CLELAND ON THE MAMMALIAN AXIS, ATLAS, AND OCCIPITAL BONE. 151 



work received whilst writing out these pages, in which, for instance, 

 Begoniaceae, Melastomaceae, Gesneriaceae, Burnianniaceae, andOrchideae, 

 are collected into one series, whilst Memecyleae, Bignoniaceae, and Iri- 

 deae, are far removed from them. 



It appears to me, therefore, that whilst in an artificial or analytical 

 system for finding out the name of a plant, one prominent character is 

 selected to mark out each division ; in a natural or synthetical system, 

 on the contrary, for the arrangement and study of plants, the affinities 

 according to which they are grouped should be judged of by the com- 

 bination of as many and as constant characters as possible, derived from 

 all parts of the .plants ; but that, in both cases, characters must be as- 

 signed. " Character non facit genus," it is true; but a genus without 

 a character is of no assistance to the mind of the naturalist. 



XYI. — On the Serial Homologies of the Articular Surfaces of the 

 Mammalian Axis, Atlas, and Occipital Bone. By John Cleland, 

 M.D., Demonstrator of Anatomy in the University of Edinburgh. 



[Read before the Royal Physical Society of Edinburgh, Nov., I860.] 



In works on human anatomy it has been customary to compare the arti- 

 cular surfaces of the atlas, and the superior articular surfaces of the 

 axis, with those of the oblique processes of other vertebrae, as if they 

 were homologous, notwithstanding the apparently anomalous manner in 

 which, according to that view, the first and second spinal nerves must 

 be considered as emerging from the spinal canal. The circumstances 

 which have led to this comparison being made, are merely the rapid di- 

 minution in size of the intervertebral discs from the. thoracic region up 

 to the axis, and a general similarity of appearance between the articular 

 surfaces of the atlas and axis and those of succeeding vertebrae : and 

 though the impropriety of this comparison has been exposed in very ex- 

 plicit terms by Prof. Henle,* there is still room for a few remarks as to 

 the precise parts of other vertebrae to which the surfaces in question 

 correspond. 



In order to arrive at a just conclusion upon this subject, we shall 

 find it advantageous to examine the atlas in the bird. In it we find on 

 the posterior aspect a pair of true oblique processes passing backwards, 

 to articulate above the intervertebral foramina with a corresponding 

 pair of processes of the axis, similar to those of succeeding vertebrae ; 

 while inferiorly there is a cartilaginous surface which forms, with 

 the body of the axis and its odontoid process, a joint similar to those be- 

 tween the succeeding bodies of vertebrae. On the anterior aspect of the 



* Henle, Handbuch der Syst. Anat. des Menschen, I., p. 42. 



