854 



THE OKGANS OF SENSE. 



Recess us 

 labyrinth! 



endolymphaticus (see note, p. 79). About the fifth week, the lower part of the vesicle is 

 prolonged forwards as a diverticulum, the future ductus cochlearis. This is at first straight, 



but as it elongates it curves on itself, 

 so that at the twelfth week all 

 three coils are differentiated. From 

 the upper part of the vesicle the 

 semicircular ducts are developed, 

 semicircular anc ^ a PP ear as three hollow, disc-like 



duct 



Posterior 



semicircular 



duct 



Lateral 



semicircular 



duct 



Cochlear 

 part 



Cochlea - 



Utricle 



Saccule 



evaginations ; the central parts of 

 the walls of each disc coalesce and 

 disappear, leaving only the peri- 

 pheral ring or canal. The three 

 ducts are free about the beginning 

 of the second month, and are de- 

 veloped in the following order, viz. : 

 superior, posterior, and lateral. The 

 intermediate part of the otic vesicle 

 represents the vestibule, and is 

 divided by a constriction into an 

 anterior part, the saccule, communi- 

 cating with the ductus cochlearis, 

 and a posterior portion, the utricle, 

 receiving the extremities of the 

 semicircular ducts. The constric- 

 tion extends for some distance into 

 the ductus endolymphaticus, and 



thus the utricle and saccule are connected by a Y-shaped tube. Another constriction 

 makes its appearance between the saccule and the vestibular end of the ductus cochlearis 

 and forms the canalis reuniens. The epithelial lining is at first columnar, but becomes 

 cubical throughout the whole labyrinth, except opposite the terminations of the acoustic 

 nerve, where it forms the columnar epithelium of the maculae of the utricle and saccule, of 

 the cristse ampullae, and of the organon spirale. On the floor of the ductus cochlearis two 

 ridges appear, of which the inner forms the limbus laminae spiralis, whilst the cells of 

 the outer become modified to form the rods of Corti, the hair cells, and the supporting 

 cells of Deiters and Hensen. 



The mesoderm surrounding the otic vesicle is differentiated into: (1) a fibrous layer, 

 the. wall of the membranous labyrinth ; (2) a cartilaginous capsule, the future petrow 

 bone ; and (3) an intervening layer of gelatinous tissue, which is ultimately absorbed, 

 leaving the perilymphatic space between the bony and membranous labyrinths. 



The development of the external and middle parts of the ear are described or 

 pp. 50-53. 



FIG. 729. 



A, Left labyrinth of a human embryo of about four weeks ; B, 

 Left labyrinth of a human embryo of about five weeks (from 

 W. His, jun.). 



OKGANON GUSTUS. 



The peripheral gustatory organ consists of groups of modified epithelial cells 

 termed calyculi gustatorii or taste buds, found on the tongue and in its immediat< 

 neighbourhood. 



Taste buds are present in large numbers around the circumference of th< 

 papillae vallatae, while some are found also on their opposing walls (Fig. 730). The;; 

 are very numerous over the foliate papillae, which correspond with the papilla 

 foliatae of the tongue of the rabbit, and are found also over the posterior part an< 

 sides of the tongue, either on the papillae fungiformes or throughout the stratifie< 

 epithelium. They exist, also, on the oral surface of the velum palatinurn and or 

 the posterior surface of the epiglottis. 



Structure of Taste Buds (Fig. 731). The taste buds are oval or flask-shapec 

 and occupy nests in the stratified epithelium of the regions mentioned. The dee 

 extremity of each is expanded and rests upon the corium ; the free end is perfoi 

 ated by a minute pore, termed the gustatory pore. They consist of two kinds ( 

 epithelial cells (a) supporting cells, and (&) gustatory cells (Fig. 732). Tl: 

 supporting cells are elongated, nucleated spindles, and are mostly arranged like t] 

 staves of a cask to form the outer envelope of the bud ; but some are found in tl 



