THE RECEPTORS AND EFFECTORS 



87 



As we shall see below (p. 92), the muscles are classified for our purposes 

 into three groups: (1) somatic muscles (the striated skeletal muscles); (2) 

 general visceral muscles (generally unstriated and involuntary) ; and (3) 

 special visceral muscles of the head which are striated and voluntary. The 

 first and third of these groups receive their motor innervation from cere- 

 bro-spinal nerves; the second, from sympathetic nerves. The classification 



Fig. 30. Muscle spindle from the muscles of the foot of a dog. Three 

 muscle-fibers are shown, and three sensory nerve-fibers, which enter the 

 muscle spindle, branch, and wind spirally around the muscle-fibers (a, 6). 

 A sympathetic nerve-fiber (Sy.n.) also enters the muscle spindle. (After 

 Huber and DeWitt, from Barker's Nervous System.) 



of the nerves of muscle sense related respectively to these three groups of 

 muscle offers some difficulties. The striated muscles of the first and third 

 groups are physiologically similar in that they act in general in response to 

 exteroceptive stimuli and they may be voluntarily excited, while the visceral 

 muscles of the second group are generally stimulated by interoceptive stim- 



Fig. 31. A teased preparation of a tendon of a small muscle from a 

 rabbit, showing the endings of the nerve-fibers of tendon sensibility, each 

 of which spreads out widely over the surface of the tendon. (After Huber 

 and DeWitt, from the Journal of Comparative Neurology.) 



uli and their functions are usually involuntary. I have, accordingly, some- 

 what arbitrarily regarded the sensory nerves of the first and third groups 

 of muscles as proprioceptors and those of the second group as interoceptors. 

 10. End-organs of Tendon Sensibility. Nerve-endings are spread 

 out over the surface of tendons and are stimulated by stretching the tendon 

 during muscular contraction (Fig. 31). 



