LABORATORY MANUAL OF PHYSIOLOGY. 



of these sensations, however, are commonly brought into the realm 

 of consciousness. The majority of such impulses, from the viscera, 

 for example, are either lost, through diffusion in the subsidiary 

 parts of the nervous system, or are transferred, as corresponding 

 efferent impulses, to complete the formation of reflex arcs. 



When such impulses become abnormally intense, so as to over- 

 come the resistance in the longer nerve pathways sufficiently to 

 reach the realm of cerebral consciousness, the subjective sensations 

 are either vague and indefinable, other than as a feeling of discom- 

 fort or pain somewhere in the region involved, or they are referred, 

 as pain, to some part of the cutaneous surface whose afferent nerve 

 distribution corresponds to the same cord segment as the efferent 

 nerve distribution of the visceral area involved. Such reference of 

 a sensory impression occurs, probably, for the reason that the sen- 

 sorium is in the habit of receiving impulses from the skin area and 

 not from the visceral area ; and where the same terminal neurons 

 transmit the impressions from the two sources, the sensation is 

 referred to the area from which the impulses more usually come. 



The nature of the conscious impression depends, not so much 

 upon the character of the stimulus applied, as upon the peripheral 

 area stimulated, the afferent nerve involved, and the brain area to 

 which the impulse goes. Thus, a stimulation of the optic nerve, 

 whether it be mechanical, electrical, or through the impact of light 

 waves upon the retina, causes a sensation of light; stimulation 

 of the olfactory nerves gives a sensation of smell; and of the taste 

 nerves, of taste. 



The cutaneous surface itself has been mapped out into areas 

 or spots which are irritable to stimuli of various kinds. Thus, 

 there are spots which respond to stimuli by a tactile sensation, 

 others which are irritable to heat, others to cold, and others to 

 stimuli which give a sensation of pain, independent of temperature 

 or tactile sensation. 



Quantitative Relation between Stimulus and Sensation. In order 

 that a stimulus may be effective in producing a sensation, its 

 intensity must exceed a certain minimum value. This minimum 



