422 INTRODUCTION TO CRANIATA [CH. 



or so-called secondary optic vesicle, the clear gelatinous connec- 

 tive tissue inside which is known as the vitreous humour. The 

 connective tissue surrounding the vesicle peripherally forms a tough 

 fibrous or even cartilaginous capsule called the sclerotic coat, 

 lined by a thin vascular tissue, the choroid coat. The sensitive 

 and nervous outer layer of the primary vesicle is known as the 

 retina, the other layer (which becomes loaded with pigment) as 

 the pigment epithelium of the retina. If we analyse the 

 structure of the retina, we find that it has fundamentally the 

 same structure as the central nervous system of which, as its 

 origin shows, it is really a part. Thus there are a number of 

 branched and cuticularised supporting cells called fibres of 

 Mil Her, extending throughout the whole thickness' of the retina, 

 and the main mass of the retina is made up of neurons. There 

 is, however, in addition a layer of characteristic visual cells; 

 that is to say, of sense-cells, with a comparatively thick striated 

 rod in place of the ordinary sense-hair. Visual rods have already 

 been described in the eyes of Anthomedusae (p. 64) and of 

 Arthropoda (p. 197); they occur wherever the capacity for vision 

 is developed. In the retina of Craniata there are two varieties of 

 visual cell, called respectively rod-cells and cone-cells. In the 

 first, the visual rod is narrow and cylindrical, and the body of the cell 

 beneath is filamentous with a rounded swelling for the nucleus ; the 

 basal process ends in an unbranched knob, that is to say, in a single 

 dendrite. In the cone- cell the rod is conical with a broad base, to 

 which the body of the cell containing the nucleus is immediately 

 applied ; the basal process ends in the normal manner in a tuft of 

 dendrites. The basal processes of both kinds of sense-cell are in 

 close relation to the receptive dendrites of a layer of neurons with 

 small cell bodies; the axis-cylinder processes of these in turn end 

 close to the receptive dendrites of a layer of neurons with large 

 cell bodies situated close to the outer basal surface of the retina, 

 which give rise to the fibres constituting the optic nerve. Taking 

 a general view therefore we may say that the retina is a sensory 

 nervous epithelium consisting of a layer of sense-cells underlaid by 

 two layers of neurons. Before its structure was thoroughly under- 

 stood, however, the appearance of the retina in transverse section 

 was a bewildering mass of fibres and nuclei, in" which for descriptive 

 purposes different layers were distinguished. These, reckoning 

 them in the order proceeding from the inner side of the eyeball 

 towards the lens, were as follows : (a) the layer of rods and 



