72 THE NERVOUS SYSTEM 



ing their myelin sheaths as they enter. They give off horizontal branches, 

 which encircle the root of the hair, and from these ascending branches arise 

 (Fig. 46). Some of these are connected with leaf-like expansions, associated 

 with cells resembling Merkel's touch-cells. 



Practically nothing is known concerning the receptors for sensations of heat 

 and cold. 



Proprioceptive Fibers and Sensory Nerve Endings. To this group belong 

 the afferent elements which receive and convey the impulses arising in the 

 muscles, joints, and tendons. Changes in tension of muscles and tendons and 

 movements of the joints are adequate stimuli for the receptors of this class and 

 excite nerve impulses which, on reaching the central nervous system, give in- 

 formation concerning tension of the muscles and the relative position of the 

 various parts of the body. For the most part, however, these impulses do not 

 rise into consciousness, but serve for the subconscious control of muscular 

 activity. The unsteady gait of a tabetic patient illustrates the lack of mus- 

 cular control that results when these impulses are prevented from reaching the 

 central nervous system. 



The proprioceptive fibers are myelinated and are associated with motor 

 fibers in the nerves to the muscles. Some follow along the muscles to reach 

 the tendons. Three types of end organs belong to this group, Pacinian cor- 

 puscles, muscle spindles, and neurotendinous end organs. Many Pacinian 

 corpuscles are found in the neighborhood of the joints. They have been de- 

 scribed in a preceding paragraph. 



Neuromuscular End Organs. The afferent fibers to the muscles end on 

 small, spindle-shaped bundles of specialized muscle-fibers (Fig. 47). These 

 muscle spindles are invested by connective- tissue capsules; and within each 

 of them one or more large myelinated nerve-fibers terminate. Within the 

 spindle the myelin sheath is lost and the branches of the axis-cylinders wind 

 spirally about the specialized muscle-fibers, or they may end in irregular disks. 

 Somewhat analogous structures are the neurotendinous end organs or tendon 

 spindles where myelinated nerve-fibers end in relation to specialized tendon 

 fasciculi. 



