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HANDBOOK OF PHV-SIOLOOV 



NEUROPHYSIOLOGY II 



(153) and only special points will he mentioned here. 

 The insertions of the muscles are near the equator 

 of the eyeball in man and the primates where there 

 is considerable eve movement, in other mammals the 

 insertions may be much closer to the cornea and in 

 birds they are nearer the optic nerve. In some ani- 

 mals, particularly the ungulates, the superior oblique 

 muscle is fleshy throughout its course, it passes 

 through a wide trochlea and the muscle fibers ex- 

 tend almost to the insertion; in birds this peripheral 

 portion forms the whole muscle which has its origin 

 just dorsal to the origin of the inferior oblique. The 

 sixth nerve innervates the lateral rectus muscle, but 

 numerous mammals ha\'e a cone or slips of a retractor 

 bulbi muscle inserted behind the equator of the eye- 

 ball and al.so innersated by this nerve. The muscle 

 pulls the eye back into the orbit and this activity is 

 apt to be overlooked by physiologists working in this 

 region. It can be seen \ery clearly in carnivores and 

 ungulates when the recti are cut. In birds the quad- 

 ratus and pyramidalis, also innervated by the sixth 

 nerve, are attached in a similar position and serve to 

 draw back the nictitatins; membrane. In some fishes 

 the lateral rectus has the peculiarity of having its 

 origin in the neck. 



Muscle Fibers 



The fibers of human eye muscles vary from 10 to 

 50 M in diameter (141, 152) and they run the whole 

 length of the muscles. The fibers are somewhat 

 smaller in the cat (7 to 35 m) and monkey (5 to 

 40 /i) (44). In the goat they are 10 to 85 ii. Fibers of 

 large diameter tend to be collected together forming 

 a central core in the muscles, while fibers of small 

 diameter form an outer coat, especially at the lateral 

 edges and on the surface away from the eyeball. 

 This arrangement is particularly striking in human 

 and inonkey material and is seen also to a lesser ex- 

 tent in the cat and goat (42). 



Motor End Plates 



In man there is a conspicuous compact band of 

 motor end plates at the junction of the proximal and 

 middle third of a rectus luuscle (33). One of these 

 end plates is illustrated in Daniel (48). The motor 

 end plate band is also conspicuous in the monkey, 

 but the end plates are more widely scattered in the 

 cat and goat. Other typical motor endings are found 

 in the outer coat of fine muscle fibers. Thev often 



consist of a single small motor end plate at tlie end 

 of a fine nerve fiber. 



Miiiilf Sftindles and Other Receptors 



The afferent endings in liml) muscles are the 

 muscle spindles, tendon organs and some fine naked 

 endings. In the eye muscles every part of the muscle 

 is richly supplied with nerve fibers, and in most 

 species, apart from the main motor end plates, the 

 endings are not typical, nor has it so far proved pos- 

 sible to cut a purely motor nerve to the muscles so as 

 to leave the sensor\- endings intact. Thus much con- 

 fusion has arisen about the sensory endings, the 

 literature on which is surveyed by Tiegs {135). AH 

 the luammalian eye muscles examined by silver and 

 gold impregnation or methylene blue techniques 

 show a rich innervation of both origin and insertion 

 tendons, with small nonencapsulated tendon endings 

 near the musculotendinous junctions (33, 42, 136). 

 Muscle spindles in the eye muscles were first seen by 

 Cre\-atin in the ox (45). Cilimbaris (31) gives a de- 

 tailed description of them in the sheep; he also 

 counted them, finding over 200 spindles in a single 

 inferior rectus muscle. Cooper et al. (36) found about 

 120 in a goat inferior oblique muscle. It is now clear 

 that typical muscle spindles are very numerous and 

 universally found in the eye muscles of the artio- 

 dactyl branch of the ungulates. Their presence in 

 man was denied for many years, but search through 

 serial sections sliowed them to be present in con- 

 siderable nuiubers (up to 50) in the proximal third of 

 all the muscles, with a few scattered ones peripheral 

 to the band of motor end plates (33, 106). Spindles 

 were also found in the chimpanzee, but not in the 

 monkey (33). 



The muscle spindles in human eye muscles (fig. i) 

 have a \ery thin connective tissue capsule; they lie 

 near the outer coat of small diameter muscle fibers 

 so that the intrafusal muscle fibers are only a little 

 smaller than the adjacent extrafusal fibers. The 

 intrafusal muscle fibers have some central nuclei, 

 but so far no nuclear bags have been seen. They 

 recall the smaller intrafusal fibers seen in the tenu- 

 issimus spindle of the cat (20) and in the lumbrical 

 spindles of man (34). 



In the eye muscles of man, some of the larger 

 nerve fibers take several spiral turns round large 

 muscle fibers, in the core of the muscle just distal 

 to the luotor endings, and then end on these fibers 

 (48). Occasional nerve fibers have been seen en- 



