Nerves have a structure of considerable complexity with features of special relevance to nerve injury and nerve regeneration. These include variations in the cross-sectional areas devoted to fascicular and epineurial tissue, the fascicular redistribution and mixing of different branch fibers brought about by fascicular plexuses, and the numbers of nerve fibers representing individual branches. The elasticity and tensile strength of nerve trunks and their capacity to resist traction deformation reside in the fascicular tissue, while the epineurium provides a protective cushion against compression. The microstructure of nerve trunks provides the basis for a classification of nerve injuries into five degrees of severity with partial and mixed types--each with a clearly defined pathology and distinguishing clinical features. Following a transection injury, changes occur in the severed axons, endoneurial tubes, fasciculi, and nerve trunk. The type of injury and the nature of these changes determine the outcome of axon regeneration.

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FOUR FIGURESIn the course of an earlier investigation by one of us (S. S., '46) on thc pupilloconstrictor pathway in the human oculomotor nerve, structures possessing the characteristic morphological features of muscle spindles were repeatedly observed in all the extraocular muscles. I n view of the generally expressed belief that eye muscles have no proprioccptive innervation it was decided to obtain niore exact information about the total number and distribution of the spindles. A preliminary report on the findings of this enquiry has already been given (Sunderland, '49).These structures were also observed by Daniel in '46 who later, in collaboration with Cooper' submitted them to a detailed exaniination which culminated in the publication of a paper "Muscle Spindles in Human Extrinsic Eye hfusclcs" ( '49). This excellent account in which the literature is comprehensively reviewed, the morphological appearance of the spindles clearly described and convincingly illustrated, and their distrihution and nuniber in the inferior rectus recorded, covers essentially the same grouiid as that surveyed by us. Though this climiriates the necessity for a further detailed account it is felt that certain features of o n r enquiry arc worth recording.In Cooper and Daniel's paper precise information concerning the distribution and number of the spindles \ Y~S confined to one specimen of the inferior rcctus muscle. Detailed counts This work mas assisted by a grant froill the National Hcalth and Illerhcal Research Council of Australia. 23 LITERATURE CITED COOPER, S., BND P. 51. DAXIEL hIusclc spindles in human extrinsic e j e musrlss. Brain, '73: 1-24. SUKDFRLAND, 8. 1949 A preliminary note 011 the presence of neurornuvrulxr spindles in the extrinsic ocular muscles in man. Anat. Rrr., lo,?: 561-562.1946 The pnpillo-constrictor patliii-aF and the nerves t o the ocular ~nuscles in ~l a~i .Grain, 69 I 301-309.