THE PERIPHERAL NERVOUS SYSTEM 



which, it will be remembered, extends from the inner angle of the eye to 

 the olfactory fossa. This thickening becomes cut off, and, as a solid cord, 

 sinks into the underlying mesoderm (Schaeffer) . Secondary sprouts, grow- 

 ing out from this cord to the eyelids, form the lacrimal canals. A lumen, 

 completed at birth, appears during the third month (Fig. 372). 



Anomalies. Lack of pigment in the retina and iris is usually associated with general 

 albinism. If the chorioid fissure fails' to close properly, there results a gaping, and hence 

 unpigmented, defect, or coloboma, in the iris, ciliary body, or chorioid. In cyclopia, a 

 single median eye replaces the usual paired conditon. All intergrades exist from closely 

 approximated, separate eyes to perfect unity. The mode of genesis, whether from the 

 fusion of separate eyes or from the inhibited separation of a common anlage into its 

 bilateral derivatives, is in dispute. In cases of cyclopia the nose is usually a cylindrical 

 proboscis, situated above the median eye. 



V. THE EAR 



The human ear consists of a sound-conducting apparatus and of 'a 

 receptive organ. The conveyance of sound is the function of the external 

 and middle ears. The end organ proper is the inner ear, with the auditory 



Hind-brain 



A caustic ganglia A ndilory placode 



Otic vesicle 



Optic vesicle 



FIG. 382. Two stages in the early development of the internal ear (after Keibel and Elze). 

 A, Horizontal section through the open neural tube of a 2 mm. human embryo, (X 27);'B 

 through the hind-brain of a 4 mm. human embryo (X 33). 



apparatus residing in the cochlear duct. Besides this acoustic function 

 the labyrinthine portion of the inner ear acts as an organ of equilibration. 

 The Inner Ear. The epithelium of the internal ear is derived from the 

 ectoderm. Its first anlage appears in embryos of 2 mm. as a thickened 

 ectodermal plate, the auditory placode (Fig. 382 A). These are developed, 

 dorsal to the second branchial grooves, at the sides of the hind-brain 

 opposite the fifth neuromeres (Fig. 383). The placodes are invaginated 



