846 



THE ORGANS OF SPECIAL SENSE. 



cornea ; it is called the plica semilunaris. Mu'ller found smooth muscular fibres 

 in this fold, and in some of the domesticated animals a thin plate of cartilage has 

 been discovered. This structure is considered to be the rudiment of the third eyelid 

 in birds, the membrana nictitans. 



The Lachrymal Apparatus (Fig. 454). 



The lachrymal apparatus consists of the lachrymal gland, which secretes the 

 tears, and its excretory ducts, which convey the fluid to the surface of the eye. 

 This fluid is carried away by the lachrymal canals into the lachrymal sac, and along 

 the nasal duct into the cavity of the nose. 



The lachrymal gland is lodged in a depression at the outer angle of the orbit, 

 on the inner side of the external angular process of the frontal bone. It is of an 

 oval form, about the size and shape of an almond. Its upper convex surface is in 

 contact with the periosteum of the orbit, to which it is connected by a few fibrous 

 bands. Its under concave surface rests upon the convexity of the eyeball and 

 upon the Superior and External recti muscles. Its vessels and nerves enter its 

 posterior border, whilst its anterior margin is closely adherent to the back part of 

 the upper eyelid, where it is covered to a slight extent by the reflection of the con- 

 junctiva. The fore part of the gland is separated from the rest by a fibrous septum ; 

 hence it is sometimes described as a separate lobe, called the palpebral portion of 

 the gland (accessory gland of Rosenmuller). Its ducts, from six to twelve in number 

 run obliquely beneath the mucous membrane for a short distance, and, separating 

 from each other, open by a series of minute orifices on the upper and outer half 



FIG. 454. The lachrymal apparatus. Right side. 



of the conjunctiva near its reflection on to the globe. These orifices are arranged 

 in a row, so as to disperse the secretion over the surface of the membrane. 



Structure of the Lachrymal Gland. In structure and general appearance the 

 lachrymal resembles the serous salivary glands (page 885). In the recent state the 

 cells are so crowded with granules that their limits can hardly be defined. They 

 contain an oval nucleus, and the cell-protoplasm is finely fibrillated. 



The lachrymal canals commence at the minute orifices, puncta lachrymalia, 

 on the summit of a small conical elevation, the lachrymal papilla, seen on the 

 margin of the lids at the outer extremity of the lacus lachrymalis. The superior 

 canal, the smaller and shorter of the two, at first ascends, and then bends at an 

 acute angle, and passes inward and downward to the lachrymal sac. The 

 inferior canal at first descends, and then, abruptly changing its course, passes 

 almost horizontally inward to the lachrymal sac. These canals are dense and 



