THE SENSE ORGANS 



56s 



of an elongated process. Usually the outer termination of these cells 

 also is beset with one or more hair-like processes. Neurosensory cells 

 of this sort have a wide distribution in the animal kingdom. Those of 

 the olfactory epithelium of vertebrates are at this stage. 



A third stage is represented by sensory neurons which have lost 

 their connexion with the external surface, but retain "free" nerve termina- 

 tions in the epidermis. Such a cell is bipolar in form with both cutaneous 

 and central connexions. A neuron of this sort may also terminate 

 within the underlying corium, branch freely in the connective tissue, 



A. NEUROSENSORY CELLS 

 OF ANNELID. 



B. t^UROSENSORY CELLS 

 OF OLFACTORY MEM- 

 BRANE OF MAN. 



C. PRIMARY SENSORY CELL D. SECONDARY SENSORY 

 WITH FREE NERVE CELLS CONVEY IMPULSES 



TERMINATION. TO THE PRIMARY SENSORf CELL. 



Fig. 466. — Stages in the hypothetical evolution of secondary sense cells. A and B, 

 diagrams of neurosensory cells in invertebrate (.4) and vertebrate (B). C, a sensory 

 cell (neuron) with free nerve terminations. D, secondary sense cells convey impulses 

 to the primary sensory cell. The series assumes that the definitive receptor cells are 

 secondary. The possibility that the neurosensory cells become the definitive sense cells 

 and that the sensory nerve is secondary must be admitted. (Redrawn after Kahn's 

 "Das Leben Des Menschen," W. Keller & Co.) 



end between tactile cells, or become surrounded by a connective tissue 

 capsule. 



A definitive evolutionary stage is attained when a secondary sensory 

 cell becomes the receptor element by means by which a stimulus is 

 transferred to the dendrites of a neuron. The receptor cells of the 

 taste-buds and the hair-cells of the cochlea represent a final stage of 

 this sort. The substitution of a secondary sensory cell for the primary 

 neurosensory one presents a problem which has never been satisfactorily 

 solved. That such an evolutionary change has occurred seems indisput- 

 able, but transitional stages are wholly conjectural. 



Among the factors which have led to the formation of the sense 

 organs of higher animals, may be noted the tendency of the neurosensory 



