VISUAL RECEPTORS AS BIOLOGICAL TRANSDUCERS 37 



Fig. 2. Simultaneous recording of potentials recorded from a micropipet te elec- 

 trode inserted into a Limulus photoreceptor cell (upper trace) and from the attached 

 nerve bundle (lower trace). The black mark below the lower trace indicates the dura- 

 tion of the stimulating illumination (.02 sec). Time marks indicate fifths of a second. 

 Upward deflection of the top trace indicates positivity of the micropipette. (From 

 Hartline et al, 1952). 



impulses to pass from the receptor along the nerve to the recording electrodes, 

 it is evident that the responses are synchronous. Before discussing the electrical 

 properties of the eye further, let us take a brief look at the histology since it is 

 important in interpreting the results. 



The lateral eyes of Limulus are of the compound variety like those of flies 

 but many times larger. They contain less than 1000 sensory elements or om- 

 matidia which is far less than the number in the fly's eye. The sensory elements 

 are also very much larger. Fig. 3 is a photograph of a stained section through 

 one of the eyes. 5 The original was colored with Mallory's stain. The light colored 

 carrot shaped objects are transparent in life and are made of a crystalline 

 material which acts as the optical system of the eye and conducts the light into 

 the bundles of cells at the base which are the retinulae, or receptor units. Com- 

 ing from these are nerve fibers which converge to form the optic nerve. 



A schematic section of an ommatidium is shown in Fig. 4. 



There are a number of large elongated cells known as the retinula cells, shaped 

 and grouped like the segments of an orange. Through the center of this struc- 

 ture runs a central canal containing the distal process of a large spherical cell 

 known as the eccentric cell. The eccentric cell has a large axon but the retinula 

 cells also give rise to smaller axons. Fig. 5 is a drawing of a transverse section 

 through an ommatidium showing the grouping of the retinula cells about the 

 central canal and the distal process of the eccentric cell passing through it. 



In our preparation the excised eye was mounted in a stainless steel clip and 

 sectioned perpendicular to the surface of the cornea with a razor blade. The 

 upper portion of the eye was discarded and the lower portion held in the clip 

 transferred to a chamber filled with physiological solution. The appearance of 

 the cut surface of the eye, as viewed through a dissecting microscope, is very 



5 Histological photographs courtesy of William Miller, Rockefeller Institute. 



