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Henry Gray (1825–1861).  Anatomy of the Human Body.  1918.
 
cancellous tissue is called the diploë, and this, in certain regions of the skull, becomes absorbed so as to leave spaces filled with air (air-sinuses) between the two tables. The flat bones are: the occipital, parietal, frontal, nasal, lacrimal, vomer, scapula, os coxæ (hip bone), sternum, ribs, and, according to some, the patella.

Irregular Bones.—The irregular bones are such as, from their peculiar form, cannot be grouped under the preceding heads. They consist of cancellous tissue enclosed within a thin layer of compact bone. The irregular bones are: the vertebræ, sacrum, coccyx, temporal, sphenoid, ethmoid, zygomatic, maxilla, mandible, palatine, inferior nasal concha, and hyoid.

Surfaces of Bones.—If the surface of a bone be examined, certain eminences and depressions are seen. These eminences and depressions are of two kinds: articular and non-articular. Well-marked examples of articular eminences are found in the heads of the humerus and femur; and of articular depressions in the glenoid cavity of the scapula, and the acetabulum of the hip bone. Non-articular eminences are designated according to their form. Thus, a broad, rough, uneven elevation is called a tuberosity, protuberance, or process, a small, rough prominence, a tubercle; a sharp, slender pointed eminence, a spine; a narrow, rough elevation, running some way along the surface, a ridge, crest, or line. Non-articular depressions are also of variable form, and are described as fossæ, pits, depressions, grooves, furrows, fissures, notches, etc. These non-articular eminences and depressions serve to increase the extent of surface for the attachment of ligaments and muscles, and are usually well-marked in proportion to the muscularity of the subject. A short perforation is called a foramen, a longer passage a canal.
 
1. Development of the Skeleton
 

The Skeleton.—The skeleton is of mesodermal origin, and may be divided into (a) that of the trunk (axial skeleton), comprising the vertebral column, skull, ribs, and sternum, and (b) that of the limbs (appendicular skeleton).

The Vertebral Column.—The notochord (Fig. 19) is a temporary structure and forms a central axis, around which the segments of the vertebral column are developed. 1 It is derived from the entoderm, and consists of a rod of cells, which lies on the ventral aspect of the neural tube and reaches from the anterior end of the mid-brain to the extremity of the tail. On either side of it is a column of paraxial mesoderm which becomes subdivided into a number of more or less cubical segments, the primitive segments (Figs. 19 and 20). These are separated from one another by intersegmental septa and are arranged symmetrically on either side of the neural tube and notochord: to every segment a spinal nerve is distributed. At first each segment contains a central cavity, the myocœl, but this is soon filled with a core of angular and spindle-shaped cells. The cells of the segment become differentiated into three groups, which form respectively the cutis-plate or dermatome, the muscle-plate or myotome, and the sclerotome (Fig. 64). The cutis-plate is placed on the lateral and dorsal aspect of the myocœl, and from it the true skin of the corresponding segment is derived; the muscle-plate is situated on the medial side of the cutis-plate and furnishes the muscles of the segment. The cells of the sclerotome are largely derived from those forming the core of the myocœl, and lie next the notochord. Fusion of the individual sclerotomes in an antero-posterior direction soon takes place, and thus a continuous strand of cells, the sclerotogenous layer, is formed along the ventro-lateral aspects of the neural tube. The cells of this layer proliferate rapidly, and extending medialward surround the notochord; at the same time they grow backward on the lateral aspects of the neural tube and eventually surround it, and thus the notochord and neural tube are enveloped
Note 1.  In the amphioxus the notochord persists and forms the only representative of a skeleton in that animal. [back]

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