THE ORGANS OF SENSE. 



THE cells directly receiving the stimuli producing the sensory impressions of 

 touch, smell, taste, sight and hearing are all derivations of the ectoblast 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 mesoblastic 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 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, the correspondence of its structure with that 

 of the mucous membranes, with which it is directly continuous at the orifices on the 

 exterior of the body, emphasizes the close relation of the skin to the alimentary and 

 other mucous tracts. 



This general investment, the tegmentum commune, includes the skin proper, 

 with the specialized tactile corpuscles, and its appendages the hairs, the nails and 

 the cutaneous glands. Its average superficial area is approximately one and a half 

 square meters. 



The skin (cutis), using the term in a more restricted sense as applied to the 

 covering proper without its appendages, everywhere consists of two distinct portions 

 a superficial epithelial and a deeper connective tissue stratum. The former, the epi- 

 dermis, is devoid of blood-vessels, the capillary loops of which never reach farther than 

 the subjacent corium, as the outermost layer of the connective tissue stratum is called. 



The thickness of the skin, from .5-4 mm., varies greatly in different parts of 

 the body, being least on the eyelids, penis and nymphae, and greatest on the palms 

 of the hands and soles of the feet and on the shoulders and back of the neck. In 

 general, with the exception of the hands and feet, the skin is thicker on the extensor 

 and dorsal surfaces than on the opposite aspects of the body. Of the entire thick- 

 ness, the proportion contributed by the epidermis is variable, but in most localities 

 it is about . i mm. Where exposed to unusual pressure, as on the palms of laborers 

 or on habitually unshod soles, the epidermis may attain a thickness of 4 mm. 



As seen during life, the color of the skin results from the blending of the in- 

 herent tint of the tissues with that of the blood within the superficial vessels. When 

 the latter are empty, as after death, the skin assumes the characteristic pallor and 

 ashen hue. Where the capillaries are numerous and the overlying strata thin, the 

 skin exhibits the pronounced rosy color of the lips, cheeks, ears and hands. Where, 

 on the contrary, the contents of fewer vessels shimmer through the epidermis, the 

 paler tint of the limbs and trunk is produced. 



In certain localities especially over the mammary areolae after pregnancy, the 

 axillae, the external genital organs and around the anus the skin presents a more or 

 less pronounced brownish color owing to the unusual quantity of pigment within the 



1381 



