The intramuscular distribution of the accessory and the cervical nerves was studied in the human sternocleidomastoid muscle, with special regard to muscle architecture. The innervation of the sternomastoid and cleidomastoid muscles of the rabbit was investigated by electrical stimulation of the nerves and also by persuing degenerative changes in the nerve fibers and motor end-plates following nerve section. 1. The human sternocleidomastoid is composed of four more or less separate portions: the sternomastoid, the sternooccipital, the cleidomastoid and the cleidooccipital. The boundary between the sternomastoid and the sternooccipital portions was determined, differing from that of previous workers, by a fissure, into which the terminal branch of the muscle nerve enters. (Fig. 1). 2. The cleidooccipital portion was entirely lacking in 6 of the 56 muscles examined (10.7 % ). 3. The accessory nerve enters the muscle in two different ways: in 29 of the 66 muscles examined (43.9 % ), it goes through the upper part of the cleidomastoid portion and then anastomoses with the branches of the cervical nerves, which enter laterally to the cleidomastoid (Figs. 2, 3, 5 and 6); in the other 37 muscles (56.1 % ), both the sternocleidomastoid branch of the accessory and the cervical branches enter laterally to the cleidomastoid, anastomosing with each other in their course. (Figs 4 and 7). 4. The branches of the accessory and the cervical nerves are distributed in the human muscle after they have anastomosed with each other and are to a great extent mixed. Therefore, the boundary of their distribution is not to be recognized. This condition agrees with the general view, that the accessory nerve chiefly conveys motor fibers to the muscle and the cervical branches are sensory for the most part. In this respect, the sternocleidomastoid, together with the trapezius, differs greatly from the other double-innervated muscles in the region of the spinal nerves. They are to be regarded as belonging to the same category as other cranial muscles, which receive motor and sensory fibers from separate nerves. 5. The sternomastoid, the medial part of the sternooccipital and the cleidomastoid are estimated to receive more fibers from the accessory, while the lateral part of the sternooccipital and the cleidooccipital are supplied more abundantly with the cervical nerves. Furthermore, it is highly probable, that the lateral part of the cleidooccipital portion is innervated solely by the cervical branches. This suggests that the sternocleidomastoid receives some motor fibers from the cervical nerves. 6. On three sides of two rabbits, the cervical nerves were divided and the pripheral stumps were stimulated electrically. In only one of the cases, a very weak contraction of the cleidomastoid muscle was observed by stimulation of the second cervical nerve. 7. Thirty-six to ninety-six hours after severing the accessory and/or cervical nerves (C(3)-C(4), in most cases), the steronmastoid and cleidomastoid muscles of rabbits were examined by silver-impregnation method of Agduhr. In the rabbits, in which the cervical nerves were sectioned, some degenerated nerve fibers running between the muscle fibers and degenerated motor end-plates were found in both muscles. Thus in the rabbit, also, some fibers of the cervical nerves are demonstrated to be motor in nature.