Fig. 32.—A type of vertebra. (Leidy.) 1, Body; 2, pedicle; 3, lamina; 4, spinal foramen; 5, spinous process; 6, transverse process; 7, articular process.
Fig. 33.—The sacrum, from before.
(Drawn by D. Gunn.)
Some of the dorsal vertebræ are peculiar in the arrangement of their facets and demi-facets, while among the cervical vertebræ are several whose peculiarities should be more carefully noted. Thus, the first cervical vertebra or atlas supports the head and has practically no body, the place of the body being taken by a narrow anterior arch of bone and an opening, continuous with the spinal foramen, into which the odontoid process of the axis fits, being held in place by ligaments. At either side on top is a facet for articulation with the occipital bone. There is almost no spine. The second vertebra or axis has surmounting the body the odontoid process, with a facet in front for articulation with the atlas and one behind for the transverse ligament to move over. The seventh cervical vertebra or vertebra prominens has a very long spinous process—hence name—to which is attached the ligamentum nuchæ. It can be felt very distinctly on the living.
Running from the skull down through the spinal column into the sacral vertebræ and formed by the joining of the [spinal foramina] of the individual vertebræ is an opening called the spinal canal, which holds the cord. The cord, however, stops practically at the first lumbar vertebra, where it splits up into the cauda equina, only the filum terminate extending farther down.
Occasionally the laminæ do not form completely and the membranes of the cord may bulge out and form a tumor, or the cord itself may come out also. This generally occurs in the lumbar region, where it is known as spina bifida. If in case of fracture of a vertebra there is paralysis of the parts below due simply to the pressure of a fragment of bone upon the cord, it may be completely cured by removal of the fragment. If, however, the cord suffers injury, the paralysis will remain. Humpback or Pott’s disease is caused by the tubercle bacillus, which eats away the bodies of the vertebræ so that the column caves in and the spinous processes are thrown out in a hump or kyphos.
Fig. 34.—Muscles of the right side of the head and neck: 1, Frontalis; 2, superior auricular; 3, posterior auricular; 4, orbicularis palpebrarum; 5, pyramidalis nasi; 6, compressor naris; 7, levator labii superioris alæque nasi; 8, levator labii superioris; 9, zygomaticus major; 10, orbicularis oris; 11, depressor labii inferioris; 12, depressor anguli oris; 13, anterior belly of digastric; 14, mylohyoid; 15, hyoglossus; 16, stylohyoid; 17, posterior belly of digastric; 18, the masseter; 19, sternohyoid; 20, anterior belly of omohyoid; 21, thyrohyoid; 22, 23, lower and middle constrictors of pharynx; 24, sternomastoid; 25, 26, splenius; 27, levator scapulæ; 28, anterior scalenus; 29, posterior belly of omohyoid; 30, middle and posterior scalenus; 31, trapezius. (Dorland’s Dictionary.)
Muscles of the Neck.—Before speaking of the muscles of the back a few of those of the neck had best be taken up. They are numerous but mostly of minor importance. Largest and most important is the sterno-cleido-mastoid muscle, which has its origin on the upper part of the sternum and the inner third of the clavicle and is inserted into the mastoid process of the temporal bone. It passes obliquely across the side of the neck and serves to flex the head to the side and to draw the face in the opposite direction. When both muscles contract the head is flexed on the neck and the neck on the chest. In wry neck or torticollis this muscle is constantly contracted. The platysma myoides arises from the fascia over the pectoral, deltoid, and trapezius muscles and is inserted into the lower jaw, the angle of the mouth, and the loose tissue in the lower part of the face. It wrinkles the skin of the neck and depresses the lower jaw. In the cow and horse it is so highly developed that by it the skin can be contracted all over the body to drive off flies. The rectus capitis anticus major arises from the third to the sixth cervical vertebræ and is inserted into the occipital bone, serving to flex the head. The [scalenus muscles] have their origin on the lower cervical vertebræ and are inserted into the first and second ribs, thus aiding in the elevation of the ribs as well as in lateral flexion of the neck. The head is held upright by the ligamentum nuchæ, which rises from the external occipital protuberance and is inserted into the spinous processes of all the cervical vertebræ except the first.