SPINAL COLUMN HEAD AND FACE. 253 



parts aro framed. At its first appearance it is a cartilaginous 

 cylinder, surrounding and protecting the primitive trace of the 

 nervous system ; but as the embryo increases in growth, points of 

 ossification are developed corresponding to each vertebra, the whole 

 tube being finally divided into distinct pieces called vertebr&e, to 

 which the bones of the head are a prolongation, corresponding in 

 their nature, though differing outwardly in form. 



The vertebrae, are divisible into true and false, the former reach- 

 ing from the head to the pelvis, and the latter extending thence 

 backward, being respectively called the sacrum and coccyx. 



The true vertebrae, comprise the 7 cervical, 18 dorsal, and 6 lum- 

 bar vertebrae. Each consists of a body, from which two laminae 

 or plates project upwards, terminating in a spinous process. In 

 addition to these are two lateral projections (transverse processes), 

 which serve the purpose of firmly connecting the vertebrae together 

 by means of the muscles attached to them, and also to the ribs and 

 extremities below. Lastly, each vertebra has two small surfaces 

 before and the same number behind (articular surfaces), which 

 form distinct joints between them. 



Between the body, the laminae, and the spinous process, is an 

 opening, more or less triangular in shape, in which lie the spinal 

 cord and its investments. The edges of this opening are attached 

 to those before and behind by ligamentous tissues (ligamenta sub- 

 flava.), which, opposite each intervertebral space, are pierced by 

 openings on each side to give exit to the vertebral nerves passing 

 out to the exterior of the body and to the extremities. Opposite 

 to these openings the bone is notched above and below, and these 

 intervertebral notches complete the parts common to the whole 

 series. Thus the vertebral or spinal column serves as a firmlv se- 

 cured but flexible tube for the lodgment of the spinal cord, * -hile 

 at the same time it gives passage to its nerves. By this forn ation 

 it is far less liable to injury, and also more useful as an aid to loco- 

 motion, than if it were made of one solid piece of bone, which, from 

 its length, would be readily broken. 



OF THE HEAD AND FACE, AND OF THE HYOID ARCH. 



MODERN ANATOMISTS, following out the idea first suggested by 

 Maclise and Owen, consider the head as made up of six vertebrae ; 

 the posterior one, or that nearest to the neck, being the occipital 

 bone, the next two being made up of the temporal bone, and the 

 ultimate vertebrae consisting of the sphenoid and aethmoid bones. 

 This is a somewhat fanciful hypothesis, when worked out in detail ; 

 but it is obvious that the several bones of the skull subserve the 

 same purposes as the vertebrae, and resemble those parts of the 

 skeleton in forming a series of irregular arches to protect the 

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