10 THE EYE. 



upper end is closed and rounded, forming a cul-de-sac ; the lower end gradually 

 narrows into the nasal duct. On the outer side, and a little in front, it receives 

 the lachrymal canals ; and here it is placed behind the internal tarsal ligament, and 

 some of the inner fibres of the orbicular muscle of the lids ; while on its orbital 

 surface is the tensor tarsi muscle. The nasal duct, very variable in length (12 

 to 24 mm.), and 3 or 4 mm. wide, grooving the upper maxilla, descends to the 

 fore part of the lower meatus of the nose, the osseous canal being completed by 

 the lachrymal and lower turbinate bones. Both sac and duct are composed of 

 fibrous and elastic tissues, adhering closely to the bones above mentioned, and 

 strengthened in the case of the lachrymal sac by a fibrous process sent from the 

 internal tarsal ligament, which crosses it a little above its middle. The inner 

 surface is lined by a mucous membrane, which is continuous through the canaliculi 

 with the conjunctiva, and through the nasal duct with the mucous membrane of the 

 nose. 



At the opening into the nose the lining membrane is often arranged so as to form 

 an imperfect valve (Hasner). Other valvular folds have been often noticed and 

 described, but they appear to be less constant. The nasal duct is rather narrower 

 in the middle than at either end ; its direction is not quite vertical, but inclined 

 slightly backwards. Its direction is indicated by a line joining the mesial 

 canthus of the eyelids with the anterior edge of the first molar of the upper jaw 

 (Testut). The lower orifice of the nasal duct is very variable in position, but is 

 usually from 80 to 35 mm. behind the posterior margin of the anterior nasal opening 

 (Arlt). It may open by a simple round orifice close under the inferior turbinate. 

 or by an oblique or slit-like orifice or groove in the mucous membrane somewhat 

 lower. In rare cases two lower openings have been described. This condition is 

 always present in some animals (e.g., dog). 



The nasal sac and duct are lined by a columnar epithelium, which may be 

 ciliated here and there, but does not appear to be covered everywhere with cilia as is 

 the case on the adjacent mucous membrane of the nose. The lower part of the 

 nasal duct has numerous glands similar to those in the nasal meatus into which it 

 opens. The arteries come from the nasal and inferior palpebral. The veins are 

 very large and numerous on the nasal duct (as in the adjacent nasal mucous mem- 

 brane). The nerves are derived from the infratrochlear branch of the nasal 

 division of the ophthalmic. 



THE GLOBE OF THE EYE. 



The globe or ball of the eye is supported by a quantity of fat and loose connective 

 tissue in the fore part of the orbital cavity, somewhat nearer its lateral and inferior 

 walls than its mesial and superior. The recti and obliqui muscles closely surround 

 the greater part of the eyeball, and are capable of changing its position within 

 certain limits : the lids, with the plica semilunaris and caruncle, are in contact with 

 its covering of conjunctiva in front ; and behind it receives the thick stem of the 

 optic nerve. 



The eyeball is composed of segments of two spheres, of which the anterior is the 

 smaller and more prominent ; the segment of the larger posterior opaque sphere 

 corresponds with the limit of the sclerotic coat, and that of the smaller sphere with 

 the cornea. The junction of the two is marked by a broad shallow groove which 

 has been named sulcus sderce. 



The eyeball measures nearly an inch (24*5 mm.) across from side to side, but 

 slightly less from before back (24 mm.), and still less from above down (23'5). 

 It weighs about 7 grammes, and has a volume of about 6 -5 cubic centimetres. 



