VOL. LXXVII.] PHILOSOPHICAL TRANSACTIONS. 345 



its anterior side near the bottom, for the passage of the portio dura, and which 

 is continued backward to the cavity of the tympanum near the stapes, and 

 emerges near the posterior and upper part of this bone. 



Of the organ of seeing. — The eye in this tribe of animals is constructed on 

 nearly the same principle as that of quadrupeds, differing however in some cir- 

 cumstances; by which it is probably better adapted to see in the medium through 

 which the light is to pass. It is on the whole small for the size of the animal, 

 which would lead to the supposition, that their locomotion is not great; for I 

 believe animals that swim are in this respect similar to those that fly; and as this 

 tribe come to the surface of the medium in which they live, they may be con- 

 sidered in the same view with birds which soar ; and we find birds that fly to 

 great heights, and move through a considerable space, in search of food, have 

 their eyes larger in proportion to their size. 



The eyelids have but little motion, and do not consist of loose cellular mem- 

 brane, as in quadrupeds, but rather of the common adipose membrane of the 

 body; the connection however of their circumference with the common integu- 

 ments is loose, the cellular membrane being less loaded with oil, which allows of 

 a slight fold being made on the surrounding parts in opening the eyelids. This 

 is not to an equal degree in them all, being less so in the porpoise than in the 

 piked whale. The tunica conjunctiva, where it is reflected from the eyelid to the 

 eyeball, is perforated all round by small orifices of the ducts of a circle of glan- 

 dular bodies lying behind it. The lachrymal gland is small ; its use being sup- 

 plied by those above-mentioned; and the secretion from them all I believe to be a 

 mucus similar to what is found in the turtle and crocodile. There are neither 

 puncta nor lachrymal duct, so that the secretion, whatever it be, is washed off 

 into the water. 



The muscles which open the eyelids are very strong: they take their origin 

 from the head, round the optic nerve, which in some requires their being very 

 long, and are so broad as almost to make one circular muscle round the whole of 

 the interior straight muscles of the eye itself. They may be divided into 4 ; a 

 superior, an inferior, and one at each angle: as they pass outwards to the eye- 

 lids, they diverge and become broader, and are inserted into the inside of the 

 eyelids almost equally all round. They may be termed the dilatores of the eye- 

 lids; and, before they reach their insertion, give off the external straight 

 muscles, which are small, and inserted into the sclerotic coat before the trans- 

 verse axis of the eye: these may be named the elevator, depressor, adductor, and 

 abductor, and may be dissected away from the others as distinct muscles. Be- 

 sides these 4 going from the muscles of the eyelid to the eye itself, there are 2 

 which are larger, and inclose the optic nerve with the plexus. As these pass 



VOL. XVI. Y Y 



