THE FROG 



69 



THE SENSE ORGANS 



If one marks a series of spaces on the volar ( ) 



surface of the fore arm of a human being about a millimeter square, and 

 such person is then blindfolded, it will be found that when a cold needle 

 touches certain squares he will feel a sensation of cold, whereas if it 

 touches certain other squares, he will feel a sensation of heat. From 

 this experiment it is learned that a great many, if not all, nerves have a 

 very special and definite work to perform. 



Where a great mass of such specialized nerve endings is grouped in 

 one place, it produces an organ of special sense such as the eye, the ear, 

 the nose, the tongue. All of these organs are groups of nerves whose 

 endings are on the surface of some part of the body, and carry sensations 

 inward to the central nervous system. These are called sensory nerves. 



The nerves which begin in the central nervous system and go out- 

 ward to some of the muscles, producing various movements of those 

 muscles, are called motor nerves. 



Both sensory and motor cells may unite in a ganglion and have 

 both types of fibers run in the same sheath from there on; these are 

 called mixed nerves. 



m.n. p. sup. d.n.l. gl.n.t. 

 I'lr 



B 



Fig. 18. The Eye. 



A. Eye in position. d.n.l., lachrimal duct leading from eye to interior of 

 nose ; gl.n.L, lachrimal gland ; m.n., nictitating membrane ; no,., nares ; p.i., lower 

 eye-lid; p.sup., upper eye-lid. (After Schimkewitsch.) 



B. Diagrammatic section through the optical axis of the eye of the frog. 



C. Diagrammatic horizontal section of the eye of man. (After Guyer.) 



THE EYE 



Probably the most important special sense organ is the eye (Fig. 

 18). Practically only one type of sensation is carried by the nerves of 

 this special sense organ, and that is light perception. The eye of the 

 frog is a large spherical organ similar to the eye of all of the higher 

 animals. The walls of the organ are opaque, with the exception of a 

 transparent portion directly in the foreground occupying about one- 

 third of the eye ball and called the cornea ( ). 



