ioo8 THE MUSCULAR SENSE. 



The last-named gave them the name here adopted. They were not known to be 

 sensorial until Ruffini (1892) succeeded in historically analysing their nerve- 

 endings, and I (1893) in tracing the majority of the nerve fibres to the spinal 

 ganglia. The muscle spindle is a fusiform organ, in which for clearness of 

 description I distinguish an equatorial and two polar regions. Along its axis lies 

 a bundle of striated muscle fibres, two to ten in number. In the monkey and 

 man it is rare to find a single iiitrafusal muscle fibre only. I have seen in 

 man as many as twelve muscle fibres in one spindle. In the lizard and 

 snake, 1 however, the muscle spindle contains usually but a single fibre. 

 The intrafusal muscle fibres differ from ordinary muscle fibres in being 

 smaller and not undergoing any marked degenerative change after destruction 

 of the nerve trunk innervating the muscle. 2 They seem in some respects 

 extreme instances of the red variety of muscle fibre, but the intrafusal 

 fibres are even in the very reddest muscles of the rabbit smaller than 

 are the ordinary fibres of those muscles. They contain many central nuclei, 

 and at the equatorial regions of the spindle a richly nucleated core of non- 

 striated material encroaches on the striated structure of the fibre, leaving only 

 a peripheral striped sheet, which recalls that of a developing fibre ; and in 

 places even that peripheral zone may be wanting. The intrafusal muscle 

 bundle emerges from, the capsules sometimes at both ends, often only at the 

 distal pole, it having in the latter case ended in tendon while still within the 

 capsule. The capsule consists of fibrous membranes, which externally are 

 connected with the well-developed sheath of Henle of the entering nerve 

 fibres. At the poles the capsule merges into perimysium internum. It is 

 separated internally from the intrafusal muscle bundle by a space, the periaxial 

 space, widest in the equatorial region. The intrafusal muscle bundle is 

 supplied by numerous nerve fibres, some of which are probably motor. The 

 most obvious nerve-ending is one in which a large myelinated fibre having 

 crossed the periaxial space, lost its myelin, and divided dichotomously more 

 than once, terminates in a ribbon-shaped axis cylinder, which winds many times 

 round an intrafusal muscle fibre forming close coils. This is the " annulo-spiral 

 ribbon " ending of Rufnni. Another form of ending, also sensorial, is Ruffini's 

 4 ' flower-spray " ending, which coexists with the annulo-spiral. 3 Both these 

 kinds of endings are situated in the equatorial region, but outside that region 

 are found nerve-endings, somewhat like ordinary motor end-plates ; these are 

 probably motor. The motor nerve fibres of skeletal muscle are as a rule larger 

 than sensory nerve fibres ; in the spindles the sensory nerve fibres are larger 

 fibres than the motor. Cipollone 4 has recently shown that an intrafusal muscle 

 fibre in the lizard commonly has not one but two motor end-plates, one at each 

 end of it. Just as, unlike other fibres of skeletal muscle, the intrafusal muscle 

 fibres do not undergo degeneration after destruction of the nerve trunk, 5 so 

 also, unlike other muscle fibres, they do not participate in the hypertrophy 

 produced by functional exercise. 6 Muscle spindles have now been studied in 

 various animal types (e.g. frog, lizard, pigeon, rat, guinea-pig, rabbit, cat, dog, 

 monkey, man, etc.). Those of the frog and lizard are smallest and structurally 

 simplest, and often contain but one muscle fibre apiece. 7 Those of the monkey 

 and man are largest and most complex, and rarely contain but a single muscle 

 fibre. In the cat and monkey I found it not uncommon for the spindles to be 

 conjoined in pairs and even triplets. Their tendon-bundles with special 

 frequency enter into Golgi-organs, 8 with these again possess accessory Pacini 



Cipollone, "Ricerche sull' anat. d. terminazioni nervose nei muscoli striati," Roma, 

 1897. 



2 Sherrington, Journ. PhysioL, Cambridge and London, 1894, vol. xvii. p. 211. 



3 See also an excellent account by C. Sihler, Arch. f. mifcr. Aiiat., Bonn, 1895, Bd. 

 xlvi. S. 709. 



4 Lot. cit. 5 Sherrington, loc. cit. 



6 Morpurgo, Virehow's Archiv, 1897, Bd. cl. S. 554. 



7 Cipollone, loc. cit. a A. Cattaneo, Arch. ital. dc bioL, Turin, 1888, vol. x. 



