This muscle draws the head back on the spinal column.
In the muscles of this layer there are no very striking differences to be observed in the other mammalia, nor in birds. Reptiles and fishes differ too widely to allow of a com parison.
Sixth layer.—On raising the complexus and trachelo-mastoideus we observe a beautiful series of muscles for moving the head, viz. the inferior oblique, the superior oblique, the rectus capitis posticus major and minor. These, with the spinalis colli, form a sixth layer.
The spinalis, or rather semi-spinalis colli, arises by four or five fasciculi from the trans verse processes of as many superior dorsal vertebrae; these pass upwards and in wards, to be inserted into the second, third, fourth, and fifth cervical spines, forming a thicker muscle than the spinalis or semi-spinalis dorsi.
This muscle commences between the longis simus and semi-spinalis dorsi, then it lies be tween this last and the complexus. It is almost concealed by the complexus. It ex tends the cervical vertebrae and inclines them to its own side.
The obliquus capitis inferior arises from the spine of the second vertebra, passes outwards and a little upwards and forwards, to be in serted into the transverse process of the first. Its origin is connected with that of the rectus posticus major, and the insertion of the spi nalis colli. Its insertion is blended with the origin of the obliquus superior. It is fusi form in shape, the largest of the four muscles to be met with here, and is often called obli quus major. It covers the vertebral artery and the lamina of the second vertebra, and is itself covered by the complexus and trachelo-mas toideus, and by the posterior branch of the first cervical nerve.
It rotates the first vertebra on the second, thus turning the face to its own side.
The obliquus capitis superior (or minor) has a pointed origin from the transverse process of the atlas; runs upwards, inwards, and back wards, becoming broader,. and is inserted into the os occipitis between its transverse ridges, just above the insertion of the rectus posticus major. This muscle is covered by the splenius capitis, trachelo-mastoideus and complexus. It covers the vertebral artery and the interval between the atlas and occiput.
Its action is to extend the head, giving it some inclination to its own side.
The rectus capitis posticus major is triangu lar; its apex arises from the spine of the den tata; it passes upwards and a little outwards, to be inserted by its base into the inferior transverse ridge of the os occipitis. This muscle and its fellow arise close together; passing up they separate. The insertion is overlapped by that of the superior oblique. The complexus covers the greater part of it.
This muscle draws back the head, turning the face a little to its own side.
The two obliqui, with this last muscle, en close a triangular space, in which we see the posterior branch of the sub-occipital nerve enveloped in adipose tissue, the vertebral artery, the posterior half ring of the atlas, and the thin ligament which connects this last with the edge of the foramen magnum. Here we
find the nerve dividing into three branches for these three muscles.
On removing some cellular tissue from between the recti majores, we observe the Rectus posticus minor, shaped like the last, but much smaller. It arises close to its fellow from a little tubercle on the back of the atlas, passes upwards, outwards, and backwards, to be inserted into the os occipitis between the inferior oblique ridge and the foramen mag num. It is partly concealed by the rectus major. This muscle can draw the head back wards.
In quadrupeds these four muscles are pro portionally larger than in man. The inferior oblique and the rectus major are considerably larger. Birds have three recti, and only one oblique—the inferior. Reptiles and fishes may be said to want them, as the analogy is very remote.
On removing the spinalis colli and all the muscles of the fifth layer, we observe nume rous fasciculi of muscular fibres, which are named inter-spinalis, in ter-transversalis, and 2nultifidus spina?. These might be considered a seventh layer ; but they are very analogous to the small muscles just described, and nearly on the same plane.
The inter-spinules are short bundles of fleshy fibres placed between the spinous processes of contiguous vertebrae. They are in pairs in the neck, where the spine consists of two laminae. Here also they are well marked. In the dorsal region they are scarcely visible, and in the loins they are not easily distinguished from an interspinal ligament. They are analogous to the recti postici. They extend the spine.
On the lips of the spinous processes of the neck some fibres may be shown, to which the name supra-spinal muscles has been given. They extend farther than from one vertebra to the next.
The inter-transversales are similar fibres, scarcely to be demonstrated except in the neck, where they are in pairs, corresponding to the divided transverse processes.
The nzultifidus spinae consists of separate bundles of fibres, extending from each trans verse process obliquely upwards and inwards to the spinous process of the vertebra next above, or sometimes to the second above. The first bundle runs from the side or transverse process of the sacrum to the spine of the last lumbar vertebra; the last from the transverse process of the third cervical to the spine of the second. They are smaller as we ascend, and are not easily separated from the spinales and semi spinales. They support the spine, and rotate one vertebra on the other slightly.
In the article SP I N E, the practical utility of a knowledge of the muscles of this extensive region will be demonstrated.