mentally round the tooth-like process of 
the vertebra dentata, and as the head is 
closely connected to the atlas, it is carried 
round at the same time. Therefore the la- 
teral or rotatory motions of the head are 
performed by a different joint from that 
which performs the nodding motions. Nei- 
ther of these articulations admits of very ex- 
tensive motion ; but the deficiency is com- 
pensated by the mobility of the vertebrae, 
which enable us to carry the head freely in 
any direction we may wish. The head rests 
nearly in equilibrio on the spinal column ; 
yet, if left to itself, it would fall forwards, 
as the joint is not precisely in the centre of 
the basis cranii. To counteract this ten- 
dency there is a ligamentous substance ex- 
tended from the spinous processes of the 
cervical vertebra* to the occiput, and cal- 
led the ligamentum nucliae. In quadrupeds 
this can be best seen, as the weight of the 
head is there supported to a much greater 
disadvantage. The muscles also contribute 
to keep the head upright; and hence, when 
a man drops asleep sitting, the relaxation of 
the extensor muscles causes the head to nod 
forwards. 
Joints of the spine . — The spine, or back- 
bone, is a chain of joints of very wonderful 
construction. Various, difficult, and almost 
inconsistent offices were to be executed by 
the same instrument. It was to be firm, 
yet flexible ; firm, to support the erect po- 
sition of the body; flexible, to allow of the 
bending of the trunk in all degrees of cur- 
vature. It was further also to become a 
pipe or conduit for the safe conveyance of a 
most important part of the animal frame, 
the spinal marrow; a substance, not only of 
the first necessity to action, if not to life, 
but of a nature so delicate and tender, so 
susceptible, and so impatient of injury, as 
that any unusual pressure upon it, or any 
considerable obstruction of its course, is 
followed by paralysis or death. It was also 
to afford a fulcrum, stay, or basis for the in- 
sertion of the muscles which are spread 
over the trunk of the body, in which trunk 
there are not, as in the limbs, cylindrical 
bones to which they can be fastened; and 
likewise, which is a similar use, to furnish a 
support for the ends of the ribs to rest 
upon. 
The breadth of the bases, upon which the 
parts severally rest, and the closeness of the 
junction, give to the chain its firmness and 
stability ; the number of parts, and conse- 
quent frequency of joints, its flexibility ; 
which flexibility, yve may also observe, va- 
ries in different parts of the chain; is least 
in the back, where strength more tha® 
flexure is wanted ; greater in the loins, which 
it was necessary should be more supple than 
the back ; and greatest of all in the neck, 
for the free motion of the head. Then, 
secondly, in order to afford a passage for 
the descent of the medullary substance, each 
of these bones is bored through in the mid- 
dle in such a manner, as that, when put to- 
gether, the hole in one bone falls into a line 
and corresponds with the holes in the two 
bones contiguous to it ; by which means the 
perforated pieces, when joined, form an en- 
tire, close, uninterrupted channel. But, a*, 
a settled posture is inconsistent with its use, 
a great difficulty still remained, which was 
to prevent the vertebrae from shifting upon 
one another, so as to break the line of the 
canal as often as the body moves or twists, 
or the joints gaping externally whenever the 
body is bent forwards, and the spine there- 
upon made to take the form of a bow. 
These dangers, which are mechanical, are 
mechanically provided against. The verte- 
brae, by means of their processes and pro- 
jections, and of the articulations which some 
of these form with one another at their ex- 
tremities, are so locked in and confined, as 
to maintain in what are called the bodies or 
broad surfaces, of the bones the relative po- 
sition nearly unaltered ; and to throw the 
change and the pressure produced by flexion 
almost entirely upon the intervening carti- 
lages, the springiness and yielding nature of 
whose substance admits of all the motion 
which is necessary to be performed upon, 
them, without qny chasm being produced 
by a separation of the parts. I say of all 
the motion which is necessary ; for, although 
we bend our backs to every degree almost 
of inclination, the motion of each vertebra 
is very small : such is the advantage which 
we receive from the chain being composed 
of so many links. Had it been composed of 
three or four bones only, in bending the 
body the spinal marrow must have been 
bruised at every angle. 
The substances which connect the bodies 
of the vertebrae to each other, called the in- 
tervertebral cartilages, are thick, firm, and 
elastic. They are similar in shape, and 
nearly so in size to the bones which they 
join. They are thicker before than behind, 
so that, when we stoop forwards, the com- 
pressible cartilage, yielding to the force, 
brings the surfaces of the adjoining vertebrae 
nearer to a state of parallelism than they 
were before, instead of increasing the inch- ’ 
nation of their planes, which must have oc- 
casioned a fissure or opening between them : 
