EYE 465 



From these sacs it passes through the naso-lachrymal ducts into the nasal 

 cavity. The lachrymal ducts are lined with stratified squamous epithe- 

 lium, resting upon a tunica propria containing an abundance of cells and 

 elastic fibers. Externally these ducts are surrounded by striated muscle 

 fibers, chiefly longitudinal. The lachrymal sac, .which is provided with 

 small branched tubular glands, and the naso-lachrymal duct are both 

 lined with two-rowed columnar epithelium, surrounded by a lymphoid 

 tunica propria. They are separated from the underlying periosteum by 

 a thick plexus of veins. 



EAR. 



Development and General A natomy. The ear is divided into three parts : 

 (i) the external ear, which includes the auricles projecting from the surface 

 of the body, and the external acoustic meatus leading from the surface to 

 the tympanic membrane; (2) the middle ear, including the tympanic cavity 

 or "drum" and the chain of three bones extending across it; and (3) the 

 internal ear, which is a system of epithelial ducts and surrounding tissue 

 spaces, imbedded in the temporal bone, and connected with terminal 

 branches of the acoustic nerve. 



On either side of the body, the internal ear first appears as a local 

 thickening of the epidermal ectoderm near that portion of the medullary 

 tube which later becomes the pons. The thickened areas are invaginated 

 as shown in Fig. 471 A and B, and the pockets thus produced become 

 separated from the epidermis in the form of auditory vesicles (otocysts). 

 The place where they become detached from the epidermis is marked 

 by a slight elevation on the medial surface of the vesicle, which soon 

 elongates, producing the tubular endolymphatic duct (Fig. 471, C). The 

 blind upper end of the duct becomes enlarged to form the endolymphatic 

 sac, which, however, is only slightly developed in man; it appears in the 

 models of the embryonic vesicle shown in side view in Fig. 472, A-C. In 

 the adult the endolymphatic duct is a very slender tube, terminating 

 blindly (or perhaps with secondary apertures) just beneath the dura. 



In two places the medial and the lateral walls of the upper half of the 

 vesicle approach one another, and after fusing, the epithelial plates thus 

 produced become thin and rupture, so that two semicircular ducts are formed 

 (Fig. 472, B and C). The space encircled by each duct may be regarded 

 as a hole through the vesicle. The two ducts are the superior and posterior 

 semicircular ducts respectively . Th e third or lateral semicircular duct forms 

 soon afterward. In Figs. 471, D and 472, B it is a horizontal shelf-like 

 projection of the vesicle, the center of which is to become perforated so 

 that its rim will become the duct. The portion of the vesicle which re- 

 ceives the terminal openings of the three semicircular ducts is called the 

 30 



