DEVELOPMENT OF THE ALIMENTARY SYSTEM. 919 



ye. 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 any central opening. As the pupil appears, it is 

 closed by a vascular membrane which probably belongs to the capsule of the crystal- 

 line lens called the pupillary membrane. This membrane gradually disappears by an 

 atrophy extending from the centre to the periphery. It attains its maximum of develop- 

 ment 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 tegumentary layer. At the tenth week, we observe 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 fcetal 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 forma- 

 tion 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 subsequently to 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 appear- 

 ance. 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 each 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 curious structure is known as the cartilage of Meckel. 



There are no special points for description in the development 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 later than the parts con- 

 nected with vision and audition. The vesicles themselves become filled with ganglionic 

 matter, and constitute the olfactory bulbs, their pedicles being the so-called olfactory 

 nerves, or commissures. The development of some of the parts of the central nervous 

 system is illustrated in Plates I. and II., facing page 920. 



As far as the functions of the nervous system of the foetus are concerned, it is probable 

 that they are restricted mainly to reflex phenomena depending upon the action of the 

 spinal cord, and that perception and volition hardly exist. It is probable that many reflex 

 movements take place in utero. When a foetus is removed from the uterus of an animal, 

 even during the early periods of pregnancy, movements of respiration occur, a fact which 

 we have often demonstrated to medical classes ; and it is well known that efforts of respi- 

 ration sometimes occur within the uterus. This we believe to be a reflex action excited 

 by the want of oxygen in the tissues, when the placental circulation is interrupted. "We 

 have already discussed these phenomena in connection with the subject of respiration. 



Development of the Alimentary System. 



The intestinal canal is the first formation of the alimentary system. As we have 

 already seen, this is at first open in the greatest part of its extent, presenting, at either 

 extremity of the longitudinal gutter, in front of the spinal column, a rounded, blind ex- 



