1. 100 cases of them. scalenus anterior in 50 cadavers of adult Japanese (37 male, 13 female) are observed (Tab. 1).
2. The m. scal. ant. arises most frequently from the transverse process of the 4th-6th cervical vertebrae in three slips (49 cases of 36 cadavers-Tab. 2).
3. The slips of the m. scal. ant. lie in front of or on those of the m. scal, medius, and both of them tend to form a common slip at the upper cervical vertebrae, and to be separated from each other at the lower (Fig.1). The m. scal. ant. takes origin not only on the anterior tubercle, but also on the so-called costo-transverse lamella (Fig. 2 a). The slip of the m. scal. ant. arising from the 6th cervical vertebra is most thick (Tab.6) and fleshy and shows almost a sickle-shape or a triangle in a cross section (Fig.2 b). The upper slips are more tendinous, thinner and flattened.
4. The m. scal. ant. descends in front of the ventral rami of the cervical nerves and subclavian artery, and is inserted on the tuberculum m. scaleni anterioris. In three cases, a portion of the m. scal. ant. passes over to the wall of the subclavian artery, being, in one of these three, the muscle penetrated by this artery.
5. The fascicle arising mainly as the split of the m. scal. med. from the 2nd-5th cervical vertebrae (Tab. 7), and descending behind the cervical nerves joins to the m. scal. ant. in 35 cases of 27 cadavers (posterior fascicle-Fig. 3, Photo 6).
6. Between the mm. scaleni ant. and med. there exists the m. scal. minimus in 32 cases of 24 cadavers. This muscle arises from the 6th-7th (18 cases) or from the 7th cervical vertebra (14 cases), and the upper limit of its origin is the same with (C6: 18, C7: 4 cases) or lower than the lower limit of origin of the m. scal. ant.
7. The m. scat. ant. can be separated from the mm. intertransversarii antt. longi (Testut), in 20 cases of 18 cadavers, in which the upper limit of the m. scal. ant. comes never higher than the 4 th cervical vertebra. In the other cases, the posterior part of the intertransv. antt. longi arise from uppermost slip of the m. scal. ant. which arises from the 4th or 5th cervical vertebra (42 cases of 32 cadavers), or continues tendinously into this slip (23 cases of 18 cadaveas). In the remaining cases, the lateral part of the m. scal. ant. ascends fleshly and attaches tendinously to the 3rd or rarely to the 2nd and 3rd cervical vertebrae forming a part of the intertrnsv. antt. longi (15 cases of 11 cad.). The uper part of these 15 cases is regarded as the arising slip of the m. scal. ant., and also in these cases almost of the uppermost slips show the connection with the intertransversarii antt. longi in above mentioned manner (11 cases of 7 cadavers). These connections between both muscles are observed in 76 cases of 47 cadavers in all.
8. The m. scal. ant. is innervated by branches from the one-four of the ventral rami of the 4th-8th cervical nerves (most frequently three from the 5th, 6th and 7th-11 among 20 cases).
9. It is certified that the limits of the origin of the m. scal. ant. can be varied according to different criteria to regard a fascicle as the arising slip of the m. scal. ant.

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