230 SENSE OF SIGHT. 



also, superior orbitar and spTieno-maxillary fissures , through which many 

 vessels and nerves proceed to the eye and its appendages. The base 

 of the orbits is not directly opposite the apices, but tends outwards; so 

 that the axes of these cavities, forming an angle of about 90 with 

 each other, if prolonged, would meet at the sella turcica. The eye, 

 however, is not placed in the direction of the axis of the orbit, but 

 straight forward ; and as it is nearly spherical, it is obvious that it 

 cannot completely fill the conical cavity. In Fig. 108, muscles 9 and 

 13 indicate the shape of the upper and lower surfaces of the cavities; 

 the whole of the space between the posterior part of the orbit and the 

 muscles, which is not occupied by the optic nerve, being occupied by 

 an adipous, areolar tissue, on which the eye is placed as it were on a 

 cushion. Under special morbid circumstances, this deposit becomes 

 greatly augmented, so as to cause the eye to start from its socket, 

 constituting the disease called exophthalmos. 



The parts, however, that are more immediately reckoned amongst 

 the protectors of the organ tutamina oculi are the eyebrows, eyelids, 

 and lachrymal apparatus. The eyebrows or supercilia are situate im- 

 mediately on the superciliary ridge of the frontal bone. They con- 

 sist of hair, varying in colour according to the individual, and turned 

 towards the outer angle of the eye; of common integument; sebaceous 

 follicles, situate at the root of each hair ; and muscles to move them, 

 namely, the frontal portion of the occipito-frontalis, the upper edge of 

 the orbicularis palpebrarum, and the corrugator supercilii. The pal- 

 pebr& or eyelids are, in man, two in number, an upper and a lower, 

 or a greater and a less, palpebra major vel superior, and palpebra 

 minor vel inferior, the former covering three-fourths of the eye; hence 

 the transverse diameter of the organ is not represented by their union, 

 the latter being much below it, and therefore improperly termed, by 

 Haller, sequator oculi. By the separation of the eyelids, we judge, 

 but inaccurately, of the size of the eye, one, who is capable of sepa- 

 rating them largely from each other, appearing to have a large eye ; 

 and conversely. 



The edge of the eyelids is thick; rounded; and furnished with hairs, 

 which resemble generally, in colour, those of the head. These are the 

 eyelashes or cilia. On the upper eyelid they are curved upwards ; on 

 the lower downwards. The eyelids are formed of four membranous 

 layers in superposition; and of a fibro-cartilage, which extends along 

 the whole edge, and keeps them tense. The outermost of these layers 

 is the common integument, the skin of which is delicate and semitrans- 

 parent, yielding readily to the motions of the eyelids, and having nu- 

 merous transverse folds. The areolar tissue beneath the skin is very 

 loose; and, under particular circumstances, is infiltrated by a serous 

 fluid, which gives the eyelids, especially the lower, a dark appearance ; 

 but they never contain fat. Beneath the common integument is the 

 muscular stratum, formed, in the lower eyelid, by the orbicularis palpe- 

 brarum; in the upper, by the same muscle, and the levator palpebrse 

 superioris, (Fig. 108,) which arises from above the foramen opticum, 

 and is inserted into the superior edge of the fibro-cartilage of the 

 tarsus. Beneath the orbicularis palpebrarum, again, is a fibrous layer, 



