70 THE OSTEOLOGY AND MYOLOGY 



a little behind the posterior margin of the neurapophyses. The so-called " body " 

 (hypapophysis) of the atlas, unlike that of some marsupials, in which it does not advance 

 beyond the cartilaginous stage, is completely ossified. The ossification appears to take 

 place at an early age, as I find the bony ring perfect in a specimen in which the epiphyses 

 of the long bones are still distinct. The I'ing, unlike the broad neural arch, is slender and 

 delicate. Within, it is scooped away for the lodgment of the odontoid, which appears 

 but loosely confined, so far as lateral motion is concerned. The contour of the inside of 

 the ring and neural arch together is, in a general way, circular ; but on looking through the 

 aperture, a slight hour-glass contraction is observed, dividing the canal into two parts. 

 The upper conducts the spinal mari'ow, the odontoid revolves in the lower ; they are of 

 nearly equal size ; the upper is wider and flatter. The constriction is produced partly by 

 the little bony eminences that serve for the attachment of the check ligament ; but more 

 by the inward expansion of the well developed post-zygapophyses. These articular plates 

 are raised above the general level of the bone, with their sharp, well-defined edges above ; 

 below they gradually subside into the articular surface for the odontoid process. The con- 

 dylar articular facets appear as large, well marked depressions. There is no vertebrarterial 

 foramen ; the vessel merely grooves the bone as it passes by the root of the transverse process. 

 Anteriorly, the neural arch is perforated on either side by a venous canal, running directly 

 transverse, and opening on the outer surface into a deep, distinct groove that winds around 

 the front of the base of the transverse process. Inside the arch, the canal terminates 

 abruptly at the inner margin of the condylar fiicet, but varies in different specimens ; 

 sometimes it is little more than a simple perforation, terminating a long groove ; in others, 

 the greater part of this groove is bridged over by bone. In one individual I observed a 

 difference between the two sides of the bone. The left presented the groove and sunjDle 

 perforation ; the right, a long cjinal, whose bony arch was deficient in the middle, disjilay- 

 ing the vessel after it had been once hidden. This interruption caused the most lateral 

 part of the canal — that at the root of the transverse process — to simulate the vertebrarte- 

 rial foramen in everything but position. 



The neural spine of the axis is the remarkable part of that bone. It is an enormously 

 expanded, upright lamina, thicker l)ehind than before, reaching its greatest height pos- 

 teriorly, where it rises vertically above the line of articulation of the next vertebra, and 

 thence slopes downward and forward to reach the anterior border of the atlas. This for- 

 ward projection of the spine lies upon the neural arch of the atlas, riding freely from side 

 to side. The post-zygapophyses appear as two flaring ovals at the back part of the root 

 of the spine, though really borne, as usual, upon the neui^apophyses. The pre-zygapoj^hy- 

 ses seem situated upon the anterior projection of the body of the vertebra ; they are stout, 

 prominent processes, whose articular facets are well defined by a sharp border, except just 

 in front where they appear more or less continuous with the articular surface of the odon- 

 toid process. The latter presents no special peculiarity, being simply a conical knob. The 

 vertebrarterial canal is perfect. The greatly produced posterior prolongation of the trans- 

 verse process projects outward, downward and backward ; there is no corresponding ante- 

 rior prolongation, such as exists in the other cervical vertebrae ; this portion of the process 

 subsiding into the body of the vertebra just at the termination of the arterial foramen, 

 which it bounds below. 



The third cervical is the opossum's " vertebra prominens " ; the spine here reaches its 



