The erector spinas is composed of two planes of longitudinal fibres, aggregated together, below, to form one mass at their point of origin, from the spines and posterior surface of the sacrum, from the sacro-iliac ligament, and from the posterior third of the iliac crest. It divides into two portions, the sacro-lumbalis, and the longissimus dorsi.

The former, arising from the iliac crest or from the pleurapophysis (rib) of the first sacral vertebra, is inserted by short flat tendons into (1.) the apices of the stunted lumbar ribs, close to the tendinous origins of the tranversalis abdominis ; (2.) the angles of the eight or nine inferior dorsal ribs; (3.) it is inserted, through the medium of the musculus accessorius, into the angles of the remaining superior ribs, and into the long and occasionally distinct pleurapophysial element of the seventh cervical vertebra ; and (4.) through the medium of the cervi-calis ascendens, into the pleurapophysial elements of the third, fourth, fifth, and sixth cervical vertebrae. In other words, the muscular fibres extend from rib to rib, from the sacrum to the third cervical vertebra.

The longissimus dorsi, situated nearer the spine than the sacro-lumbalis, is inserted, (1.) into the me-tapophysial spine of the lumbar diapophyses ; (2.) into the diapophyses of all the dorsal vertebrae, near the origin of the levatores costarum; (3.) through the medium of the transversalis colli into the diapophyses of the second, third, fourth, fifth, and sixth cervical vertebrae, and (4.) through the medium of the trachelo-mastoid into the mastoid process, or the only element of a transverse process possessed by the parietal vertebra. In other words, its fibres extend from diapophysis to diapophysis, from the sacrum, upwards, to the parietal vertebra.

The proper connections of the occipital haemal arch (scapulo-coracoid), can be better studied in fish than in warm-blooded animals, where it is displaced to the dorsal region. Cuvier seems to have entertained some doubt as to the propriety of comparing the pectoral extremity of fish with that of other vertebrate animals ; "Comme le membre pectoral des poissons ne peut être comparé qu' avec quelque doute à celui des autres animaux vertébrés, nous avons cru devoir en traiter particulièrement et dans son ensemble.*" But M. Dumeril, under whose revision the above quoted posthumous work has appeared, speaks much more clearly as to the homologies of the scapulo-coracoid arch. "The pectoral fins are attached to a bony ring which surrounds the trunk behind the branchial apparatus, limits its orifice behind, and forms a kind of sill upon which the operculum strikes when it closes. This ring, when it is complete, is composed of three bones of each side, united by the squamous suture, or rather by imbrication, articulated to the posterior superior angle of the cranium, and descending under the throat to unite, most frequently by means of ligament, but sometimes by suture, with those of the opposite side. This apparatus can be regarded as the bones of the shoulder. Behind and below, there adhere to them two or three other bones, holding the place of the bones of the arm and forearm, and supporting the pectoral fin which represents the hand."+

* Leçons d'Anatomie comparée, t. i. p. 366.

+ Op. Cit., p. 460.

The haemal arch is again completed in the parietal vertebra by the styloid processes, the stylo-hyoid ligaments, the lesser cornua, and the body of the os hyoides, the latter sending back two long thyro-hyals (great cornua) which support the organ of voice or larynx and the succeeding rings of the trachea. The tenacity which the parietal pleurapo-physis exhibits to affix itself to its proper vertebral segment is illustrated in the cranium of the ass, where the mastoid bone (parietal diapophysis) sends a process down between the tympanic bone and the par-occipital to unite with the proximal extremity of the styloid process.

The special homologies of haemal arches of the frontal and nasal vertebrae, have been pointed out in the description of the cranial bones. The arch-like form of the inferior maxilla or the mandible will be recognized at once; that of the superior maxilla is obscured by the development of appendages, which bind the pleurapophysis (palate bone) to the base of the skull, and the hsemapophysis (superior maxilla) to the cranial walls.