THE ORGANS OF THE SENSES. 



THE cells directly receiving the stimuli producing the sensory impres- 

 sions of touch, smell, taste, sight and hearing are all derivations of the ecto- 

 derm the great primary sensory layer from which the essential parts of the 

 organs of special sense are differentiations. The olfactory cells nervous 

 elements that correspond to ganglion-cells retain their primary relation, 

 since they remain embedded within the invaginated peripheral epithelium 

 lining the nasal fossae, sending their dendrites towards the free surface and 

 their axones into the brain. Usually, however, the nerve-cells connected 

 with the special sense-organs abandon their superficial position and lie at 

 some distance from the periphery, receiving the stimuli not directly, but 

 from the epithelial receptors by way of their dendrites. In the case of the 

 most highly specialized sense-organs, the eye and the ear, the percipient 

 cells lie enclosed within capsules of mesodermic origin, the stimuli reaching 

 them by way of an elaborate path of conduction. 



THE SKIN. 



Since the extensive integumentary sheet that clothes the exterior of the 

 entire body not only serves as a protective investment, an efficient regulator 

 of body temperature and an important excretory structure, but also contains 



Epidermis 



Papillary stratum 



Reticular stratum 



Hair follicle 



Retinaculum 



Fat 



FIG. 360. Section of skin, showing its chief layers epidermis, corium and tela subcutanea. X 17. 



the special end-organs and the peripheral terminations of the sensory nerves 

 that receive and convey the stimuli producing tactile impressions, the skin 

 may be appropriately considered along with the other sense-organs, of which 

 it may be regarded as the primary and least specialized. On the other hand, 



