20 



PRACTICAL PHYSIOLOGY 



is present all the lines will not be seen with equal distinctness at 

 one time. 

 (Read the Physiology of the Dioptric Mechanism in a Text Book.) 



II. Action of the Retina and Brain 



1. Are Visual Sensations Produced by Light only ? 



Press upon the eye-ball far back, and note the effect of such 

 mechanical pressure on the retina. Formulate your conclusion. 



This is one of the most important experiments in Physiology, for 

 from it may be deduced the conclusion that the stimulation of a 

 receptor by any kind of change produces the same kind of sensation, 

 and the converse that the same kind of stimulation of different kinds 

 of receptors will produce different and specific changes of sensation. 



This is the law of Specific 

 Nerve Energy. 



2. Is the whole Retina Stimu- 

 lated by Light ? 



METHODS. (i) Mariotte's 

 Experiment. Make two 

 marks horizontally about 4 

 inches apart upon a piece 

 of plain paper. 



With the head about 18 

 inches from the paper and 

 with the left eye closed, fix 

 the right eye on the left- 

 hand mark. 



Are both marks visible ? 

 Does any change take place 

 as the paper is gradually 



brought towards the face ? 

 FIG. 12. To show the demonstration of 

 the Blind Spot by method 2 (i). 

 (From Noel Paton's Essentials.) 



Make a diagram of the results of 

 the experiment. 



(ii) Make a mark on the left side 

 of a piece of plain paper. Holding 

 the head firmly fixed at about a foot 

 from the paper and closing the left 

 eye, keep the right eye fixed upon the 

 mark, and move the point of a pencil, 

 held nearly horizontally, slowly to- 

 wards the right side of the paper. 

 Note any change in the appearance of 

 the point of the pencil you may observe. 



Make a diagram of the experiment and draw your conclusions. 



(iii) Map out the blind spot by moving the point of the pencil 



method 2 (ii). 

 (From Noel Paton's Essentials.) 



