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  "text": "Section 9 of Grey's Anatomy, Part 1. This is a LibriVox recording. All LibriVox recordings are in the public domain. For more information or to volunteer, please visit LibriVox.org. Recording by Morgan Scorpion. Anatomy of the Human Body, Part 1 by Henry Grey. Osteology. Introduction. Development of the Skeleton. The general framework of the body is built up mainly of a series of bones, supplemented, however, in certain regions by pieces of cartilage. The bony part of the framework constitutes the skeleton. In the skeleton of the adult, there are 206 distinct bones, as follows. The axial skeleton, divided into the vertebral column with 26 bones, the skull with 22 bones, the hyoid bone, 1, the ribs and sternum, 25. 74 bones in total. The appendicular skeleton, upper extremities with 64 bones, lower extremities with 62 bones. 126 altogether, and the auditory ossicles, 6. Total of 206. The patellae are included in this enumeration, but the smaller sesamoid bones are not reckoned. Bones are divisible into four classes, long, short, flat and irregular. Long bones. The long bones are found in the limbs, and each consists of a body or shaft and two extremities. The body, or diaphysis, is cylindrical, with a central cavity termed the medullary canal. The wall consists of dense, compact tissue of considerable thickness in the middle part of the body, but becoming thinner toward the extremities. Within the medullary canal is some cancellous tissue, scanty in the middle of the body, but greater in amount towards the ends. The extremities are generally expanded for the purpose of articulation and to afford broad surfaces for muscular attachment. They are usually developed from separate centers of ossification termed epiphyses, and consist of cancellous tissue surrounded by thin compact bone. The medullary canal and the spaces in the cancellous tissue are filled with marrow. The long bones are not straight but curved, the curve generally taking place in two planes, thus affording greater strength to the bone. The bones belonging to this class are the clavicle, humerus, radius, ulna, femur, tibia, fibula, metacarpals, metatarsals, and phalanges. Short bones. Where a part of the skeleton is intended for strength and compactness combined with limited movement, it is constructed of a number of short bones, as in the carpus and tarsus. These consist of cancellous tissue covered by a thin crust of compact substance. The patelli, together with the other sesamoid bones, are by some regarded as short bones. Flat bones. Where the principal requirement is either extensive protection or the provision of broad surfaces for muscular attachment, the bones are expanded into broad flat plates, as in the skull and the scapula. These bones are composed of two thin layers of compact tissue enclosing between them a variable quantity of cancellous tissue. In the cranial bones, the layers of compact tissue are familiarly known as the tables of the skull. The outer one is thick and tough, the inner is thin, dense and brittle, and hence is termed the vitreous table. The intervening cancellous tissue is called the diploae, 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 coxae, 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 vertebrae, 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, for tuberance 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 fossae, 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 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 is a temporary structure and forms a central axis around which the segments of the vertebral column are developed. Note 11. In the amphioxus, the notochord persists and forms the only representative of a skeleton in that animal. End note. It is derived from the enterderm 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 midbrain 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. These are separated from one another by intersegmental scepter 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 myocoil, 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. The cutis plate is placed on the lateral and dorsal aspect of the myocoil 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 myocoil and lie next to the notochord. Fusion of the individual sclerotomes in an anteroposterior direction soon takes place and thus a continuous strand of cells, the sclerotogenous layer, is formed along the ventrolateral aspects of the neural tube. The cells of this layer proliferate rapidly and extending medialwards 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 by a continuous sheath of mesoderm which is termed the membranous vertebral column. In this mesoderm the original segments are still distinguishable but each is now differentiated into two portions, an anterior consisting of loosely arranged cells and a posterior of more condensed tissue. Between the two portions the rudiment of the intervertebral fibre cartilage is laid down. Cells from the posterior mass grow into the intervals between the myotomes of the corresponding and succeeding segments and extend both dorsally and ventrally. The dorsal extensions surround the neural tube and represent the future vertebral arch while the ventral extend into the body wall as the costal processes. The hinder part of the posterior mass joins the anterior mass of the succeeding segment to form the vertebral body. Each vertebral body is therefore a composite of two segments being formed from the posterior portion of one segment and the anterior part of that immediately behind it. The vertebral and costal arches are derivatives of the posterior part of the segment in front of the intersegmental septum with which they are associated. This stage is succeeded by that of the cartilaginous vertebral column. In the fourth week two cartilaginous centres make their appearance, one on either side of the notochord. These extend around the notochord and form the body of the cartilaginous vertebra. A second pair of cartilaginous foci appear in the lateral parts of the vertebral bow and grow backward on either side of the neural tube to form the cartilaginous vertebral arch and a separate cartilaginous centre appears for each costal process. By the eighth week the cartilaginous arch has fused with the body and in the fourth month the two halves of the arch are joined on the dorsal aspect of the neural tube. The spinous process is developed from the junction of the two halves of the vertebral arch. The transverse process grows out from the vertebral arch behind the costal process. In the upper cervical vertebrae a band of mesodermal tissue connects the ends of the vertebral arches across the ventral surfaces of the intervertebral fibre cartilages. This is termed the hypochordal bar or brace. In all except the first it is transitory and disappears by",
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