DEVELOPMENT OF THE NERVOUS SYSTEM 825 



appear very early, as lateral offshoots of the anterior cerebral vesicle. 

 These gradually increase in size and advance anteriorly as development 

 of the other parts progresses. The eyes are situated at first at the sides 

 of the head, gradually approaching the anterior portion. At the ex- 

 tremity of each of these lateral prolongations, a rounded mass appears, 

 which becomes the globe of the eye. The superficial portions of the 

 globe are developed into the sclerotic and the cornea, which seem to 

 be formed of a process from the dura mater. The pedicle attached to 

 the globe becomes the optic nerve. The iris is developed at about 

 the seventh week and is at first a simple membrane without a central 

 opening. As the pupil appears, it is closed by a vascular membrane 

 which probably belongs to the capsule of the crystalline lens called 

 the pupillary membrane. This membrane gradually disappears by an 

 atrophy extending from the centre to the periphery. It attains its 

 maximum of development at the sixth month and disappears at the 

 seventh month. The vitreous humor is formed of the liquid contents 

 of the optic vesicle. The crystalline lens is regarded as a product of 

 the epiblast. At the tenth week there is the beginning of the formation 

 of the eyelids. These meet at about the fourth month and adhere to- 

 gether by their edges. In many mammals the eyelids remain closed 

 for a few days after birth ; but they become separated in the human 

 subject in the later periods of foetal life. 



It is probable that the vesicle which is developed into the internal 

 ear is formed independently ; and cases have been observed in which 

 there was congenital absence of the auditory nerves, the parts of the in- 

 ternal ear being perfect. Soon after the formation of the auditory vesi- 

 cle, however, it communicates with the third primitive cerebral vesicle, 

 the filament of communication being developed into the auditory nerve. 



The auditory vesicle, which appears later than the organ of vision, 

 is eventually developed into the vestibule. The next formations are 

 the arches, or diverticula, which constitute the semicircular canals. 

 The membranous labyrinth appears long before the osseous labyrinth ; 

 and it has been found perfectly developed at three months. The bones 

 of the middle ear, which have no connection, in their development, with 

 the nervous system, but which it is convenient to mention here, are 

 remarkable for their early appearance. They appear at the beginning 

 of the third month and are as large in the foetus at term as in the adult. 

 A remarkable anatomical point in relation to these structures is the 

 existence of a cartilage attached to the malleus on either 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, although, in the commencement, it 



