34 THE SKELETON. 



skull ; the outer one is thick and tough ; the inner one thinner, denser, and more 

 brittle, and hence termed the vitreous table. The intervening oancellous tissue 

 is called the diploe. The flat bones are : the occipital, parietal, frontal, nasal, 

 lachrymal, vomer, scapula, os innominatum, sternum, ribs, and patella. 



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 vertebrce, sacrum, coccyx, 

 temporal, sphenoid, ethmoid, malar, 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 following 

 names. 



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 designated 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 surface, a ridge or line. 



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

 as fossa?, grooves, furroios, fissures, notches, etc. These non-articular eminences 

 and depressions serve to increase the extent of surface for the attachment of liga- 

 ments and muscles, and are usually well marked in proportion to the muscularity 

 of the subject. 



A prominent process projecting from the surface of a bone, which it has never 

 been separate from or movable upon is termed an apophysis (from djibtyuatz, an 

 excrescence) ; but if such process is developed as a separate piece from the rest of 

 the bone, to which it is afterward joined, it is termed an epiphysis (from Ixiyuotz, 

 an accretion). The main part of the bone, or shaft, which is formed from the 

 primary centre of ossification, is termed the diaphysis, and is separated, during 

 growth, from the epiphysis by a layer of cartilage, at which growth in length 

 of the bone takes place. 



THE SPINE. 



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

 vertebrce (from vertere, to turn). 



The Vertebrae are thirty-three in number, and have received the names 

 cervical, dorsal, lumbar, sacral, and coecyyeal, 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 vertebrae in the upper three 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 anterior solid segment or 

 body, and a posterior segment or arch. The arch (neural") is formed of two pedi- 

 cles and two lamince, supporting seven processes viz. four articular, two trans- 

 verse, and one spinous. 



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



