no HUMAN BIOLOGY 



accurate but for skin sensations it is commonly so. Deeper 

 sensations are less accurately referred. It is not always easy 

 to tell exactly which tooth aches and deeper pains, as the 

 physician well knows, are systematically mislocated. Never- 

 theless the reference is more or less trustworthy and always 

 to some spot either in the body or more commonly on it. 

 Since most of our daily sensations originating through 

 the cord are skin sensations this sensory reference is usually 

 to organs on the surface such as those of touch and temper- 

 ature. These sense organs may therefore be called surface 

 receptors. All the skin sense organs connected with the cord 

 belong to this class and represent a primitive and very 

 ancient type of mechanism. 



In strong contrast with the spinal cord and its surface- 

 receptors is the brain with its sensory equipment. The brain 

 through its own nerves possesses a full outfit of surface 

 receptors which are located in the skin of the face as the 

 cord receptors are in that of the trunk. But in addition to 

 these surface receptors the brain also has three pairs of 

 special receptors of its own, the nasal cavities, the eyes, 

 and the ears. In all these the sensory activities are usually 

 referred not to the bodily locat on of the organ itself but 

 to some distant point outside the body and commonly far 

 away. The odor of the morning coffee is not referred to the 

 nose where the stimulation occurs but to the coffee percolator 

 across the table. Similarly the form of an approaching 

 friend is not seen in the eye where the image is but far away 

 down the street, and the overture played by the orchestra 

 is not heard in the ear but as coming from the distant band 

 of musicians. All these sense organs differ from the surface 

 organs in that the sensations called forth by them are 

 referred to distant points far beyond the body. They are 

 therefore called distance receptors and in most animals 

 they are, as in man and other vertebrates, peculiar to the 

 head. They are undoubtedly the most important single 

 factor in the evolution of the vertebrate brain for without 

 them we would have remained simply spinal-cord animals. 



The three distance receptors in the vertebrates have 

 without question arisen separately and at quite different 

 times. They are modified surface receptors that have evolved 



