116 



OSTEOLOGY 





shape, smallest in the cervical and upper part of the thoracic regions, and gradually 

 increasing in size to the last lumbar. They transmit the spinal nerves and 'are 

 situated between the transverse processes in the cervical region, and in front of 

 them in the thoracic and lumbar regions. 



Vertebral Canal. The vertebral canal follows the different curves of the column; 

 it is large and triangular in those parts of the column which enjoy the greatest 

 freedom of movement, viz., the cervical and lumbar regions; and is small and 

 rounded in the thoracic region, where motion is more limited. 



Abnormalities. Occasionally the coalescence of the laminae is not completed, and conse- 

 quently a cleft is left in the arches of the vertebrae, through which a protrusion of the spinal 

 membranes (dura mater and arachnoid), and generally of the medulla spinalis itself, takes place, 

 constituting the malformation known as spina bifida. This condition is most common in the 

 lumbosacral region, but it may occur in the thoracic or cervical region, or the arches throughout 

 the whole length of the canal may remain incomplete. 



; ' First thoracic 



FIG. 112. The thorax from in front. (Spalteholz.) 



THE THORAX. 



The skeleton of the thorax or chest (Figs. 112, 113, 114) is an osseo-cartilaginous 

 cage, containing and protecting the principal organs of respiration and circulation. 



