y-o.i.] SHUFELDT ON THE OSTEOLOGV OF SPEOTYTO, 95 



"cularity of tlie dorsal vertebra). The vertebral canal begins, circular, on 

 either side at the third cervical vertebra, most of its length beiug im- 

 mediately beneath the prezygapophyses of each segment. It is formed 

 in the usnal manner by the di- and par-apophysial processes uniting 

 laterally with the pleurapophysial elements. Small at the cephalic 

 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 reuiform 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 case until the odontoid 

 process of the vertebra next below, the 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, however, always stretches across this interspace, 

 separating the extremity of the odontoid from the condyle of the oc- 

 ciput. This is not invariably the case, either, as in many of the 

 individuals we have examined a minute vacuity usually 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 j)ostzygapophyses 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 prodnced from the ridge on the 

 anterior aspect of the centrum of the bone. The odontoid process arises 

 vertically from the posterior margin of the upi^er 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 

 €onditions in the prezygapophyses being exactly the opposite ; this is 

 the rule throughout the cervical portion of the column. After we pa«8 

 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 



