SPINAL NERVES IN VERTEBRATES 181 



Muscle spindles 



As has been noted pre\dously, there are eight or more muscle 

 or neuro-muscle spindles (fig. 1, M.S.) in the ventro-cephaHc 

 end of the M. cordis caudalis of the 20 cm. Polistotrema, im- 

 mediately outside the ventral veno-lymphatic trunk. This 

 part of the muscle is composed mainly of large muscle bundles 

 similar to those of the myotomes. Further cephalad there is a 

 suggestion of muscle spindles in certain of the constrictor mus- 

 cles of the mucous sacs, and a study of the specialized mandibular 

 muscles might reveal their presence. This series happened to 

 be deeply and favorably stained for bringing out the spindle 

 placques so that the non-appearance of muscle spindles in the 

 other more lightly stained series probably signifies midifferentia- 

 tion rather than absence. Unfortunately silver and methylen 

 blue preparations were not available for this study. 



Muscle sense endings have been described for most verte- 

 brates, but so far as known Polistotrema (Bdellostoma) is the 

 lowest vertebrate in which muscle spindles have been found. 

 Huber and DeWitt ('00) quote Pansini as finding neuro-tendinous 

 end-organs in the bony fish Hippocampus and in a selachian. 

 Johnston ('08) has described free endings in the intermuscular 

 septa in connection with the dorsal spinal nerves of Petromyzon. 

 If a cerebellum is absent in Polistotrema, as is usually main- 

 tained, muscle sense must be correlated entirely in the spinal 

 cord, brain stem, and cerebral hemispheres. 



Ordinarily the muscle spindles of the caudal heart muscle in 

 Polistotrema are enclosed in a lamellated connective tissue 

 sheath or capsule, which may be poorly developed or even 

 absent in some cases. These spindles may occur centrally on 

 the muscle fibers, but more often they are near their tendinous 

 attachments. Portions of the spiral or annular nerve placque 

 are usually present in longitudinal sections of these muscle 

 spindles. A section through the cephalic end of the M. cordis 

 caudalis of the 20 cm. Polistotrema series (fig. 14) shows three 

 muscle spindles. No nervous elements are visible in the upper- 

 most spindle. This spindle is composed of three muscle fibers 

 {M.F.) surrounded by a well-developed lamellated connective 



