CLELAND ON THE MAMMALIAN AXIS, ATLAS, AND OCCIPITAL BONE. 153 



in the cervical region the arches are bulged outwards at the points where 

 they are placed (Fig. 5). The axis is shaped altogether like one of the 

 succeeding vertebrae, except only that the odontoid process is super- 

 added to the centrum : and the bulging of the arch on each side behind 

 the transverse process is well marked, and bears the inferior articular 

 surface on its under side. On the other hand, the superior articular 

 surface is placed partly on the odontoid process, but principally on the 

 most anterior part of the arch, viz. that part which, in all the succeed- 

 ing vertebras, forms the posterior angle of the body (Fig. 3). So also 

 on the anterior extremity of the arch are placed the articular surfaces 

 (both superior and inferior) of the atlas (Fig. 4) ; and also, in the dor- 

 sal region, the surfaces for the heads of the ribs. The occipital con- 

 dyles are placed upon the most anterior parts of the arch of the occipital 

 bone, and to a small extent upon the centrum. 



The foregoing examination of vertebral articulations leads us to ob- 

 serve, that, when surfaces for a synovial joint are present upon the body 

 of a vertebra, however little of the body they may cover, they are never 

 absent from those angles which are formed by the arches. 



The synovial articulations between the bodies of vertebras in mam- 

 mals are arranged in the following manner : In the dorsal region are the 

 synovial capsules for the heads of the ribs, which always occupy the angles 

 of the bodies, but are also, in many animals, united across the middle 

 line between the intervertebral disc and the conjugal ligament; while in 

 some cases, as in the horse and the sheep, a small line of cartilage is 

 stretched along the superior margin of the posterior of the two vertebras 

 concerned in each joint. In the cervical region in the human subject, 

 the minute joints described by Luschka,*" are situated between those 

 parts of the bodies which are formed by the arches. Lastly, in the 

 atlo-axoid and atlo-occipital articulations, the principal parts of the 

 articular surfaces are placed upon those parts of the arches which cor- 

 respond to the angles of the bodies of succeeding vertebrae, while the 

 intervertebral discs have disappeared. 



I may here remark that, if the odontoid process be regarded as the 

 centrum of the atlas, — a view which seems to be supported by its very 

 large comparative size in the young condition, long before the anterior 

 tubercle of the atlas makes its appearance — then we must recognise in 

 the odontoid ligaments the terminal member of the series to which the 

 transverse ligament of the atlas and the ligamenta conjugalia belong : 

 and indeed the arrangement of their fibres, some of which are continu- 

 ous from side to side, is favourable to this supposition, and reminds one 

 of the ligamentum conjugate in the sheep. 



Note. — Since writing the above, my attention has been called to 

 Rathke's work "Ueber die Entwickelung der Schildkroten," in which 

 (page 77), the view that the odontoid process is the centrum of the atlas 

 is strenuously urged, and strong evidence brought forward in its favour. 



* Luschka, Die Halbgelenke des Mensohlichen Kdrpprs, 18J8, p. 71, and Tab. I., 

 fig. 1. 



VOL. I. N. H. R. X 



