SPINE. 



contraaed towards the middle, and narrow below. They 

 are formed by the feries of vertebral laminx, feparated by 

 larger or fmaller fpaces, which are occupied by the yellow 

 ligaments. They arc hlled by the vertebral mufcles, par- 

 ticularly by the multifidus fpina;. 



The feries of tranfverfe procefl'es occupies each lateral re- 

 gion ; but they have not all the fame direftion : they are 

 more anterior in the neck and loins, and thrown backwards 

 in the back. Between them in the neck, and in front of 

 them in the back and loins, there is a fucceffion of large 

 holes, increafing in iize from above downwards, and giving 

 pafFagc to the vertebral nerves. In front of thefe, m the 

 back, are the furfaces for articulation of the heads of the 



ribs. 



Tiie fuperior extremity, articulated to the occiput, torms 

 a riglit angle with it on each fide ; and an anterior and pof- 

 terior angle, which vary according to the flexion or extenfion 

 of the head. This extremity prefents, from before back- 

 v^ards, the anterior arc of the atlas, the tooth-hke procefs, 

 the commencement of the canal, and the polterior arc of 

 the atlas. On each fide of the canal, towards the front, 

 there is the concavity for the occipital condyle, and the large 

 tranfverfe procefs. The inferior extremity prefents merely 

 the fmall rounded point formed by the laft bone of the 

 coccyx. . 



The length of the fpine does not vary greatly in different 

 individuals"; the differences of ftature depending more on 

 the limbs than on this column. In malformations, however, 

 where it is curved in various direftions, the ftature is much 

 affefted ; and the hnibs, although not exceeding their ordi- 

 nary fize, appear difproportionately long. We have already 

 obferved, that the compreflion of the fibro-cartihges by the 

 weight which they naturally fupport, diminifhes the length 

 of the fpine after long ftanding ; and that the column re- 

 covers its former length by repofe in the horizontal pofture. 

 This effeft is lefs ftrongly marked in the old fubjeft, where 

 the fibro-cartilages are very denfe, and coafequently do not 

 eafily yield to prefTure. 



The fize of the column increafing from above downwards, 

 in proportion to the greater weight and more powerful eflefts 

 it has to fupport, it may be regarded as a pyramid, of which 

 the bafis is below, and the apex above. Yet this augmenta- 

 tion is not gradual throughout ; and the column may be con- 

 fidered as the affemblage of four pyramids. The upper 

 end, including the atlas and axis, deftined for articulation 

 with the head, which it fupports, is larger than the imme- 

 diately fucceeding part of the neck. From the third to the 

 feventh cervical vertebra the fpine is gradually expanded : 

 thence, to the third or fourth dorfal vertebra, it contradls 

 . again ; and is gradually augmented from the latter to the bafis 

 of the facrum : it contrafts again from the bafis of the fa- 

 cruin to the apex of the os coccygis. Meafured tranfverfely, 

 the fpine increafes from the third cervical to the fifth dorfal 

 vertebra, while the remaining dorfal and the lumbar vertebra; 

 are nearly of the fame breadth, or increafe very flightly 

 downwards. The depth of the vertebrse is alfo increafed ; 

 but the inferior cervical are all nearly of one depth. 



The intervertebral intervals are nearly of uniform fize, 

 down to the third dorfal ; from which they "increafe down- 

 wards. The flat furfaces of a vertebra are always parallel 

 to each other, and the intervals are confequently of the fame 

 depth before and behind. There is an exception, however, 

 of the parts in which the column is bent ; the intervals of 

 the third, fourth, fifth, :ind fixth dorfal vertebrae being 

 narroweft in front, while thofe between the fourth and fifth 

 lumbar, and the facrum, are broadelt in front. 



The fpinal column is itraight, when viewed before or be. 



hind ; and might be divided by a perpendicular line into » 

 right and left correfponding half: but, wlien we view it 

 laterally, we find that it is bent in fevcral parts, fo as to de- 

 fcribe a waving line, in which each anterior prominenci' is 

 accompanied by a correfponding poftcrior convexity. The 

 cervical portion projefts flightly in front ; the dorfal is de- 

 cidedly convex behind. The concavity of the dorfal part 

 of the fpine combines with the inclination backwards of the 

 tranfverfe procefl'es to increafe the capacity of the thorax- 

 The greateft depth of this concavity is oppofite to the 

 lower bone of the fternum. In the loins, the fpine is again 

 convex in front, while the facrum and coccyx flant back- 

 wards remarkably, fo as to prefent a deep concavity. The 

 middle of the firil convexity correfponds to the fourth cer- 

 vical vertebra ; that of the fucceeding concavity to the 

 feventh or eighth dorfal ; the middle of the lumbar projec- 

 tion to the junftion of the fifth lumbar vertebra with the fa- 

 crum ; and that of the facral concavity to the middle of the 

 bone. About the third, fourth, and fifth dorfal vertebra, 

 the fpine is fometimes a little convex towards the right. 

 This has been afcribed to the prefence of the aorta ; but 

 that caufe is manifeilly inadequate. The explanation, which 

 is grounded on the greater ufe of the right arm, and a con- 

 fequent inchnation of the trunk to the left fide, in order to 

 afford a firmer point of lupport, appears rather more pro- 

 bable. Any habitual attitude, particularly when perfiiled 

 in from childhood, will affeft the diretlion of the vertebral 

 column. Under various difeafed ftates, remarkable curva- 

 tures are produced ; in which we find that the vertebrx and 

 their fibro-cartilages become accommodated to the unnatural 

 pofition, being greatly diminilhed in depth on the concavity 

 of the curve, fo that the two furfaces are no longer parallel. 

 It is alfo conftant, that a bend in one part renders curvature 

 neceflary in another, in order to preferve the centre of gra- 

 vity in a Itraight line. The pofition of the head and upper 

 limbs is alfo calculated to maintain the equilibrium of the 

 body ; and thus nearly the whole frame is influenced by a 

 caufe originally affefting only a part of the fpine. 



The canal of the fpine extends throughout the column, 

 excepting a fmall portion of the lower end of the facrum 

 and the os coccygis : it is continuous, above, with the cavity 

 of the cranium, and ends below on the back of the facrum. 

 It does not occupy the centre of the column, being placed 

 much nearer to the pofterior part. In the neck it is large ; 

 more ample in the upper than in the lower part of the back, 

 the narroweft part being from the fixth to the ninth : it exr 

 pands a little in the eleventh and twelfth dorfal vertebrae, is 

 very large in the lumbar, and again fuddenly contracted in 

 the facrum. Its figure is triangular, with the angles round- 

 ed ; fo that it has an anterior, and a right and left furface ; 

 it IS more nearly round in the middle, and quite triangular in 

 the facrum. It partakes of all the curvatures of the fpine. 

 It is formed in front of the backs of the vertebral bodies 

 and fibro-cartilages, covered by the poft«rior ligaments. 

 Behind, the lammse and the yellow ligaments compofe it. 

 On each fide it prefents a feries of twenty-nine large fora- 

 mina for the paflage of the nerves ; or there are thirty, if 

 we include the exit of the firft cervical pair, between the oc- 

 ciput and the atlas. 



This canal is lined by a prolongation of the dura mater, 

 including the medulla fpinalis, covered by its arachnoid coat 

 and pia mater. (SeeBRAiv and Nervous Sj//lem.) The 

 dura mater is furrounded by a loofe cellular tillue, contain- 

 ing ferous fluid. 



Developiment of the Spine. — Few parts of the flceleton un- 

 dergo more remarkable changes in the progrefs of oflification 

 than the vertebral column. It differs much in the child 



and 



