THE SPINE 5 



nearly vertical plane of compact tissue projects into the neck of the l)one from the 

 inferior cervical tul)ercle towards the great troclianter. This is ])laced in the line 

 through which the Aveight of the l)ody falls, and adds to the stability of the neck 

 of the Ijone: it is said to be liable to absorption in old age. In the lower end of 

 the bone, the vertical and horizontal tibres are so disposed as to form a rectangular 

 niesliwork. 



Tlie i)lan of construction exhil)ited by the femur is the most complex in the 

 skeleton, l)ut the principles involved are the same in all bones. An interesting 

 disposition of these curves is exhibited in the head of the humerus. The pressure 

 curves radiate in two directions: one set at right angles to the articular surface of 

 the head of the bone; the other at right angles to the greater tuberosity. The 

 last set, like those in the greater trochanter, are the result of tension exerted by the 

 muscles attached to these })rominences. 



The shafts of long bones at the time of -birth are mainly cylindrical and free 

 from ridgt'S. The majority of the lines and ridges so conspicuous on tlie shafts of 

 long l)ones in adults are due to the; ossification of muscle-attachments. The more 

 developed the muscles, the larger the ridges become. 



The surfaces of bones are variously modified by environing conditions. Pres- 

 sure at the extremities causes enlargement, and movement renders them smooth. 

 The two causes combined produce an articular surface, ^^'hen rounded and sup- 

 ported upon a constricted portion of bone, an articular sin-face is termed a head, 

 sometimes a condyle ; when depressed, a glenoid fossa. Blunt, non-articular j»ro- 

 cesses are called tuberosities ; smaller ones, tubercles ; sliarp prtijections, spines. 

 Slightly elevated ridges of l)ones are crests ; when narrow and i)ronounced, lines 

 and borders. A shallow depression is a fossa ; when narrow and deep, a groove ; 

 a perforation is usually called a foramen. The majority of terms — such as canal, 

 spine, notch, sulcus, and the like — are so obvious as to render explanation need- 

 less. 



( 



THE SPINE 



The spine (vertebral column) consists of thirty-three superimposed ])ones 

 termed vertebrae. Of these the upper twenty-foiu* remain separate throughout life 

 and form three groups. The first seven are called cervical, the succeeding twelve 

 thoracic (dorsal), and the next five lumbar. In adult life the last nine verteljrte 

 ankylose to form two composite bones named the sacrum and the coccyx. The 

 sacrum is formed by the fusion of five vertebra? from tlie twenty-fifth to the twenty- 

 ninth inclusive; the four terminal are vestigial, and form tlie coccyx. In order to 

 gain a general notion of the characters of a vertebra, it is desirable to select a bone 

 from the middle of the thoracic series. 



A vertebra consists of a body and an arc-h: The body or centrum is a solid disc 

 of ])one slightly concave on its superior and inferior aspects, and wider transversely 

 than antero-posteriorly. The upper and lower surfaces are rough for intervertebral 

 discs, and the margins are slightly lipped. The circumference of the body is, in 

 front, concave vertically, but convex from side to side; posteriorly it is excavated, 

 and presents foramina for the escape of veins from the cancellous tissue. On the 

 sides of the body, at the upi)er and lower angles, there are four demi-facets; wlien 

 two vertebne are superimposed, the adjacent demi-facets form a complete articular 

 facet for tlie head of a rib. 



The arch is formed liy two pedicles and two lamina\ and has connectcfl with 

 it seven processes — one spinous, two transverse and four articular. The pedicles are 

 two constricted short piers of l)one prcijecting horizontally backwards from the 

 upper angles of the posterior surface. The lower liorder of each jx'dicle is deeply 

 notched; hence, when two vertebra? are in jwsition the notches are converted into 

 inter\^ertebral foramina for the transmission of spinal nerves and vessels. 



