378 THE VERTEBRATE SKELETON. 



A. THE VERTEBRAL COLUMN. 



This consists of a series of about forty vertebrae arranged 

 in succession so that their centra form a continuous rod, and 

 their neural arches a continuous tube, surrounding a cavity, 

 the neural canal. 



The vertebrae may be readily divided into five groups : 



1. The cervical or neck vertebrae. 



2. The thoracic or chest vertebrae which bear ribs. 



3. The lumbar vertebrae which are large and ribless. 



4. The sacral vertebrae which are fused with one another 

 and united with the pelvis. 



5. The caudal or tail vertebrae which are small. 



Except in the sacral region the vertebrae are movably 

 articulated to one another, while their centra are separated 

 from one another by cartilaginous intervertebral discs. 



GENERAL CHARACTERS OF A VERTEBRA. 



Take as a type the fourth lumbar vertebra. It may 

 be compared to a short tube whose inner surface is smooth 

 and regular, and whose outer surface is thickened and drawn 

 out in a variety of ways. The basal part of the vertebra 

 is the centrum or body which forms the thickened floor of 

 the neural canal. Its two ends are slightly convex and 

 are formed by the epiphyses, two thin plates of bone which 

 are at first altogether ^distinct from the main part of the cen- 

 trum, but fuse with it as the animal grows older ; its sides are 

 drawn out into a pair of strong transverse processes, which 

 project forwards, outwards, and slightly downwards. The 

 neural arch forms the sides and roof of the neural canal, and 

 at each end just above the centrum bears a pair of interver- 

 tebral notches for the passage of the spinal nerves, the pos- 

 terior notches being considerably deeper than the anterior. The 

 neural arch is drawn out into a series of processes. Arising 



