470 SPECIAL PHYSIOLOGY. 



pressions, these being either annular, star-like, or square. But, ac- 

 cording to Serre, the shape of these phosphenes is determined by that 

 of the body which exercises the pressure, and by the extent to which 

 the retina is acted upon by it. When the pressure is communicated by 

 the finger, the spot is either roundish, or when the pressure is strong 

 and widely spread, it is annular, forming a ring with a dark centre. 

 When the pressure is made by a circular, triangular, or square object, 

 the luminous figure is circular, triangular, or square, provided that the 

 part compressed is entirely over the retina. But if the pressure be 

 applied over the anterior margin of the retina, it gives rise to an in- 

 complete figure, which becomes more so, the smaller the portion of the 

 retina covered by the compressing body. A second spectrum is often 

 perceived, in a direction corresponding with the point of pressure. 

 This is produced by a change in the form of the coats of the eyeball, 

 on the side opposite to the seat of the primary pressure; it is always 

 a complete figure, because the retina itself is there continuous ; such 

 counter pressure acts on some portion of the deeper part of the eye- 

 ball. Lines connecting the primary and secondary phosphenes, or the 

 points of the retinal surface by the stimulation of which these are pro- 

 duced, traverse a common centre of direction, which lies in the middle 

 of the crystalline lens, and may be said to correspond with the visual 

 centre of direction. 



Light is the proper homologous stimulus of the retina, but various 

 other, or heterologous stimuli, such as irritation, blows, or pressure 

 on the eyeball, or electrical shocks, excite luminous, and even colored 

 sensations or spectra of various kinds. Even the action of the orbic- 

 ular muscles of the eyelids, or the sudden movement of the eyes from 

 side to side, will excite luminous impressions. Visual sensations, like 

 all other sensations, may also be excited by internal stimuli. Thus, 

 when the eyes are closed, and the retina is at rest, the field of vision 

 is dark ; but, if the optic nerve, or sensorial centre, is in a state of 

 excitement, impressions of luminous rays, or of bright colors, are per- 

 ceived. During congestion of the brain, each arterial pulse is accom- 

 panied by an alteration in the degree of light perceived by the eyes, 

 owing to a pulsating illumination of the field of vision (Muller). Cer- 

 tain chemical agents, when absorbed into the circulation, also give rise 

 to the sensation of light, either by producing some alteration in the 

 condition of the retina, the effects of which are transmitted, through 

 the fibres of the optic nerves, to the brain, or perhaps by affecting the 

 nerve, or the sensorial centre itself. The action of opium, digitalis, 

 and belladonna, in producing spectra, is well known, and the vivid and 

 horrible spectral illusions seen in fever, and especially in delirium tre- 

 mens, have often been described. Again, the motion of the blood is 

 sometimes observed, especially after gazing at bright surfaces, such as 

 the sky, or after looking for a certain time at a white surface. In- 

 distinct movements in the luminous visual field, are then noticed. The 

 appearance of dark bodies in rapid and constant motion, seen in cases 

 of congestion of the brain, and also on suddenly rising from a stooping 

 posture, are dependent on the movement of the blood, exciting lumin- 

 ous impressions. As elsewhere stated, no instance of the occurrence 



