566 COMPARATIVE ANATOMY 



cells, which were primarily scattered and separate, to become concentrated 

 in clusters to form the sense organs. Secondary sense cells show the 

 same tendency, and the cephalization of the nervous system is correlated 

 with the concentration of the sense organs in the head region. Attention 

 has already been called to the fact that the three primary divisions of 

 the brain are associated with three major sense organs, the nose, the eye, 

 and the ear. 



CUTANEOUS SENSES 



At least four senses, pressure or touch, pain, warmth, and cold, 

 are based upon nerve terminations in the skin. The experiments of 

 Goldschneider and others demonstrate that corresponding with these 

 four sensations are four different sorts of nerve terminations. These 

 may be either free or encapsuled. Usually, the free nerve terminations 

 are located in the basal layers of the epidermis, and are therefore more 

 superficial, while the encapsuled terminations lie in the corium below. 



Evolution of Cutaneous Sense Organs 



It may be assumed that the various cutaneous sense organs were 

 derived from neurosensory cells of the epithelium, which by the outgrowth 

 of a neurite become connected with the nerve net or cord. When the 

 body of the cell gradually migrated into the underlying connective tissue 

 and the epidermis became many-layered, all connexion with the surface 

 was lost, and the neurosensory cell was converted into a sensory neuron, 

 with free nerve terminations in the lower layers of the epidermis and 

 central connexions with the nerve cord. As a last step, connexion with 

 the epidermis was lost, and the peripheral termination of the sensory 

 cell was buried in the corium. 



All these stages are represented in chordates. The skin of amphioxus, 

 for example, is beset with many neurosensory cells, both single and in 

 clusters, many of which possess a stiff terminal bristle which projects 

 above the general surface. Amphioxus also has sensory nerves with 

 free nerve terminations branching among the epithelial cells. 



The encapsuled nerve terminations have apparently followed two 

 independent lines of evolution. On the one hand, free nerve terminations 

 in the corium have become encapsuled by concentric layers of connective 

 tissue cells, as represented in the corpuscles of Pacini, Krause, and Golgi- 

 Mazzoni. On the other hand, some nerve terminations are associated 

 with tactile cells, which primarily were located in the basal layer of the 

 epidermis, but which later migrated into the corium. In some instances, 

 a single lenticular tactile cell may rest upon a cup-shaped termination 

 of a sensory neurite, or the nerve may branch among a cluster of such 

 cells. As a final evolutionary stage, a cluster of tactile cells connected 



