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The incidence and patterns of association of tendon organs (TO's) with muscle spindles was studied in serial histological sections of the extensor digitorum brevis (EDB) muscles of an adult cat. Some observations were obtained from the medial gastrocnemius and soleus muscles of a second cat, and the EDB muscle of a two-toed sloth. An intimate relation of a TO to a nearby spindle, wherein the “spindle tendon” passes alongside the TO to attach to the aponeurosis, was found for 50% of cat EDB TO's, and 20 to 25% of the TO's in the other muscles. In cat EDB samples both TO's and spindles found in “tendon organ-spindle dyads” had larger cross-sectional areas at the equator than “solitary” units, and dyad TO's received the attachments of more extrafusal fibers than did solitary TO's. A number of the “solitary” TO's were found to be in line at a greater distance with spindles that lay along the extrafusal fibers inserting onto the TO. The possible developmental and functional significance of these findings is discussed.

The manner of insertion of intrafusal (IF) fibers of muscle spindles has been studied in serial histological sections of spindles from hindlimb muscle of the cat, opossum, mouse, rat, bat and gibbon. Four basic types of IF terminations in relation to the capsule were recognized: (A) direct insertion into the capsule; (B) strong lateral attachments to the capsule, though the fiber may continue beyond the capsule; (C) termination in the connective tissue beyond the limits of the capsule; (D) free extension of the fiber beyond the capsule. Bag fibers in all species generally extended beyond the capsule. The pattern of chain fiber insertion, however, varied with the species. Most rat spindles, for example, followed the D pattern, those in the cat pattern A, and the gibbon chiefly A and B. The bearing of these findings upon a postulated mechanism by which spindles may detect intramuscular pressures, as changes in length, is discussed.