SPINE. 



and the adult ; and in old age is again very different from 

 both. 



It is one of the firft parts formed in the embryo, where it 

 feem.s to be a kind of foundation, on which the Itrufture of 

 the body is erefted. Its form is difcernible at the end of the 

 third week after conception, when we can juft fee the head 

 and a curved line connefted to it. Long before the extremi- 

 ties can be fcen, the fpine continues viiible. The commence- 

 ment of its olTification by .numerous points may be dated 

 about the end of the fi'cond month. The gelatinous fpine, 

 or carina, as it has been called from its incurvated keel-like 

 ihape, is the firil trace of developement in the incubated 



The formation of the fpine is more advanced, and its 

 fize more confiderable, in comparifon with other parts, par- 

 ticularly with the pelvis and lower limbs, at the time of 

 birth. Hence the trunk is proportionably longer. The ge- 

 neral (tature too, at this lime, depends on the fpine, while 

 in the adult it is influenced more by the lower limbs. This 

 \jreat length of the Ipine correfponds to the depth of the 

 oranium, which is much more confiderable proportionally 

 in the fcEtus and young fubjefts than in the adult. The ver- 

 tebral canal being alfo large, the breadth of the column, 

 which depends much on the fize of the canal, is confiderable. 

 All parts which relate to this excavation are much ad- 

 vanced. On the contrary, whatever has reference to fup- 

 port or locomotion, is little developed at this time. The 

 fpinous proceiles do not exill ; confequently the points of 

 attachment for the mufcles are weaker. The bodies are ilill 

 in great meafure cartilaginous ; behmd, where they contri- 

 bute to the canal, they are the moft formed ; but there is 

 much foft fubllance in front, where, when the fpine is dried, 

 we fee merely a feries of rounded bony tubercles. Such a ftate 

 is very unfavourable to the fohd fupport whicli is neceflary 

 for preferving the ereft attitude. At this age, the tranfverfe 

 proceffes are fmall in the loins, where they relate to the 

 mufcles only ; they are much more confiderable in the back 

 and neck, on account of their connexion with the ribs and 

 the vertebral artery. 



The column, at this time, does not form a pyramid, of 

 which the bafis is downwards and the apex upwards. The 

 cervical portion is larger than the lumbar, on account of the 

 developement of the tranfverfe proceiles, which, being 

 placed nearly on the fame line with the bodies, give to the 

 former region a confiderable volume ; and of the compa- 

 ratively imperfeft ftate of the lumbar vertebrae. The dor- 

 fal and lumbar regions are nearly equal in fize ; but in forae 

 inltances the former vertebra; are largeft. As the principal 

 effort, m Handing, is fupported by the loins, the imperfeft 

 Hate of the lumbar vcrtebrx is very unfavourable to the 

 maintenance of that attitude in the newly born child. The 

 imperfeftly developed Hate of the bones coincides with the 

 inconfiderable volume of the pelvis. 



The direftion of the column is remarkable in the foetus : 

 it is itraight, or at leaft exhibits the hardly fenfible com- 

 mencement of thofe curves, which it will prcfcnt in the fc- 

 ([uel. In confequence of this llraightnels, the vacillations 

 which occur in the ereft pollure of the body, and in all the 

 motions executed in th.at polture, are much more fenfible, 

 and the centre u{ gravity is more eafiiy carried beyond the 

 bafis of fupport. Hence another realon why newly born 

 children cannot keep themfelves upright, or move in that 

 attitude. 



The vertebral column, in early age, is fmall and narrow 

 in front, particularly at the back and loins. In confequence 

 of the want of fpinous proceiles, there is great flexibility, 

 particularly backwards. The latter property is favoured by 



the large quantity of foft fubftance in the column ; the car- 

 tilaginous portion of the vertebral bodies being added to 

 the fibro-cartilages. The vertebral channels are very fhal- 

 low, in confequence of the ftate of the fpinous proceiles. 

 The lateral holes arc very large : the laft lumbar vertebra 

 does not form fo marked an angle with the facrum. The 

 upper end of the column does not differ materially from its 

 ftate in the adult. 



As offification proceeds, all the peculiarities juft enume> 

 rated gradually diiappear. The erect attitude is more fecure, 

 and progrcftion i^ performed with greater facility. Hence 

 the fupport of the trunk by all the limbs, is a refult of the 

 organization of the fcEtus. The child, having crawled 

 about for fome time, gradually experiences thofe develope- 

 ments of its bony and mufcular fabric, which enable it to 

 appear in the erect poiture as a biped. The vertebral co- 

 lumn, however, arrives at the ftate which we have defcribed 

 as exifting in the adult, by a very flow progreflion. At the 

 time of puberty, in both fexes, the atlas and the coccyx are 

 complete. In the fix lower cervical vertebra, the proceffes 

 are not yet conlolidated to the body ; and the vertebrse of 

 the back ap.d loins are ftill lefs complete, having a bony plate 

 at each furface of the body, and a bony nucleus in the fpi- 

 nous procefs. Slight marks of the original feparation are 

 ftill vifible in the facrum. In a few years thefe imperfeftions 

 diiappear; the apices of the fpinous procefles of the back 

 and loms being among the laft parts that are confolidated. 



In the old fubjeft, the intervertebral fibro-cartilages Ihrink 

 and become harder : fometimes, but very rarely, they offify. 

 The fpine, at this age, generally bends forwards, apparently 

 from debility of the pofterior mufcles : thus, the force which 

 fliould counterbalance that tendency to fall forwards, arifiiig 

 from the pofition of the vifcera in front of the fpine, is gra- 

 dually loft. To counteraft this inclination of the fpine 

 forwards, and fupport the centre of gravity, the pelvis re- 

 cedes, and the knees and legs projeft in front. The latter 

 circumltance is more ftrongly marked in proportion as the 

 inclination of the fpir.e forwards is more confiderable. The 

 weight of the trunk being carried towards the front, the bafe 

 of fupport muft move forwards alfo. There is always an 

 exadl proportion between the bending of the fpine, the re- 

 ceding of the pelvis, and the advance of the knees. In this 

 cafe, the trunk and lower limbs together exhibit the fame 

 phenomenon as the fpine itfelf in its particular curves, which 

 balance each other in the neck, back, and loins. 



Mechanifm of the Spine. — This muft be confidered in three 

 points of view: lit, with reference to the canal ; zdly, as 

 the point of fupport of the trunk ; and 3dly, as the centre 

 of the motions performed bv the latter. 



1. Of the Canal. — All the arrangements about the verte- 

 bral column are calculated to cnfurc the fafety of this part. 

 The breadth of the vertebral bodies ; the multiplicity and 

 llrength of the uniting ligaments ; the parts lying in front 

 of the fpine in the neck, cheft, and abdomen ; and the thick 

 midcular malfes that fill the vertebral hollows behind ; all 

 concur in protcfting the contents of the canal from external 

 injury. Hence the refiftance which the fpine oppoles to all 

 efforts direfted againft it ; a rcfiltancc which renders lux- 

 ation impoffible. The individual vertebra: are capable of 

 fo very flight a motion, that the medulla fpinalis cannot be 

 comprefled in any inflexions of the fpine. 



2. Mechantfm of the Spine in Standing. — Here tlu- fpine 

 is the point of fupport ot the trunk, of which it tranfmits 

 all the might, together with that of the head, to the pelvis. 



The mode in which the head is fupported on the fpine, is 

 confidered under the article Head. The part in front of 

 the articulation certainly preponderates over that behind ; 



but 



