70 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



G. VERTEBRAL COLUMN OF REPTILES 



The vertebral column of reptiles reaches a degree of differentiation consider- 

 ably in advance of that of the Amphibia and fishes. 



i. The vertebral column of the alligator.— Study the whole mounted skeleton 

 of the alligator/ Observe that the column is differentiated into five regions: 

 the cervical or neck region, much longer than in Amphibia; the thoracic region 

 bearing long ribs; the lumbar region, without ribs; the sacral region, composed 

 of two fused vertebrae, supporting the hind limbs; and the caudal region. The 

 tail vertebrae are the most primitive in form and will be studied first. Each 

 consists of a large centrum, a neural arch with a high neural spine, prominent 

 transverse processes directed straight out from the centrum, and a haemal arch, 

 missing on the first caudal vertebra. The haemal arches are relatively small and 

 situated at the posterior end of the centrum. They are often referred to in 

 texts as chevron bones. Toward the end of the tail the various processes project- 

 ing from the vertebrae tend to become reduced and finally vanish altogether, so 

 that the last vertebrae consist of centra only. The sacral region consists of two 

 vertebrae, each bearing a stout sacral rib, to which the supports of the hind 

 limbs are articulated. The sacral vertebrae have high neural spines but lack the 

 haemal arches. The lumbar vertebrae, anterior to the sacrum, are five in 

 number, possess high neural spines and broad transverse processes. There are 

 ten thoracic vertebrae, similar in form to the lumbar vertebrae, but bearing 

 long ribs, which reach the median ventral line. There are nine cervical vertebrae, 

 of which the two first, known as the atlas and the axis, are somewhat different 

 from the other seven. These latter have strong neural arches with long neural 

 spines and short transverse processes bearing ribs. Most of these cervical ribs 

 are of peculiar form, being V-shaped and attached to the cervical vertebrae by 

 the two ends of the V. Cervical, thoracic, lumbar, and sacral vertebrae lack 

 haemal arches, but the cervical and anterior thoracic vertebrae possess short 

 ventral projections, the hypapophyses, on the centra. All of the vertebrae are 

 provided with well-developed pre- and postzygapophyses. By moving the 

 vertebrae apart note that the centra are procoelous, i.e., concave in front, con- 

 vex behind. 



The first two cervical vertebrae, the atlas and the axis, are very remarkable 

 and deserve further attention. They are remarkable in that the parts which 

 compose the primitive vertebra are retained in these two vertebrae almost 

 unchanged (Fig. 22F). The atlas or first cervical vertebra is composed of four 

 separate pieces, which together form a ring. The ventral side of the ring is 

 composed of a median piece, the fused basiventrals. It bears a pair of long 

 movable ribs. The sides of the ring are curved bones, the basidorsals. The 

 ring is completed above by the neural spine. The atlas apparently has no 

 centrum but the centrum is really present and has become attached to the anterior 

 end of the second cervical vertebra or axis. This centrum is composed of the 



