A TURTLE 145 



Exercise 19. Draw the ventral aspect of the three anterior and the two 

 posterior vertebrae of the carapace, with their ribs and the dermal 

 plates with which they are fused. 



Separate the plates of the carapace from one another and observe 

 carefully the difference in structure between the first and second 

 vertebrae. Note on the first vertebra the articular surfaces with 

 which the posterior cervical vertebra articulates. Study also the 

 costal and marginal plates. Note the sutures on all these plates. 



The Plastron. Peel off the horny plates. Two rows of bony 

 plates will be seen, between which is a single small median plate. 

 Exercise 20. Draw an outline of the plates of the plastron. 



The Cervical Vertebrae. Disarticulate the head from the neck, 

 being careful not to injure the hyoid apparatus, the cartilaginous 

 support of the tongue which is embedded in the muscles of the 

 lower jaw. Thoroughly clean the vertebrae of the neck, and string 

 them on a cord in the proper order so as not to lose the sequence. 

 The neck contains eight vertebrae which are remarkable for the 

 variety of form which characterizes them. They fall into two 

 groups, one of which contains the first and second vertebrae ; the 

 other contains the remaining six. The first two vertebrae form the 

 support of the skull. The first one is called the atlas. It is a ring- 

 shaped bone which is made up of three pieces,— two dorsolateral 

 portions representing the neural arch, and a ventral portion. On 

 its anterior surface is a depression in which the condyle of the 

 skull articulates. The second vertebra is called the axis. It is 

 made up of a cylindrical centrum and a neural arch through which 

 the spinal cord runs. At the anterior end of the neural arch is a 

 pair of articular processes, the prezygapophyses, and at its pos- 

 terior end are the corresponding postzygapophyses. Short trans- 

 verse processes and no ribs are present. Projecting forward from 

 the anterior end of the centrum is the odontoid process. It lies 

 within the arch of the atlas, the centrum of which it represents. 



The six posterior cervical vertebrae are similar to the axis, ex- 

 cept that they do not have the odontoid process. Study each one 

 separately and note the points in which they differ from one an- 

 other, especially in the form of the end surfaces of the centra. 



