384 



THE PERIPHERAL NERVOUS SYSTEM 



and near the external angle of the eye. The anlages are at first knob-like, but 

 rapidly lengthen into sohd epithehal cords. They begin to branch in 30 mm. 

 embryos. At stages between 50 and 60 mm. (C R) additional anlages appear 

 which also branch. 



In 38 mm. (C R) embryos a septum begins to partition the gland into orbital a,nd. pal- 

 pebral portions. This septum is complete at 60 mm. (C R), the five or six anlages, first de- 

 veloped constituting the peripheral orbital part. Lumina appear in the glandular ^ords in 

 fetuses of 50 mm. (C R) by the degeneration of the central cells. -Accessory lacrimal glands 

 appear in 300 mm. (C R) fetuses. The lacrimal gland is not fully differentiated at biirth, being 

 only one-third the size of the adult gland. In old age marked degeneration occurs. 



The N aso-lacrimal Duct arises in 12 mm. embryos as a ridge-like thickening 

 of the epithelial Kning of the naso-lacrimal groove (Fig. 149), 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 sohd cord, sinks into the underlying meso- 

 derm (Schaeffer). Secondary sprouts growing out from this cord to the eyehds 

 form the lacrimal canals. A lumen, completed at birth, appears during the third 

 month (Fig. 372). 



V. The Development of 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. 



Hind-brain 

 Auditory ganglion Auditory placode 



Optic vesicle 



A B 



Fig. 382. — Two stages in the early development of the internal ear (after Keibel and Elze). A, 

 Horizontal section through the head and open neural tube of a 2 mm. human embryo showing the 

 auditory placode and ganglion (X 27); B, section through the hind-brain and otic vesicles of a 4 mm. 

 human embryo (X 33). 



The end organ proper is the inner ear with the auditory apparatus residing in the 

 cochlear duct. Besides this acoustic function the labyrinthine portion of the inner 

 ear acts as an organ of equilibration. 



