the retina of the human eye, do not 

 hold good in the case of animals which have their eyes situated 

 laterally in the head and at some distance apart. 



The pupil of the horse is a horizontal slit, which in full sunlight 

 becomes so contracted that, with the presence of the corpora nigra, it 

 is difficult to understand how sufficient light finds its way to the 

 retina. A monocular retinal image in full sunlight must be a broken 

 or imperfect one, for, filling up the centre of the horizontal pupil, are 

 some large, black, soot-like bodies, the corpora nigra just referred to. 

 These are attached principally to the superior pupillary margin of 

 the iris, and completely block out the centre of the pupil when the 

 latter is closely contracted. The horse is, I believe, the only animal 

 possessing corpora nigra, and the function of these bodies is quite 

 obscure. 



It is essential that the horizontal pupil should always be kept 

 horizontally placed, no matter what position the head may occupy ; 

 every upward and downward movement of the head necessitates a 

 rotation of the eyeballs to ensure the pupils remaining horizontal, 



