1 86 ZOOTOMY. 



popliyses are very small and placed low down on the arch : 

 the anterior face of the centrum is somewhat excavated and 

 gives off dorsally a conical peg, the odontoid process, 

 which bears an oblique articular facet on the ventral surface 

 of its free end, for articulation with the atlas (§ 20). 



In the young bird the odontoid is a separate bone. 



20. The characters of the first vertebra or atlas : it is 

 very small and is reduced to a mere ring, the centrum being 

 no thicker than the arch : the centrum is crescentic, the 

 concavity of the crescent being upwards, and bearing an 

 articular facet for the odontoid process of the axis, the two . 

 together forming a concave articular cavity for the occipital 

 condyle (§ 35): there are no anterior zygapophyses and the 

 posterior zygapophyses are small. 



21. The mode of articulation of contiguous centra ; they are united 

 by rings of cartilage, their adjacent faces being separated by a closed 

 space (synovial capsule) ; each intervertebral ring is produced inwards 

 into a thin plate of cartilage, the meniscus, \^hich is perforated in the 

 centre, and forms a vertical partition dividing the synovial space into an 

 anterior and a posterior division : a delicate ligamentous cord, the 

 ligamentum suspensorium, passes from centrum to centrum through 

 the aperture in the meni.-.cus. 



V. In the thoracic vertebrae note : 



22. The fusion of the first four into a single mass by 

 the ankylosis of the centra, zygapophyses, and transverse 

 processes. 



23. The fusion of the fifth with the compound sacrum : 

 its large transverse processes abut against the ilia, with 

 which the proximal ends of its ribs are also united. 



24. The large hypapophysis of the first thoracic 

 vertebra, dividing below into two divergent laminae. 



25. The thoracic ribs: each is divided into two parts 

 by a transverse articulation, — a dorsal section, the vertebral 



