NECTURUS 89 



cartilage called the anteorbital cartilage; it will be lost in the 

 preparation of the skull, unless great care is taken. 



Exercise 23. Draw the dorsal aspect of the skull on a scale of 2, 

 showing accurately the outlines of the bones and cartilages. By 

 shading, indicate the cartilages, cartilage bones, and membrane 

 bones. 



Exercise 24. Draw the ventral aspect in the same way. 



Boil the skull until the bones can be separated from one another 

 and from the chondrocranium. Carefully observe the extent of the 

 latter and its relation to the cartilage and to the membrane bones. 



The vertebral column is composed of about forty-six vertebrae. 

 Four regions may be distinguished in it : a cervical region, consist- 

 ing of the first vertebra, the atlas ; a thoracolumbar region, con- 

 sisting of about eighteen vertebrae ; a sacral region, consisting of 

 one vertebra, usually the nineteenth, which forms the sacrum and 

 supports the pelvis ; and a caudal region, consisting of the remain- 

 ing vertebrae. 



A typical vertebra is made up of the following parts : a cylin- 

 drical centrum, or body ; a flattened neural arch, on the dorsal side 

 of the centrum, in which the spinal cord lies ; and a pair of large, 

 irregular transverse processes, at the sides of the centrum and 

 neural arch, which bear the ribs. 



Note the shape of the centrum. It is biconcave, having at each 

 end a deep depression which, in a fresh condition, is filled with 

 the soft notochord. The neural arch is made up of a pair of neural 

 processes, which form its lateral walls, and the neural spine, which 

 forms the roof. At each end of the neural arch is a pair of hori- 

 zontally flattened prolongations called zygapophyses, by means of 

 which the vertebrae articulate with one another. 



Study the regions of the vertebral column. The first vertebra, 

 which forms the cervical region, is called the atlas. It is about 

 half as long as those immediately behind it, and has very short 

 transverse processes. On its anterior surface is a pair of depres- 

 sions into which the occipital condyles of the skull fit. The 

 thoracolumbar vertebrae are the largest, and all bear ribs. The 

 sacral vertebra, which is usually the nineteenth, is similar to 



