828 THE APPARATUS OF THE SENSES. 



(The chief function of this humour appears to be to maintain the con- 

 vexity of the cornea, and to facilitate the movements of the iris and lens ; as 

 well as to assist, to some extent, in refracting the light that passes through it 

 to the lens and retina. The rapidity with which this fluid can be regenerated 

 is very striking ; absorption also takes place very rapidly in the anterior 

 chamber of the eye. The frequency of adhesions between the iris and lens, 

 after attacks of ophthalmia, is accounted for by the small quantity of this 

 fluid that exists between them, as owing to the smallness of the posterior 

 chamber this is reduced to a mere film.) 



ARTICLE II. ACCESSORY ORGANS OF THE VISUAL APPARATUS. 



ORBITAL CAVITY. 



Situated at the side of the head, at the point corresponding to the union 

 of the cranium and face, the orbital cavity is circumscribed by a bony 

 margin, in the formation of which the orbital process, frontal, lachrymal, 

 malar, and a small portion of the zygomatic process of the temporal bone, 

 concur. Posteriorly, however, there are no bony walls, and the cavity, in 

 the skeleton, is confounded with the temporal fossa. A fibrous membrane 

 completes this cavity in the domesticated animals, and keeps it distinct 

 from the fossa. 



Designated the ocular sheath (ocular membrane or periorbita), this fibrous 

 structure is attached, posteriorly, to the border of the orbital hiatus, and 

 anteriorly to the inner face of the orbit ; being prolonged beyond the 

 external lip of this osseous rim to form the fibrous membrane of the eyelids. 

 Strong externally, the ocular sheath is thin within the cavity. It is 

 traversed by vessels and nerves, and is composed of a mixture of elastic and 

 inelastic fibres. (Unstriped muscular fibres have also been described as 

 existing in this orbital periosteum.) 



Thus completed, the orbital cavity has the form of a regular hollow 

 cone, open at its base, and closed- at the apex, which corresponds to the 

 orbital hiatus. 



In the ordinary position of the head, the opening of this cone is directed 

 forwards, downwards, and outwards. 



Independently of the globe of the eye, this cavity lodges the muscles that 

 move it, the membrana nictitans, and the lachrymal gland. 



MUSCLES OP THE EYE. (Fig. 391.) 



These are seven in number : five termed recti muscles, and distinguished 

 as posterior, superior, inferior, external, and internal ; two named oblique a 

 large and small. 



(Preparation. Detach the eyelids from the margin of the orbit, cutting away the lower, 

 but leaving the upper. Saw through the zygomatic process of the temporal bone, in front 

 of the temporo-maxillary articulation, also through the temporal process of the zygo- 

 maticus, and the base of the orbital process of the frontal bone ; remove the excised 

 piece of bone, and the temporal fossa and ocular sheath are exposed. Cutting through 

 the latter, the muscles of the eye are seen disposed in a conical manner around the 

 globe ; dissect away the fat lodged among them, in order to isolate them.) 



1. POSTERIOR EECTUS MUSCLE (or retractor oculi). This muscle com- 

 pletely envelops the extra-cranial portion of the optic nerve, being a 

 muscular sheath resembling in shape the fibrous lining of the orbit. Its 

 fibres are disposed longitudinally, arise around the optic foramen, and are 



