116 THE SKELETON. 



The Irregular or Mixed Bones are such as, from their peculiar form, cannot 

 be grouped under either of the preceding heads. Their structure is similar to 

 that of other bones, consisting of a layer of compact tissue externally, and of 

 spongy, cancellous tissue within. The irregular bones are, the vertebrae, sacrum, 

 coccyx, temporal, sphenoid, ethmoid, superior maxillary, inferior maxillary, palate, 

 inferior turbinated, and hyoid. 



Surfaces of Bones. If the surface of any bone is examined, certain eminences 

 and depressions are seen, to which descriptive anatomists have given the fol- 

 lowing names. 



A prominent process projecting from the surface of a bone, which it has 

 never been separate from, or movable upon, is termed an apophysisfaom dn6<t>vois, 

 an excrescence"); but if such process is developed as a separate piece from the 

 rest of the bone, to which it is afterwards joined, it is termed an epipliysis (from 

 iiti$M<Hs, an accretion). 



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. Non-articular eminences are desig- 

 nated according to their form. Thus, a broad, rough, uneven elevation is called 

 a tuberosity; a small rough prominence, a tubercle; a sharp, slender, pointed 

 eminence, a spine ; a narrow rough elevation, running some way along the sur- 

 face, a ridge, or line. 



The non-articular depressions are also of very variable form, and are de- 

 scribed as fossae, grooves, furrows, fissures, notches, etc. These non-articular 

 eminences and depressions serve to increase the extent of surface for the attach- 

 ment of ligaments and muscles, and are usually well marked in proportion to 

 the muscularity of the subject. 



THE SPINE. 



The Spine is a flexuous and flexible column, formed of a series of bones 

 called Vertebrae. 



The Vertebra are thirty-three in number, exclusive of those which form the 

 skull, and have received the names cervical, dorsal, lumbar, sacral, and coccygeal, 

 according to the position which they occupy ; seven being found in the cervical 

 region, twelve in the dorsal, five in the lumbar, five in the sacral, and four in 

 the coccygeal. 



This number is sometimes increased by an additional vertebra in one region, 

 or the number may be diminished in one region, the deficiency being supplied 

 by an additional vertebra in another. These observations do not apply to the 

 cervical portion of the spine, the number of bones forming which is seldom 

 increased or diminished. 



The vertebra in the three uppermost regions of the spine are separate 

 throughout the whole of life; but those found in the sacral and coccygeal 

 regions are, in the adult, firmly united, so as to form two bones five entering 

 into the formation of the upper bone or sacrum, and four into the terminal 

 bone of the spine or coccyx. 



GENERAL CHARACTERS OF A VERTEBRA. 



Each vertebra consists of two essential parts, an interior solid segment or 

 body, and a posterior segment or arch. The arch is formed of two pedicles 

 and two laminae, supporting seven processes, viz., four articular, two transverse, 

 and one spinous process. 



The bodies of the vertebrae are piled one upon the other, forming a strong 

 pillar, for the support of the cranium and trunk; the arches forming a hollow 

 cylinder behind for the protection of the spinal cord. The different vertebrae 

 are connected together by means of the articular processes, and the interverte- 



