S i6 



PRINCIPLES OF GENERAL PHYSIOLOGY 



The sea anemone appears to possess photo-receptors of this simple kind (see von 

 Uexkiill, 1909, p. 71), and according to Parker (1903 and 1905) the power is 

 also present in the skin of fresh-water fish, such as the Ammom-tr, and in 

 Numerous amphibia (see also the monograph by Nagel, 1896). Parker does not 

 think that the elaborate eye of the vertebrate has been formed from this primitive 

 sensibility of the skin to light. The receptor mechanism of the vertebrate eye 

 is, embryologically, an outgrowth from the central nervous system, so that it 

 seems more probable that the photo-receptors concerned may have been formed 

 in the central nervous system itself of a transparent animal. 



However this may be, it is obvious that a mere sensibility to light and shade 

 is of comparatively little value, until a mechanism is developed by which images 

 of external objects are formed on a sensitive surface composed of a multitude of 



elements connected with separate nerve 

 fibres. By this means a picture is, so to 

 speak, conveyed to the brain. 



A fully developed eye consists, then, of 

 some dioptric mechanism, corresponding to 

 the lens of a photographic camera, together 

 with a layer of a photo-chemically active 

 substance, like the silver bromide of the 

 plate. In the eye, we have also endings 

 of a large number of nerve fibres, attached 

 to complex receptors, which serve to pro- 

 duce nerve impulses from the photo-chemical 

 changes. This part is called a retina. It 

 is also necessary that stray light should be 

 kept out by an arrangement like that of 

 the camera bellows and the dark slide. This 

 is done by pigment cells, which absorb the 

 light. Fig. 160 shows a simple eye. The 

 complex structure of the retina of the 

 vertebrate may be seen in Fig. 161 (from 

 the monograph by Ramon y Cajal, 1894). 

 It is to be remembered that the peripheral 

 receptor mechanism proper consists of the 

 rod and cone, and perhaps of the pigment, 

 layer ; the other layers, as will be seen from 

 the figure, consist of neurones, interposed 

 between the actual receptor neurones and 

 the nerve centres. The retina thus con- 

 sists in great part of nerve centres, owing 

 to its mode of development. In the eye 

 of the Cephalopod, which is a highly differ- 

 entiated one, similar to that of the verte- 

 brate, these intermediate . neurones form a 

 distinct ganglionic mass, outside the eye 

 itself. In these organisms, also, the light impinges directly on the receptors, 

 the retina not being inverted, as in the vertebrate, where the incident light passes 

 through the nerve layers before reaching the rods and cones. 



Arrangements of the simple kind described above are found in some of the 

 very primitive organisms, as may be seen from Fig. 160, but it is doubtful 

 whether such simple dioptric mechanisms can do more than serve to concentrate 

 the light on the sensitive cells. In Pecten, we have a number of eyes of an 

 elaborate nature, shown in Fig. 162, from Dakin's monograph (1909). There 

 are here a number of separate nerve fibres, and, in consequence, the possibility 

 of an appreciation of something approaching an image. It is interesting to note 

 that the arrangement noted above in the vertebrate retina, namely, the passage 

 of the light through the nerve layer before reaching the sensitive substance, is 

 also met with in Pecten. The fact suggests that there may be some reason for 



Fn;. 160. OCKLLUS OF LIZZIA 

 KM K i.i. IK KI; i. Seen from the side. 

 After treatment with dilute osmic 

 acid. Obj. F., Oc. 2. 



I, Lens. (After Hertwig.) 



oc, Receptive inechaniMii. 



