602 GEOLOGICAL SURVEY OF THE TERRITORIES. 
extremity of the column, its calibre gradually increases in each ver- 
tebra as we proceed toward the thoracic extremity, until it attains its 
maximum capacity at the eleventh vertebra. In the twelfth the in- 
tegrity of its walls is lost by a parting of the par- and pleur-apo- 
physial elements, with a disappearance of the former, leaving it no 
floor, so that in this vertebra it ceases to be a closed canal. The most 
prominent object presenting itself for examination in the atlas, superi- 
orly, is the deep reniform cavity for articulation with the occipital con- 
dyle of the basi-cranii. It makes up to the entire superior articulating 
surface of what would first appear to be the centrum. of this vertebra, 
unless we should not consider such to be the ease until the odontoid 
process of the vertebra next below, tbe true centrum of the atlas, lends 
its assistance, in which event the surface of this articulation is only 
complete when made so by the extremity of the process just alluded 
to. A membrane sometimes stretches across this interspace, sepa- 
rating the extremity of the odontoid from the condyle of the ocei- 
put; this is not invariably the case, however, as in many of the 
individuals we have examined a minute vacuity exists, allowing the 
process to come in immediate contact with the condyle at one point. 
Below and posteriorly there is another articulating surface, convex for 
the centrum of the axis and concave for its odontoid process, accurately 
meeting the opposed surface of this vertebra and forming the atlo-axoid 
articulation. A lip of bone, a portion of the hypapophysis of the verte- 
bra we are now describing, projects downwards and shields this joint in 
front, overlapping, indeed, a good part of the axis. The neurapophyses 
of the atlas are slight in structure. The concave posizygapophyses ar- 
ticulate with the convex prezygapophyses of the axis. The bone is de- 
void of a neural spine. In the axis we find both an hypapophysis and 
neural spine developed, the former being produced from the ridge on the 
anterior aspect of the centrum of the bone. The odontoid process arises 
vertically from the posterior margin of the upper surface of the centrum. 
Its summit and anterior face are convex and articulating, while behind 
it is flat and continuous with the spinal canal. The facet for articula- 
tion with the centrum of the third vertebra looks downwards and in- 
wards, is convex from side to side and concave in the opposite direction. 
The postzygapophyses are concave, look downwards and outwards, the 
conditions in the prezygapophyses being exactly the opposite; this is 
the rule throughout the cervical portion of the column. After we pass 
the atlas and “axis, we find in the third cervical vertebra, here, as in 
most vertebrates, parts that are common to the series of this portion of 
the column, deviating but slightly from each other as we examine them 
in seriatim ; but gradually as this deviation, proceeds, some requisite con- 
dition is brought about when the climax is attained. The fact of the 
presence of a neural spine on the axis is conveyed, though in a less 
marked degree, to the third or next vertebra below, where it occupies a 
position about in the middle of the bone. As we descend, this process 
becomes less and less prominent, being found set further back on each 
successive vertebra; it disappears about entirely at the tenth, after 
which it rapidly begins to make its appearance again, assuming its former 
position in the middle of the vertebra, being quite evident in the twelfth 
in the shape of a pointed spine, while in the fourteenth it bears the quad- 
rate form, with extended crest, being the first step towards an assumption 
of that notorious feature found further on in the dorsals. In the third 
_ vertebrathe space between the pre- and post-zygapophyses is almost» en- 
tirely filled in, a minute foramen on either side alone remaining, by a 
lumina of bone extending from one process to the other, giving to this 
