72 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



On the ventral side of the lumbar region will be noted a series of riblike bones 

 not attached to any other part of the skeleton. These are the so-called abdomi- 

 nal ribs. They are membrane bones and therefore belong to the exoskeleton, 

 being in fact homologous with the dermal plates of the turtle's plastron. They 

 are likely to occur in long-bellied animals. 



Draw a cervical or anterior thoracic vertebra of the alligator with all of its 

 parts, showing particularly the relation of the ribs. 



3. The vertebral column of the turtle. — In our study of the carapace of 

 the turtle we already noted certain peculiarities of the vertebral column of these 

 animals. These peculiarities arise from the circumstance that part of the verte- 

 bral column is fused to the exoskeleton. The vertebral column of the turtle 

 consists of cervical, trunk, sacral, and caudal regions. As usual, the caudal 

 region is the most primitive. The caudal vertebrae have neural and haemal 

 arches and transverse processes, of which the most anterior are ribs, as shown by 

 the suture at their bases. The first caudal vertebra is fused to the sacrum, which 

 consists of two sacral vertebrae bearing sacral ribs. The first caudal, the two 

 sacral, and the ten trunk vertebrae are all fused to the carapace. As already 

 explained in connection with the exoskeleton, the neural arches of these ver- 

 tebrae are broadened to form the vertebral plates of the carapace, and the ribs 

 of the trunk vertebrae are similarly expanded to form the costal plates of the 

 carapace. The first and second pairs of ribs are fused distally. The ribs of the 

 turtle possess a single head, the capitular head, which articulates at or near the 

 boundary between two successive centra. There are eight cervical vertebra which 

 are devoid of ribs and which are very flexibly articulated by ball-and-socket 

 joints. The first two are differentiated as in the alligator into atlas and axis. 

 The odontoid process, which is the centrum of the atlas, is the large mass 

 attached to the anterior end of the axis. 



H. VERTEBRAL COLUMN OF BIRDS 



The vertebral column of birds, like their entire structure, is highly special- 

 ized in relation to the flight habit. With a prepared skeleton of the bird- 

 chicken or pigeon — before you, note the following points. The vertebral column 

 is divided into cervical, thoracic, lumbar, sacral, and caudal regions, but all of 

 these except the first are fused together in order to strengthen the back. The 

 cervical vertebrae are numerous (sixteen in the chicken, thirteen or fourteen in 

 the pigeon) and have very flexible articulations, birds being the only vertebrates 

 that can turn their heads halfway round. This flexibility is due to the shape 

 of the centra. On isolated vertebrae or by slightly separating some of the cer- 

 vical vertebrae on the mounted skeleton note that the ends of the centra are 

 saddle shaped; this type of centrum is hclcrococloiis. The first and second 

 cervical vertebrae are the atlas and the axis; the former is a small ring-shaped 

 bone, the latter bears an odontoid process as usual. Behind the axis are the 



