THE ORGANS OF SPECIAL SENSE 269 



is considered a passive function; for example, when any 

 part of the system comes into contact with another body, 

 a sensation of its presence is given without the exercise of 

 volition. On the other hand, touch is an active sense, 

 and is exercised voluntarily, for the purpose of conveying 

 to the mind a knowledge of the qualities or properties of 

 the surfaces of bodies; as, for example, when we feel a 

 piece of cloth to ascertain its quality, or a polished surface 

 to prove its smoothness. 



In man the hand is admirably adapted for the exercise 

 of the sense of touch. The fineness of the skin, its great 

 sensibility, the cushion-like pad formed by the subcutane- 

 ous fat at the extremities of the fingers, the length and 

 flexibility of these members, and the power we possess of 

 opposing the thumb to the fingers, thus forming, as it 

 were, a pair of forceps, are properties that are essential 

 to the delicacy of touch, and that enable us to appreciate 

 with exactitude the qualities of the bodies we may feel. 



REVIEW QUESTIONS 



What constitute the organs of sight? 



How many coats has the eye? Name them. 



What is the conjunctiva? 



What is the sclerotic coat? 



Give a description of the cornea. 



What is the choroid coat? 



What is the iris? 



Give an illustration of the ciliary processes. 



What and where is the retina? 



What nerve pierces the eyeball posteriorly? 



Name the different humors of the eye. 



What is their function? 



What are the appendages of the eye? 



What are the functions of the eyebrows? 



What is the function of the eyelids? 



What do you understand by canthus? 



What denotes the entrance to the lacrimal canal? 



What is the function of the tarsal cartilages? 



