The splenii form the jburth layer. They appear as one muscle, extending from the lower cervical and upper dorsal spines obliquely upwards, outwards, and forwards, to the head and to the transverse processes of the superior cervical vertebra. Covered below by the rhomboid and serratus posticus superior, higher up by the trapezius and levator anguli scapula, and higher still by the sterno-mastoid, it is only about the middle of their course that they become distinct from each other, for they arise as one.
The splenius colli (or splenius cervicis) is the inferior portion, not so thick or broad as the superior, hut of greater length. It arises from the spines of the third, fourth, fifth, and sixth dorsal vertebra, and from the interspinal ligaments, by tendinous fibres which are long, and form an acute angle below. The flat band of fleshy substance which proceeds from this tendon passes upwards, outwards, and for wards, then divides into two or three fasciculi, which are inserted tendinous into the trans verse processes of the two or three superior cervical vertebra, blended with the attach ments of the levator anguli scapula and the transversalis colli.
The splenius capitis, the superior portion, arises from the spines of the two supeiior dorsal vertebra and of the seventh cervical, and from the ligamentum nucha as high as the fourth cervical. At the origin it is tendinous ; it coon becomes fleshy, passes upwards, out wards, and forwards, to be inserted into the back part of the mastoid process of the tem poral bone, and into the external part of the depression on the occipital, between the supe rior and inferior transverse ridges.
These two portions ought not to be con sidered distinct muscles. They are inseparable below ; their structure, direction, and uses are alike ; and they are inserted similarly—the one into transverse processes, the other into a part of the cranium perfectly analogous.
The splenii cover the longissimus dorsi, the complexus, the transversalis colli, and the trachelo-mastoideus. The splenii of opposite sides pass off from each other as they ascend, leaving a triangular space at the upper part of the neck, in which the cornplexi appear.
The action of these muscles is to incline the head to one side, and rotate it. If the sterno mastoideus of the same side act with them, the bead is inclined directly to the shoulder. If the splenii of opposite sides act together, the head and neck are kept erect, and in this they are assisted by the complexus and trapezius. They strap down the deeper muscles. Their
name is said to be derived from some resem blance to the spleen ! ( Turton's G lossary.) The splenii are generally better marked in other mammalia than in man. In the mole they are particularly strong. In carnivora there is no splenius colli. In the horse the splenius capitis is inserted into the mastoid process by a tendon common to it and to the trachelo-mastoideus. Birds have no splenius. Reptiles have analogous muscles; but fish have not.
Fifth layer.—On removing the splenii and all those previously described, we expose the fifth layer of muscles, consisting of the sacro lumbalis, longissimus dorsi, spinalis and semi spinalis dorsi, cervicalis descendens, trans versalis colli, trachelo-mastoideus, and corn plexus. These, excepting the last, are long and slender, quite different from those hitherto described. They are also less distinct from each other. The first four of them fill up the vertebral groove from the sacrum to the neck, and might well be considered as one muscle— the erector spina..
The sacro-lumbalis, placed most externally, arises from the posterior surface of the sacrum, from the margin of the ilium where the latter overlaps the former, from the sacroiliac liga ments, and from the extremities of the trans verse processes of the lumbar vertebra; pas sing upwards, and tapering in form, it is in serted by tendinous slips into the angles of all the ribs. It is reinforced in its ascent by acces sory fibres (musculi accessorii), which arise at the upper margins of the five or six lowest true ribs, internal to their angles, run upwards and outwards over one or two intercostal spaces under cover of the longer fibres, and are in serted with them into the angles of the ribs. These accessory fibres constitute almost the entire of the muscle at its upper part.
The longissimus dorsi, placed along its inner side, arises from the spinous and transverse processes of the lumbar vertebra, and from the spines of the sacrum and its posterior surface down to its apex. It forms a thick, somewhat square, mass in the loins ; on the dorsum it becomes flat and tapering, and ends in a point at the top of the thorax. It is inserted by two rows of tendinous and fleshy slips—one row into the transverse processes of all the dorsal vertebra, the other row, externally, into the lower edge of the ribs near their articulations with those processes. The costal slips are seldom inserted into all the ribs, the first two or three and the last two or three being often without them.