PHYSICAL SUPPORT AND MOVEMENT 



91 



is structurally differentiated into five regions, the cervical, thoracic, 

 lumbar, sacral, and caudal (see Fig. 79). The plan of a vertebra is shown 

 in Fig. 80. It is composed of a heavy ventral portion, the centrum, from 

 which arises a bony arch, the neural arch. The latter encloses the neural 

 canal which is occupied by the spinal cord. From the sides of the arch 

 two transverse processes project, and from the apex of the arch arises the 



VERTEBRAL COLUMN. 



Lumbar 



Sacral. 



Fig. 79. — Axial skeleton of the cat. (From Jayne, ''Mammalian Anatomy.") 



neural spine. One pair of articular processes or zygapophyses projects 

 anteriorly and another posteriorly from the sides of the arch. The 

 relations of the anterior and posterior zygapophyses and the articular 

 faces of the centra of adjoining vertebrae are made clear in Fig. 80 (right). 

 The forms of the vertebrae in different regions of the vertebral column 

 are very different, as shown in Fig. 79. In the thoracic region of an 



Fig. 80. — Diagram of a typical vertebra viewed from in front or behind and from the 

 left side: az, anterior zygapophysis; c, centrum; /, intervertebral foramen through which 

 nerves and blood vessels pass; ic, intervertebral cartilage; na, neural arch; nc, neural canal; 

 ns, neural spine; pz, posterior zygapophysis; tp, transverse process; z, zygapophysis. 



animal having ribs the vertebrae have faces for the articulation of the ribs. 

 In the sacral region the vertebrae in some animals are considerably thick- 

 ened without great change in form, while in others they are much flattened 

 and more or less fused into a platelike structure, the sacrum. In the 

 sacral vertebrae the neural canal is reduced in size and in the caudal 

 vertebrae it is entirely absent. The spinal cord does not pass into the 

 latter region. 



