16 INNERVATION. [CHAP. xvn. 



anterior portion, formed by*the cornea, is a part of a smaller sphere 

 than the rest, and is therefore slightly protuberant. Hence the an- 

 tero-posterior axis of the eye is longer than the transverse, in the 

 proportion of twenty to nineteen. 



In terrestrial quadrupeds its shape is for the most part nearly similar. In 

 animals that inhabit the water, as Cetacea and fishes, the eye is considerably 

 flattened in front ; so that, in some fishes, it is almost a half-sphere. In birds, 

 on the contrary, especially those which fly high, the cornea is very prominent 

 compared with the rest of the eye, which is of a more or less flattened form. 

 These differences have an evident reference to the density or rareness of the 

 medium through which the light passes to the organ. 



A more detailed description of the several structures composing 

 the ball of the eye will now be given, in which we shall follow the 

 order most natural to a dissector, viz. that from without, inwards. 



The sclerotic coat consists of white fibrous tissue, in which, how- 

 ever, the ultimate filaments are more distinct, and less wavy than 

 in ordinary specimens. These form numerous layers, crossing one 

 another chiefly at right angles, and thus constitute a membrane 

 capable of resisting distension, and of retaining its figure under 

 pressure. It has a white glistening aspect, especially in front 

 where it receives the insertion of the tendons of the four straight 

 muscles, and, being visible, is familiarly known as the " white of the 

 eye." The sclerotic is thickest behind, and becomes gradually thin- 

 ner in front, till nearly in contact with the cornea, where it in- 

 creases in strength a little. 



In the animal series, the sclerotic becomes of greater relative thickness behind, 

 in proportion to the flattening of the organ in front, and the pressure which it 

 will have to sustain from the surrounding medium. In aquatic mammalia this 

 is affected simply by an accumulation of the fibrous tissue in that situation, as 

 in the whale, where it is often an inch in thickness, a wonderful provision against 

 the enormous pressure to which that animal is exposed at great depths. In 

 reptiles and fishes there is a thick cartilaginous lamina included in the fibrous 

 tissue ; and in some this cartilage ossifies, as in the sea-bream, mentioned by 

 Dr. Jacob. In birds too, where the sclerotica is flattened from before backwards, 

 a thin cartilaginous plate exists in it, which confers a peculiar elasticity and 

 firmness, and is at the same time light and slender. Its anterior part is further 

 fortified by fourteen or fifteen osseous plates, disposed in a regular series round 

 the margin of the cornea. Similar plates occur in various reptiles, and are 

 especially'remarkable in those gigantic specimens of this class, the Ichthyosauri 

 and Plesiosauri, which are only known to us by their fossil remains. 



The optic nerve comes through the sclerotic behind, at a distance 

 of about its own breadth, or nearly one eighth of an inch, on the 

 inner side of the axis of the eye, by which is meant the axis of the 



