492 THE HUMAN BODY. 



which thus separates one sensation from another is its 

 modality. Sensations of the same modality may differ; but 

 they shade imperceptibly into one another, and are com- 

 parable between themselves in two ways. First, as regards 

 quality: while a high and a low pitched note are both 

 auditory sensations, they are nevertheless different and yet 

 intelligibly comparable; and so are blue, purple, and red ob- 

 jects. In the second place, sensations of the same modality are 

 distinguishable and comparable as to amount or intensity: we 

 readily recognize and compare a loud and a weak sound of 

 the same pitch; a bright and feeble light of the same color; 

 an acute and a slight pain of the same general character. 

 Our sensations thus differ in the three aspects of modality, 

 quality within the same modality, and intensity. Certain 

 sensations also differ in what is known as the " local signs" 

 a difference by which we tell a touch on one part of the skin 

 from a similar touch on another; or an object exciting one 

 part of the eye from an object like it, but in a different loca- 

 tion in space and exciting another part of the visual surface. 

 As regards modality, we commonly distinguish five senses, 

 those of sight, sound, touch, taste, and smell; to these, tem- 

 perature must be added. The varieties of common sensation 

 are also several; for example, pain, hunger, satiety, thirst, 

 nausea, malaise, Men etre (" feeling good "), fatigue. The 

 muscular sense stands on the intermediate line between 

 special and common sensations; we gather by it how much 

 our various muscles are contracted: and so learn the position 

 of various parts of the Body, on the one hand, and the re- 

 sistance opposed to bodily movement by external objects, on 

 the other. In fact, we cannot draw a sharp line between the 

 special senses and common sensations : all the Body, we con- 

 clude from observations on the lower animals, is, at an early 

 stage of its development, sensitive; very soon its cells sepa- 

 rate themselves into an outer layer exposed to the action of 

 external forces and an inner layer protected from them : and 

 some of the former cells become especially sensitive. From 

 them, as development proceeds, some are separated and 

 buried beneath the surface to become the brain and spinal 

 cord ; of those which remain superficial, some are modified 

 so that they (in the eye) become especially excited by ethereal 

 vibrations; others (in the ear) become especially responsive 

 to sound vibrations; others to slight chemical changes (in 



