1082 SPECIAL SENSE ORGANS 



The lacrimal canals and naso-lacrimal duct are formed by the growth of an epithelial 

 band which passes through the mesoderm to the nasal cavity along the naso-lacrimal gi'oove. 

 This band loses its primitive connection with the groove, and is reunited to the lid margins 

 by secondary epithelial bands which grow from the naso-lacrimal duct to the lid margin. Simi- 

 larly a secondary connection is later made with the nasal cavity at the lower end of the duct. 

 The position of the naso-lacrimal duct corresponds to the line of union of the nasal and maxillary 

 processes; but the duct does not represent a portion of the cleft between these processes, and 

 is formed secondarily between them. 



IV. THE EAR 



Under the name of the ear [organon auditus] there is included a number of 

 structures of which some, the ear proper, constitute the auditory mechanism — 

 that is, an apparatus for the collection, transmission and reception of the waves of 

 sound; while others — the semicircular ducts and associated structures — are 

 concerned in receiving and transmitting impressions produced by movements of the 

 head. These impressions constitute the basis of what may be termed the static or 

 equilibratory sense, and afford data employed in estimating movements of the 

 body in relation to surrounding objects. 



The former of these, the ear proper, consists of three main parts, each possess- 

 ing distinct structural and functional characters. The first portion, often known as 

 the external ear, consists of a receptive organ placed upon the surface of the head, 

 the auricle or pinna, and of a short tube, the external auditory meatus, which 

 leads into the interior, and is closed at its deep end by the tympanic membrane. 



The second portion, known as the middle ear, consists of the tympanic cavity, 

 a small air-containing chamber in the petrous portion of the temporal bone, con- 

 nected with the nasal part of the pharynx by a tube, the auditory (or Eustachian) 

 tube. From the tympanic chamber a recess passes posteriorly and leads to a cavity 

 in the mastoid portion of the temporal bone, the mastoid or tympanic antrum. 

 A chain of three small bones transmits the sounds across the middle ear. 



The third part, or internal ear which contains the essential sensory apparatus, 

 lies within the complex cavities in the interior of the petrous temporal bone known 

 as the osseous labyrinth. It consists of (1) the utricle and saccule, two small ves- 

 icular structures lying in the bony vestibule, and (2) the membranous semicir- 

 cular ducts and (3) the membranous cochlea, which lie within the corresponding 

 bony canals. 



These structures are filled with fluid, the endolymph, and communicate with one 

 another. They are largely separated from the bony walls by fluid, perilymph, and 

 they are lined by sensory epithelium. Closely related to the epithelial sensory 

 cells are found the terminal branches of the cochlear and vestibular nerves. 



The description of the three divisions of the ear is taken up in order from the 

 surface inward. 



1. THE EXTERNAL EAR 



The external ear consists of the auricle attached to the side of the head, and 

 the external auditory meatus leading from it to the middle ear (fig. 829). 



THE AURICLE 



The auricle, or pinna, is an irregular oval plate-like structure which lies upon 

 the lateral surface of the head. It presents a lateral and a medial surface. The 

 lateral surface is irregularly concave (fig. 827). The deepest ]iart of its concavity 

 situated near the centre, is termed the concha, and it is i)artially divided by 

 a prominent obli(iue ridge, the crus of the helix, into a superior part, the cymba 

 conchae, and a large inferior ])art, the cavum conchae. The cavum conchas leads 

 into the external auditory meatus, and is bounded ventrally by a ])rominent proc- 

 ess, the tragus, which projects posteriorly over the entrance to the meatus. The 

 tragus, is separated from the; crus of the helix by a well-marked depression, the 

 anterior incisure and has a small tubercle on it superiorly, the supratragic tuber- 

 cle, liouudiiig the cavum concluc posteriorly and inferiorly is a i)rojection, the 

 antitragus, lying opposite, l)ut inferior, to the tragus, and between the two is a 

 deep notch, the intertragic notch |incisura intertragica]. A prominent semicircular 



