318 MICROSCOPIC AXATOMY OF THE ORGANS. 



ment results. In this case at least four neurones form a part 

 of the whole path. The sensory stimulus passes through the 

 cell A upward in the white matter of the cord. The fibre a. 2 

 carries it to a neurone of the second order, (7, which transmits it 

 in turn to the cortex, where the fine branches come into contact 

 with the dendrites of a motor cell, D. The axone of this pyr- 

 amidal cell d passes down and crosses over in the pyramidal 

 tract, to come into relation with the dendrites of a motor cell 

 of the first order E in the ventral horn of the cord. From 

 this cell the motor impulse is carried out to the nerve-endings 

 in the voluntary muscle e. 



VIII. SENSE ORGANS. 



The sense organs are complex structures, each of which con- 

 sists riot only of the essential end apparatus of the sensory 

 nerve, but also of parts which support and aid this in its func- 

 tion. We distinguish five sense organs, namely : 



1. The tactile organ ; 



2. The organ of sight (visual) ; 



3. The organ of hearing (auditory) ; 



4. The organ of taste (gustatory) ; and 



5. The organ of smell (olfactory). 



The tactile sense is located in the skin, so that the latter 

 together with its various nerve-endings forms the tactile organ. 

 We shall, therefore, describe here the skin, which also acts as 

 a protective organ for the whole body. 



1. THE SKIN-THE TACTILE ORGAN. 



Here must be considered not only the outer skin (integu- 

 mentum commune), but also its appendages (the nails and the 

 hairs), and its glands (sebaceous and sweat glands). 



(a) The Outer Skin. 



The skin covers the whole outer surface of the human body. 

 It consists of two parts, a connective-tissue part (derma or cutis) 

 of mesodermal origin, and an epithelial part (epidermis) of 

 ^ctodermal origin (Fig. 241). The cutis may be divided into 



