376 THE PERIPHERAL NERVOUS SYSTEM 



rudimentary and appears as a slight elevation dorsal and cranial to the 

 inferior concha (Fig. 373). Dorsal to the inferior concha arise five ethmo- 

 turbinals, which grow progressively smaller caudally. According to 

 Peter, the ethmo-turbinals arise on the medial wall of the nasal fossa, and, 

 by a process of unequal growth, are transferred to the lateral wall (Fig. 

 372). Accessory conchae are also developed (Killian). 



In adult anatomy, the inferior concha forms from / (Fig. 373), the middle concha from 

 II, and the superior concha from /// and I V. 



In addition to the ridges formed by the conchae, there are developed in the grooves 

 between the ethmo-turbinals the ethmoidal cells. After birth the frontal recess (located 

 between / and //, Fig. 373) gives rise to the frontal sinus. During the third month the 

 maxillary sinus grows out from the inferior recess of the same groove. The most caudal 

 end of the nasal fossa becomes the sphenoidal sinus, which, as it increases in size, invades 

 the sphenoid bone. These cells and sinuses form as excavations of the bone which become 

 lined with simultaneously advancing epithelial evaginations. 



The cells of the olfactory epithelium acquire cilia, but only a small area, representing 

 the primitive epithelial invagination, functions as an olfactory sense organ. The olfactory 

 cells of this area give rise to the fibers which constitute the olfactory nerve (cf. p. 357). 



IV. THE EYE 



The anlage of the human eye appears in embryos of 2.5 mm. as a 

 thickening and evagination of the neural plate of the fore-brain. At this 

 stage the neural groove of the fore-brain has not closed (Figs. 324, 330 

 and 382). At 4 mm. the optic vesicles are larger, but still may be con- 

 nected by a wide opening with the brain cavity (Fig. 374 A, B). In 

 the section shown in Fig. 374 C, the optic vesicle is attached to the ventral 

 brain wall by a distinct optic stalk (cf. Fig. 343).. 



The thickening, flattening, and invagination of the distal and ventral 

 wall of the optic vesicle gives rise to the optic cup (Fig. 374 B-D). The 

 area of invagination also extends ventrally along the optic stalk and pro- 

 duces a groove known as the chorioid fissure (Figs. 331, 375 and 377). 

 At the same time that the optic vesicle is converted into the optic cup, 

 the ectoderm overlying the vesicle thickens, as seen in Fig. 3 74 B, forming 

 the lens plate, or optic placode. This plate invaginates to form the lens 

 pit, the external opening of which closes in embryos of 6 to 7 mm. (Fig. 

 374 D), producing the lens vesicle, which at first remains attached to the 

 overlying ectoderm. 



The invagination of the optic vesicle is a self-governed process. On the contrary, 

 contact of the optic vesicle with the overlying ectoderm stimulates the latter to lens forma- 

 tion, even in regions that normally never differentiate a lens (Lewis, 1907). It is possible, 

 however, for a lens to arise independently of this contact stimulus (Stockard, IQIO) 



