DEVELOPMENT OF THE NERVOUS SYSTEM. 821 



about the fourth month there is a deposition of nervous matter in front and 

 above, forming the pons Varolii. 



In Fig. 304 (C, 0), it is seen that the vesicles for the organs of vision 

 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, grad- 

 ually approaching the anterior portion. At the extremity 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 de- 

 veloped at about the seventh week, and is at first a simple membrane, with- 

 out any 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 max- 

 imum of development at the sixth month and disappears at the seventh 

 month. The vitreous humor is formed of the fluid 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 together 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 becomes developed into the internal 

 ear is formed independently; at least cases have been observed in which 

 there was congenital absence of the auditory nerves, the parts of the internal 

 ear being perfect. Soon after the formation of the auditory vesicle, 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 with relation 

 to these structures is the existence of a cartilage, attached to the malleus on 

 either side and extending 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 

 forms most of the mass of the bone ; it disappears at the eighth month " 

 (Meckel). This structure is known as the cartilage of Meckel. 



There are no special points for description in the development of the 



