In order to clarify the characteristics of myofibrous organization in the skeletal musculature of primates, M. tibialis anterior of adult crab-eating monkeys (5 males and 5 females) was investigated, differentiating into 3 type of muscle fibers by Sudan-Black B staining method. Following results were obtained.
1. Though the sex differences in body weight of materials was not seen, the muscle weight and the thickness of muscle fibers were greater in males, and the cross-sectional area of venter, the number of muscle fibers per sq. mm. and the total number of muscle fibers were greater in female.
2. In the 3 type of muscle fibers, white fiber were highest in about 50%, red fiber second most and intermediate fiber least. But there was a few case in female which hadn't significant differences between red and intermediate fibers or was predominant in intermediate than in red fiber.
3. Concerning the distribution of the 3 type of muscle fibers in the cross-sectional area, white fiber was predominant in lateral 1/3, red fiber in medial 1/3 and intermediate fiber in middle 1/3. And the border line of these area was lenient.
4. The thickness order for 3 type of muscle fiber was always ; largest, white ; then intermediate ; and red, smallest. All three of these types were larger in male than in female, and this tendency was predominant on the larger group of body weight.
5. Comparing the crab-eating monkey and the rat, the distribution pattern of three type of muscle fiber in the cross-sectional area and the thickness of muscle fiber were similar in both and the total number of muscle fiber were in proportion to body weight in both, and the ratio of white fiber were predominant than in rat. From these facts, it is considerd that the M. tibialis anterior of monkey is more phasic in function than in rat.