376 THE PERIPHERAL NERVOUS SYSTEM 



The Conchae are structures which are poorly developed in man. They ap- 

 pear on the lateral and median walls of the primitive nasal fossae. The inferior 

 concha, or maxillo-turbinal, is developed first in human embryos (Figs. 371 and 

 372). It forms a ridge along the caudal two-thirds of the lateral wall and is 

 marked off by a ventral groove which becomes the inferior nasal meatus (Fig. 373). 

 The naso-turbinal is very rudimentary and appears as a shght elevation dorsal 

 and cranial to the inferior concha (Fig. 373). Dorsal to the inferior concha 

 arise five ethmo-iurbinals , 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). Acces- 

 sory conchas are also developed (KiUian). 



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

 II, and the superior concha from III and IV. 



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 I and II, Fig. 373) gives rise to the frontal sinus. During the third month the maxil- 

 lary stmts 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. 



The cells of the olfactory epithelium become ciHated, 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). 



rv. The Development of 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 connected 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 invagina- 

 tion also extends ventrally along the optic stalk and produces 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 former thickens, as seen in Fig. 374 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 remains at first attached to the overljdng ectoderm. In an 



