Mechano- and Equilihriutn-Reception 



503 



Histological Types of Mechanoreceptors. The differentiation in the course 

 of evolution of certain epithelial cells into mechanoreceptors and chemo- 

 receptors probably occurred before the differentiation of conducting nerve 

 cells. In all animals with nervous systems there are specialized mechanore- 

 ceptors of one or more histological types. The simplest are free nerve end- 

 ings; these may lie between epithelial cells, as in the earthworm, or may be 

 in the deeper portion of the skin, as in vertebrates. They usually have a high 

 mechanical threshold and may be stimulated by various noxious agents; 

 that is, they are often pain receptors. Another simple type of tactile receptor 

 is a terminal bulb, as in the frog skin, or Merkel's discs, in mammalian 

 skin. Sensory hairs are widely distributed in many kinds of animals, and 

 at their base there may be free endings, knobs, or speciaHzed spirals. 

 Arthropods have many sensory hairs at joints along various appendages, 

 such as antennae, legs and wings. In insects there are also short sensory 

 spines and more pliable setae with bipolar sense cells. Frequently sense cells 



1 



Fig. 174. Schematic drawing of section of skin and its appendages. (Andrews.) 



have a short sensory hair and are grouped in organs, as in the earthworm 

 (Fig. 175). Several types of encapsulated endings have been described for 

 mammals, particularly the pacinian corpuscles of tendons and mesentery 

 (Fig. 176), and Meissner's corpuscles of the skin (Fig. 174). 



The distribution of proprioceptors is less well known than that of tactile 

 receptors, but in mammals, birds, reptiles, and amphibians there are spiral 

 structures such as the spindles of tendons and intrafusal muscle fibers which 

 respond to a pull (Fig. 177). Muscle spindles are lacking in fish but there 

 are tension receptors in fish muscle. The proprioceptors of vertebrates are 

 usually arranged to respond to tension in small bundles of muscle fibers, 

 whereas in insects they respond more to strains in the cuticle. In the cam- 

 paniform sensilla of insects a terminal filament ends in a rod under a cuticular 



