426 INNERVATION. [CHAP. XIV. 



The ceruminous (/lands of the ear resemble in their structure those 

 just described. They exist in great abundance in the skin of the 

 cartilaginous part of the external meatus, and provide an adhesive 

 secretion calculated to entangle particles of dust and small insects, 

 and to prevent their access to the delicate membrane of the tym- 

 panum.* 



Of the Functions of the Skin. Having now considered the several 

 constituents of that very complicated organ, the skin, it remains 

 for us to take a brief general view of its functions before proceed- 

 ing to a particular account of that one which brought us to this 

 structure, viz. the sense of touch. All these functions have refer- 

 ence to its external anatomical position with respect to the other 

 structures of the body. Regarded as a protective covering, the 

 skin possesses the united advantages of toughness, resistance, flexi- 

 bility, and elasticity. The areolar framework of the cutis is the 

 part chiefly conferring these properties, which are due also in some 

 measure to the epidermis. Both these structures are developed in 

 a degree proportioned to the force and frequency of external contact 

 to which different regions of the body are liable. They are thickest 

 on the palms and soles, on the back of the trunk, and the outer 

 surface of the limbs : thinner on the front of the body, and on the 

 inside of the limbs. 



These two elements also afford protection and support to the 

 other more delicate ones with which they are associated. The areola3 

 of the cutis sustain the intricate networks of blood-vessels, lympha- 

 tics, and nerves, which traverse it. The sweat-glands are imbedded 

 in cavities accurately fitted to receive them ; and their ducts, with 

 the sebaceous follicles and hairs, are all lodged in channels or spaces 

 adapted to their respective sizes. The epidermis is a defensive invest- 

 ment to the tactile organ, and, while it shields it from the injurious 

 effects of pressure, is the medium through which impressions of con- 

 tact are conveyed to it with admirable nicety and truth. The epi- 

 dermis furnishes also special organs, such as nails and hairs; which 

 are developed in particular situations, for the purposes of defence, 

 the preservation of warmth, or as aids to the sense of touch. The 

 infinite variety of modifications which the epidermis presents among 

 the lower animals, joined with others of nearly equal diversity in the 

 neighbouring textures, adapts it to very numerous and even opposite 

 uses in the animal kingdom. 



* In the sharks and rays there is a remarkable system of mucous tubes 

 opening on the skin. These tubes are nearly as large as crow-quills, and of 

 great length. They end by a blind extremity, to which a small nerve of th<- 

 fifth pair iy attached. 



