CENTRAL CONTROL OF EVE MOVEMENTS 



IO9I 



FIG. I . Longitudinal section through a human inferior rectus 

 oculi muscle. A muscle spindle is seen crossing the field from 

 top to bottom. The delicate capsule of this spindle is torpedo- 

 shaped and encloses at least two intrafusal muscle fibers. These 

 are of smaller diameter than the adjacent extrafusal muscle 

 fibers and are separated from the capsule by the periaxial space. 

 A small nerve trunk is seen at the upper right pari of the field. 

 This runs down to enter the spindle almost at its mid-point. A 

 capillary runs along just inside the capsule on the left. Paraffin 

 section; Masson's trichrome stain. [From Cooper & Daniel 

 (33)-] 



tion potentials and not to contracture (24). This 

 effect persists in tlie eye muscles in vitro (69). It is 

 not known whether the large and small muscle fibers 

 behave differently in this^ respect. Skeletal muscles on 

 the other hand only contract with close arterial in- 

 jection of acetylcholine. 



Motor Units in Eye Muscles 



The very rich nerve supply to these small muscles 

 suggests a small motor unit, i.e. with a low ratio of 

 motor nerve fibers to muscle fibers. Counts of the 

 nerve fibers supplying the eye muscles and of the 

 muscle fibers were made for human extraocular 

 muscles by Bors (18) who obtained ratios varying 

 from 1:4 to 1:7. His nerve totals appear low, judg- 

 ing by counts made by other observers (16) but, as 

 he takes no account of possible sen.sory fibers, 1:6 

 may be about the size of a human eye muscle unit; 

 a motor fiber can often be seen to divide into a little 

 cluster of 4 to 6 end plates on neighboring muscle 

 fibers. A count of nerve and muscle fibers was made 

 for sheep eye muscles by Tergast (133). He pre- 

 sumably counted the nerve fibers in the main nerves 

 to the muscles and did not include the separate 

 sensory trunks (see below). Thus in all the muscle 

 nerves except that to the inferior oblique he would 

 be counting mainly motor fibers and his ratios of i :6 

 to 1:10 may give the size of the motor unit in these 

 animals. His lower ratio of 1:3 or 4 for the inferior 

 oblique may well be explained by the fact that the 

 main nerve to this muscle is now known to be a 

 mixed nerve until verv close to the muscle. 



circling muscle fibers in the cat (42, iio). Accessory 

 nerve fibers given off from fibers going to motor end 

 plates are described in the rabbit (81, 154). These 

 fibers may run for some distance along a muscle fiber 

 giving off fine endings at intervals. Other nerve 

 fibers, which show no connection with the motor 

 fibers, also run along the small muscle fibers giving 

 off twigs to fine endings at intervals (81, 148). Some 

 of these fibers are probably sensory, but those linked 

 with the main motor supply must be motor and they 

 may form a device for shortening the rising time 

 of the muscle twitch. 



No detailed analysis of the behavior of the nerve 

 muscle junction in the eye muscles has yet been 

 made. Such analysis might prove interesting as these 

 muscles have the property of contracting in response 

 to acetvlcholine, a contraction due to hursts of ac- 



Nerve Fiber Size 



In assessing the sizes of ner\e fillers supplying a 

 muscle, consideration must be given to the age of 

 the animal and the amount of shrinkage due to the 

 technical methods used by the author. Old and 

 young animals tend to have a unimodal distribution 

 of fiber sizes (118). Distribution curves for the nerve 

 fibers to extraocular muscles of adult inan are given 

 by Bjorkman & Wohlfart (16) for nerve trunks near 

 the brain and by Rexed (118) near the muscles. A 

 bimodal distribution is described with maxima at 4 to 

 5 /i and 9 to 10 /i, the largest fibers being about 13 ix 

 (16); but the maxima are not obvious. Bjorkman & 

 Wohlfart also give figures for the sixth nerve in the 

 cat, dog, cow and sheep. Rexed gives them for the 

 fourth nerve in the rabbit and Fernand & Young 



