THE SENSORY APPARATUSES. 



3. The Aqueous Humour. 



This is a liquid that owes its name to its great fluidity ; it is contained m 

 the anterior and posterior chambera of the eye, in front of the lens. It is 

 secreted by a particular membrane — the membrane of the aqueous humour, or 

 membrane of Descemet or Demours — an extremely thin serous layer, easily dis- 

 tinguished on the posterior face of the cornea, and admitted to exist on the two 

 surfaces of the iris, the ciliary processes, and anterior face of the capsule of the 

 lens (Fig. 502, o), where it is reduced to epithelium only. 



Schwalbe has observed the glandular appearance of the epithelium on the 

 ciliary processes ; and Ehrlich has shown that when flourescene is injected beneath 

 the skin, it is eliminated by the anterior chamber of the eye when the aqueous 

 humour is evacuated by puncture of the cornea. From these facts, conclusions 

 have been drawn as to the analogy between this epithelium and a gland. Tliis 

 gland is completely innervated (Schaler and Uthof, Nicati) ; the Gasserian 

 ganglion is its moderator centre. 



(The eyeball has no proper lymph-vessels, but it has numerous small inter- 

 communicating spaces which are related to larger spaces — such as Schlemm's 

 canal, the canal of Petit, the canalis hi/aloideus, Tenon's space on the dural 

 sheath of the optic nerve ; and this arrangement allows the lymphatic system 

 of the eye to be divided into two regions — an anterior and a posterior. 



The chief function of the aqueous humour appears to be to maintain the 

 convexity 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 minute 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. 



Preparation. — The ocular cavity is prepared by clearing the temporal fossa of its musc'es 

 and adipose tissue, and removing the eyelids, also the eye and its muscles. 



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 continuous 

 with the temporal fossa. But 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 where it is in contact with the bones of the 

 cavity. It is traversed by vessels and nerves, and is composed of a mixture of 



