DEVELOPMENT OF THE NERVOUS SYSTEM. 403 



have been observed in which there was congenital absence of 

 the auditory nerves, the parts of the internal ear being per- 

 fect. Soon after the formation of the auditory vesicle, how- 

 ever, it communicates with the third primitive cerebral vesi- 

 cle, the filament of communication being developed into the 

 auditory nerve. 



The auditory vesicle, which appears subsequently to the 

 organ of vision, is eventually developed into the vestibule. 

 The next formations are the arcnes, or diverticula, which con- 

 stitute the semicircular canals. According to Meckel, the 

 membranous labyrinth appears long before the osseous laby- 

 rinth ; and he has found it perfectly developed at three 

 months. 1 The bones of the middle ear, which have no con- 

 nection, in their development, with the nervous system, but 

 which it is convenient to mention here, are remarkable for 

 their early appearance. According to Meckel, who has de- 

 scribed their development very accurately, they appear at 

 the beginning of the third month, and are as large in the 

 foatus at term as in the adult. A remarkable anatomical 

 point with relation to these structures is the existence of 

 a cartilage, attached to the malleus on each side and ex- 

 tending from this bone along the inner surface of the lower 

 jaw, the two cartilages meeting and uniting in the median 

 line to form a single cord. " This cartilage now ossifies, al- 

 though, in the commencement, it forms most of the mass of 

 the bone ; it disappears at the eighth month." a This curious 

 structure has been very elaborately described by Robin and 

 Magitot, and is known as the cartilage of Meckel. 3 



There are ho special points for description in the develop- 

 ment of the olfactory lobes, which is very simple. These 

 are offshoots from the first cerebral vesicle, appearing at the 

 inferior and anterior part of the cerebral hemispheres, a little 



1 MECKEL, Manual of General, Descriptive, and Pathological Anatomy, Phila- 

 delphia, 1832, vol. iii., p. 137. 



2 MECKEL, loc. cit. 



8 ROBIN ET MAGITOT, Memoire sur la genese et le developpement des follicules 

 dentaires. Journal de la physiologic, Paris, 1860, tome iii., p. 15, et seq'. 



