MYOFIBROUS ORGANIZATION OF THE HUMAN SUBOCCIPITAL MUSCLES

The myofibrous organization of the suboccipital muscles (rectus capitis posterior major, rectus capitis posterior minor, obliquus capitis inferior and obliquus capitis superior) was studied in 11 human subjects (6 males and 5 females) . Muscle slices were embedded in celloidin and sections were stained with hematoxylin and eosin. The following results were obtained: 1) In the total cross sectional area of the muscle belly and the total number of muscle fibers, obliquus capitis inferior was most predominant, followed by rectus capitis posterior major, obliquus capitis superior and obliquus capitis inferior in males and females. The cross sectional area in males was greater than that in females in each muscle. The total number of muscle fibers in both sexes was not different except that the obliquus capitis inferior was superior in males. 2) The number of the muscle fibers per sq. mm, was greater in the obliquus capitis muscle than in the rectus capitis posterior muscles, in general. Fibers of obliquus capitis inferior were most predominant, followed by fibers of obliquus capitis superior, and rectus capitis posterior major and minor. The number of the muscle fibers per sq.mm in the suboccipital muscles was less than the number in obliquus externus abdominis or transversus abdominis, and belonged to the moderately muscular group. 3) In fiber size, rectus capitis posterior minor was the largest, followed by rectus capitis posterior major, obliquus capitis superior and inferior. In comparison with other human muscles, the size of fiber in the suboccipital muscles was much smaller than that in the intermediate portion of the trapezius, was larger than that in psoas major, and was approximately equal to the size in muscles that functioned for maintenance between the bones. 4) The density of muscle fibers in the obliquus capitis inferior was equal to that in rectus capitis posterior minor and obliquus capitis superior, and was equal to that in rectus capitis posterior major. The former muscles exceeded the latter in males. In females, the density was equal in the suboccipital muscles. In obliquus capitis superior and rectus capitis posterior major, the density in females was superior to that in males. The density in the suboccipital muscles was lower than in obliquus externus abdominis, obliquus internus abdominis, transversus abdominis and iliacus, and showed features of support muscles in the body. 5) Histological observation revealed frequent variation in the fiber size in rectus capitis posterior minor, and muscular atrophy in obliquus capitis superior. In males, increased connective tissue at the perimysium was observed in rectus capitis posterior major and obliquus capitis inferior. 6) In the distribution pattern of muscle fiber size, most of the rectus capitis posterior major and minor had low peaks and most of the obliquus capitis superior and inferior had high peaks. Therefore, obliquus capitis muscles were more affected by muscular atrophy and tended to deviate toward the smaller fiber sizes.