90 ANATOMY OF THE RABBIT 



Receptors, or sense organs, are classified as exteroceptors, 

 which receive stimuli from outside the organism, proprioceptors, 

 which are excited by events in the organism itself, such as con- 

 traction of muscles or movements of joints, and interoceptors, 

 which are situated in visceral organs such as those of digestion, 

 respiration, etc. 



The simplest receptors, structurally, appear as free nerve end- 

 ings in epithelium. These probably transmit impulses which are 

 interpreted as pain. Several more specialized types of receptors, 

 also of microscopic size, are stimulated by touch, pressure, or 

 movement of various parts and others by heat, cold, or chemical 

 irritation. A slightly enlarged portion of the internal carotid artery 

 at the very beginning of that vessel, the carotid sinus, contains in 

 its wall receptors for changes in arterial pressure and a minute 

 organ between the bases of internal and external carotid arteries, 

 the carotid body or glomus caroticum, is a receptor for chemical 

 changes in the blood, both of these giving rise to reflex effects on 

 blood-pressure and on breathing. 



Of all receptors in the mammalian body, the most primitive, 

 as regards structure are the olfactory cells. These are neuro- 

 epithelial cells imbedded among the other elements of the nasal 

 mucous membrane. Each has a free, ciliate outer end and gives rise 

 at its inner end to an unmyelinated nerve fibre which runs through 

 the olfactory nerve to terminate in the olfactory bulb. 



The gustatory organs, or taste buds, are minute spindle-shaped 

 groups of differentiated cells imbedded at certain regions in the 

 stratified epithelial lining of the oral cavity. Sensory and support- 

 ing cells are distinguishable and round the former are the terminal 

 ramifications of gustatory nerve fibres. 



The Ear 

 The receptor for sound and for equilibratory stimuli result- 

 ing from movement or altered position of the head is the inter- 

 nal ear, which comprises two distinct though connected parts 

 serving these respective functions. The actual sensory areas occur 

 in the walls of a system of delicate canals, the membranous 

 labyrinth, which are contained within corresponding bony canals 

 imbedded in the petrosal bone. The equilibratory or vestibular por- 



