THE EXTEKNAL EAE. 827 



The spongioblasts undergo ramification and form a network or myelospongium, and also give 

 rise to the inner and outer limiting membranes ; the latter is next the original cavity of the optic 

 vesicle, and therefore corresponds to the inner limiting membrane of the spinal medulla. The 

 spongioblasts also form the groundwork of the inner and outer molecular layers into which the 

 processes of the neuroblasts grow and arborise. 



The germinal cells are always situated beneath the external limiting membrane, and by their 

 division give rise to the neuroblasts. The first-formed neuroblasts are larger than those of 

 succeeding generations, and are found in the site of the future ganglionic layer. The germinal 

 cells in the middle of the convexity of the retinal cup cease to divide at an early stage of 

 development, and become directly transformed into the rod and cone cells from which the rods 

 and cones develop as processes ; hence these structures appear first over the middle of the 

 convexity of the retina, and gradually extend towards the margin of the retinal cup. 



The molecular layers make their appearance as plexuses of myelospongium. The internal 

 molecular layer is first developed at the centre of the retinal cup, and gradually extends towards 

 the cup margin, and into it the processes from the nuclei on either side grow and ramify. The 

 rod and cone fibres, and the outer processes of the internal nuclear layer, grow into and arborise 

 within the external molecular layer. 



The condensed mesoderm surrounding the optic cup becomes the sclera and chorioid. 

 In the portion of the mesoderm which lies in front of the lens a cleft-like fissure appears, 

 and divides it into a thick anterior and a thin posterior layer. The former becomes the 

 substantia propria of the cornea ; the latter, the stroma of the iris and anterior part of the 

 vascular tunic of the lens. The fissure represents the future camera oculi anterior, 

 and its lining cells form the endothelium of this chamber. 



The eyelids arise as two integumentary folds above and below the cornea, each being 

 covered on both its surfaces by the ectoderm. By the third month the folds meet and 

 unite with each other at their edges, the eyelids being only permanently opened shortly 

 before birth; in many animals they are not opened until after birth. The ectoderm 

 forms the epithelium of the conjunctiva and the epithelium corneee. It is also invaginated 

 at the lid margins to form the hair follicles and the lining cells of the tarsal glands and 

 glands of Moll, and, at the fornix conjunctivae superior, to form the lining of the alveoli 

 and ducts of the lacrimal gland. 



The naso-lacrimal duct, lacrimal sac, and ducts represent the remains of the furrow 

 between the maxillary and lateral nasal processes (p. 49). It is at first filled with a 

 solid rod of cells, which becomes hollowed out to form the sac and ducts. 



ORGANON AUDITUS. 



The ear or auditory organ (Fig. 703) consists of three portions external, 

 middle, and internal the last constituting its essential part, since the peripheral 

 terminations of the acoustic nerve are distributed within it. 



EXTEKNAL EAE. 



The external ear l includes (a) the auricula, attached to and projecting from 

 the side of the head ; and (6) the external acoustic meatus leading inwards from 

 the most depressed part of the auricula to the tympanic membrane. 



AURICULA. 



The auricle (O.T. pinna) (Fig. 704) presents two surfaces, lateral and medial, 

 the latter forming an angle (cephalo-auricular angle) of about 30 with the side of 

 the head. The lateral surface is irregularly concave, but presents several well- 

 marked elevations and depressions. The deepest of the depressions is situated near 

 its middle, and is named the concha auriculae. It is divided by an almost transverse 

 ridge, the cms helicis, into an upper, smaller, and a lower, larger portion : the 

 former is termed the cymba conchse ; the latter, which leads into the meatus, 

 the cavum conchas. Anteriorly, the crus helicis is continuous with the margin of 

 the auricula or helix, which is incurved in the greater part of its extent, and is 

 directed at first upwards, and then backwards and downwards, to become gradually 



1 Although it is usual to speak of the external, middle, aud internal ear, it would be more correct to use 

 the terms external, middle, and internal portions of the ear. 



