SPINE. 



but the difference is lefs confiderable, than it would appear 

 from noticing merely the relation of the occipital condyles to 

 the front and back of the Iiead, in confequence of the anterior 

 portion containing the face, of which the weight does not 

 correfpond to the bulk, while all the pofterior part is folid 



and heavy. . , r ■ r 



If the pofition of the head tends to incbnc the Ipme tor- 

 wards, the fame effeft is much more ftrongly produced by 

 the fituation of the pedoral and abdominal vifcera wholly in 

 front, without any thing to counterbalance them behind. 

 The mufcles of the vertebral channels, inferted either in the 

 ribs or the vertebra:, exert a conftant effort in oppofition to 

 that of the anterior vifcera, maintaining by their exertion the 

 vertebral column, and, confequently, the trunk, in the ereft 

 attitude. The greateit effort is below, where the fpine fup- 

 ports the molt confiderable weight. Here the mufcles are 

 thicker, the bones large, and their proceffes more prominent. 

 Here, too, the fenfe of fatigue is felt after long Itanding ; 

 whence arifes the praftice of tying a firm band round tlie 

 loins, to fupport the mufcles and favour their adion. 



Thus, in Handing, the fpine is placed between two forces 

 afting in oppofite direftions ; an anterior paffivc one, which 

 is the trunk ; a pofterior aftive one, the aftion of the ex- 

 tenfor mufcles. The habitual tendency of the fpine to bend 

 forwards, produced by the weight of the vifcera, is further 

 obviated by the hip-joints, at which the trunk is fultained 

 on the lower limbs, being placed in front of the facro-iliac 

 fymphyfes. The bafis of the trunk occupies the fpace be- 

 tween the latter fymphyfes and the cotyloid cavities. It 

 will be readily underltood that falls muft have been much 

 more frequent, if the thighs had been articulated on tlie fame 

 bne with the facrum. The waving line defcribed by the 

 ▼ertebral column, and allowing extenfive motion in the 

 upper parts, without the line of gravity, which paffes through 

 all the curves, abandoning the bafe of fupport, is another 

 difpofition very favourable to the fohdity of the trunk in its 

 ereft attitude. The gradual increafe in fize of the column, 

 from above downwards, contributes to the fame end. Thus, 

 ftanding cannot be regarded, by any means, as an attitude 

 of repofe ; it requires great efforts, which naturally caufe 

 confiderable laflitude, if the pofture be long maintained. 



3. Motions of the Spine. — The general motions are, for- 

 wards, backwards, to cither fide, or in the intermediate 

 direftions. Inclination forwards, or flexion, as it is com- 

 monly termed, is the moft; extenfive, as all our principal 

 efforts have this direftion. In this motion, the anterior 

 vertebral ligament is relaxed ; the fibro-cartilages are loofened 

 in front, and ftretched behind ; the pofterior vertebral liga- 

 ment, the yellow ligaments, and tbofe of the fpinous pro- 

 ceffes, are itretched. The laminae of the fibro-cartilage?, 

 being much more numerous in front than behind, yield more 

 readily to motion in the former than in the latter direftion. 

 In extenfion, we obferve phenomena exaftly contrary to thefe. 

 The mufcles, however, which produce extenfion, being fixed 

 to the vertebral proceffes, are very near the centre of mo- 

 tion, and, confequently, aft lefs powerfully than the flexors, 

 which operate through the medium of the ribs, and, confe- 

 quently, at a diltance from the centre of motion. The fpi- 

 nous proceffes, too, particularly of the back, limit extenfion 

 very much. 



In inclination to either fide, the fibro-cartilages are prin- 

 cipally affefted ; being ftretched on one fide and relaxed on 

 the other. The ribs principally limit this motion, by coming 

 in contaft with each other. 



All thefe motions are much more fenfible at the junftion 

 of the dorfal and lumbar regions than elfewhere. When they 

 are all performed in fucceffiim, the trunk is carried round in 



a circular direftion, defcribing a cone, of which the bafis it 

 upwards, the apex downwards. 



Befides the motions jufl; enumerated, a power of rotatioa 

 has been afcribcd to the fpine, in which the vertebrse are 

 fuppofed to be twilled on their axes. Winflow admits this, 

 and Bichat does the fame ; adding, however, that it can only 

 be accompliftied with confiderable difficulty. He calls it 

 a fpecies of general twilling, in which all the ligaments are 

 ftretched, refulting from the partial twifting of each fibro- 

 cartilage. When we confider how the vertebrse are joined 

 together ; when we refleft on the nature of the fibro-carvi- 

 lages, on the yellow ligaments, on the mechanical locking 

 of the articular proceffes, and more particularly on the 

 attachments of the ribs, and the overlapping fpines in the 

 back, we Ihall begin to doubt whether this rotation be 

 poffible. Dr. Barclay decidedly denies it. (Mufcular 

 Motions, p. 337, et feq. ) If the head and (houlders be pre- 

 vented from moving on the trunk, and the pelvis be alfo kept 

 motionlefs, no rotation of the fpine can be effefted. An 

 appearance takes place, which might be afcribed to fuch a 

 rotation, from a motion of the pelvis on the hips. This may 

 eafily be detefted, by placing a rod of fome feet in length 

 acrofs the pelvis, when any motion of the latter will be itn- 

 mediately vifible at its extremity. 



The motions vary in each region : on account of the fmall- 

 nefs of the vertebrae, the obliquity of the articular proceffes, 

 the (hortnefs and horizontal projeftion of the fpinous pro- 

 ceffes, which are not tied together by interfpinal ligaments, 

 the quantity of motion in all the direftions already enu- 

 merated is confiderable. The cervical region either repre- 

 fents a fimple lever, or it may be confidered, in conneftion 

 with the head, as an angular lever, and will follow all the 

 motions of that part. In one part of this region there is a 

 true rotatory motion ; it is at the articulation of the atlas 

 with the axis. Whenever the head is twifted to one fide or 

 the other, there is a revolution of the former bone on the 

 latter, and not any general twifting of the fpine. 



Every thing concurs to limit motion in the back : for ex- 

 ample, the articulations of the ribs to the bodies and tranf- 

 verfe proceffes, and their conneftion to the fternum in front ; 

 the length and overlapping of the fpinous proceffes. 



In the loins, again, there is free motion ; particularly 

 above, at the junftion of this region with the back. 



The motions of the individual vertebras are extremely ob- 

 fcure ; but they take place in the fame direftion as the ge- 

 neral motion which we have jull defcribed. The vifible 

 movement of any part of the fpine refults, therefore, from 

 the combined effeft of feveral hardly fenfible motions : one 

 vertebra could not move fufficiently on another to produce a 

 perceptible motion of the trunk. This circumilance con- 

 curs with feveral others relating to the mode of union of the 

 vertebrse, and the mechanical confinement of the articular 

 proceffes, to render luxation impoffible : meaning by that 

 term, a difplacement of the bone without any frafture. The 

 effort is diitributed over a confiderable region, and cannot 

 therefore aft fufficiently on a fingle bone. Neither can any 

 force be eafily applied before or behind, lufficient to drive 

 out one vertebra from its place. Moreover, were fuch a 

 force aftually applied, the vertebra could not be difplaced 

 forwards without a frafture of its inferior, nor backwards, 

 without that of its fuperior articulating proceffes. 



Of the mufcles which are concerned in thefe motions, 

 fome aft direftly on the trunk, while others affeftit through 

 the medium of the ribs, the fcapulae, &c. All of them, being 

 placed on the right or left fide of the body, muft have the 

 power of moving the trunk laterally, more or lefs, accord- 

 ing to their degree of obliquity, or their diltance from the 



mefial 



