SENSITIVE SYSTEiVJ. 377 



rough part of the zygomatic arch. It is thickest and offers most 

 resistance at the outer and upper parts of the cavity, where bony 

 parietes are wanting. It serves to hold the contained parts to- 

 gether, and to protect them from external injury ; and it like- 

 wise offers resistance to the imf)ulse of the adeps whenever the 

 eyeball is retracted. 



Of the Eyeball. 



The ball, globe, bulb, or apple of the eye (so variously deno- 

 minated from its spherical form) may be regarded as an optical 

 instrument of complex but singularly beautiful construction, into 

 which the rays of light are received, and by which they are in 

 their passage in such manner refracted and inflected as to be 

 collected into focal points, and thereby to represent a correct 

 image in miniature of the object from which they are radiating. 



Defence, relative Position, and Direction. — In order that we 

 may perfectly understand the position of the eyeball in the head, 

 the manner in which it is sustained in that position, and the 

 various motions it is capable of, and thereby form correct ideas 

 of the axis and range of vision, it is necessary for us to renew 

 our acquaintance with the orbits. The orbit is an imperfect 

 socket formed by unequal pieces of bone coming from the frontal, 

 malar, lachrymal, temporal, ethmoidal, and sphenoidal bones, in the 

 following manner and proportions: — Of the external ridge: the 

 supero-anterior part, about two-fifths of the whole circumference, 

 is formed by the orbital process of the frontal bone ; the infero- 

 anterior part, about one-fifth, by the lachrymal bone ; and the 

 remaining two-fifths by the malar and temporal bones, in the 

 ratio of three to one. Internally, the fioor is constituted of the 

 orbital plates of the lachrymal and malar bones; the side by 

 that of the frontal, and by the os planum ; the back parts by the 

 ethmoidal and sphenoidal bones. The socket is of an irregular 

 figure : looking into it in front, it has the appearance of being of 

 a conoid or pyramidal form, but, on close examination, we find 

 that the roof and one side are deficient— that the eyeball is 

 guarded in thos.e places only by bony arches, which, though 

 they are firmly stayed and well placed to ward off attacks in the 

 most perilous directions, seem to leave the oroan exposed in 

 others. This, however, is far from being the case. In the 

 recent subject, not only a considerable part of the vacuity behind 

 is occupied by the condyle of the inferior maxilla, but the re- 

 mainder of the space, posterior to the orbital sheath, is filled 

 with adeps : and this serves as a bulwark to the globe behind, 

 while it freely admits of the motions of thejaw. But what prin- 

 cipally demands our attention here, is the sheath lining the orbit. 

 3 c 



