204 Journal of Comparative Neurology. 



make out a sarcolemma on the intrafusal fibers in the equatorial 

 region of the muscle-spindle, the nuclei above referred to, 

 seemed outside of this sheath. They seem to belong to a con- 

 nective tissue sheath, which surrounds each intrafusal fiber; a 

 sheath inside of the axial sheath, with which it is partly fused, 

 or to which it may be partly united by means of bands or septa 

 of fibrous tissue. In longitudinal sections of muscle-spindles, 

 the arrangement of the nuclei is such that they seem to belong 

 to endothelial cells lining the fibrous tissue sheath surrounding 

 the intrafusal fibers. On this point, however, we possess no 

 conclusive observations. The hyaline core mentioned by Sher- 

 rington, we have interpreted as sarcoplasma, the intrafusal 

 fibers, especially in the equatorial region, often showing fibrilla- 

 tion only in the peripheral zone. In the central of the intra- 

 fusal fibers, also more apparent in the equatorial region, a row, 

 or sometimes two parallel rows of nuclei are found. We can 

 confirm the statement of Christomanson and Strossner, of 

 Kerschner and Sherrington, that these nuclei are resting nuclei, 

 which show no sign of karyokinetic cell division. The main 

 differences which we have observed between the structure of 

 the intrafusal fibers in the equatorial region and that in the 

 other parts of the spindle — proximal and distal polar regions — 

 are that in the former, the cross and longitudinal striation is not 

 so apparent, the central " hyaline core " is more clearly made 

 out, and the nuclei are more numerous. The intrafusal fiber in 

 the equatorial region resembles in structure a developing stri- 

 ated muscle fiber. In cases where the muscle-spindle is situated 

 near the tendon of the muscle or near a fibrous septum, the in- 

 trafusal fiber becomes tendinous and seems to fuse with the distal 

 part of the spindle capsule. In the simpler muscle-spindles, 

 containing one intrafusal fiber, this becomes less distinctly stri- 

 ated and shows more nuclei, when within the spindle. In the 

 snake, the muscle fibers destined to become intrafusal fibers 

 are throughout very much smaller than the other striated mus- 

 cle fibers, about 8 to lo times smaller, as correctly stated by 

 Sihler. After entering the spindle, it may retain its former size 

 or become slightly larger, less distinctly striated and show more 



