42 THE SKELETON OF THE HORSE 



gradually fade out, and the vertebrae are reduced to cylindrical rods of diminishing 

 size. The last one has a pointed end. 



Variations. — The numl^cr is said by good observers to vary between fourteen and twenty- 

 one. In old age the first is often fused with the sacrum, and sometimes with the second. The 

 arch of the third may be open. 



THE VERTEBRAL COLUMN AS A WHOLE 

 In the mid-dorsal line is the series of spinous processes, which are low ridges 

 in the cervical region with the exception of the second and seventh, reach their 

 maximum height at the fourth and fifth thoracic vertebrae, and diminish to the 

 fifteenth or sixteenth thoracic. Behind this they are about equal in height as far 

 as the last lumbar and first sacral, which are somewhat lower. The second sacral 

 spine is about as high as the middle lumbar; behind this they diminish rather 

 rapidly in height and fade out about the third coccygeal. Their inclination back- 

 ward is most decided at the second thoracic, diminishes from the sixth or seventh 

 to the fifteenth or sixteenth, which is vertical and is termed the anticlinal or 

 diaphragmatic vertebra. Behind this they are inclined a little forward until the 

 sacrum is reached; here there is an abrupt change to the backward inclination, so 

 that a considerable interspinous angle is formed. 



On either side of the spinous processes is a vertebral groove which contains the 

 deep muscles of the spine. The floor of the groove is formed by the laminae and 

 articular processes. It is wide in the neck and narrows progressively in the back. 

 Viewed from the side, the column presents a series of curves. When the head 

 and neck are in the ordinary neutral position, the anterior part of the cervical spine 

 forms a gentle curve, concave ventrally. The posterior cervical and first thoracic 

 vertebrae form a more pronounced curve in the opposite direction. At the junction 

 of the cervical and thoracic regions there is a marked change of direction, forming a 

 ventral projection or angle. At the second thoracic vertebra a gentle curve, con- 

 cave ventrally, begins. This is continued to the lumbo-sacral junction, where 

 there is a change of direction, and hence a promontory. The sacrum has a variable, 

 but never very pronounced, ventral concave curvature, which is continued in a 

 much accentuated form in the coccygeal region. It may be remarked that a line 

 through the summits of the spines does not correspond to these curves formed by 

 the bodies. 



The vertebral canal, of course, corresponds in curvature to the bodies. Its 

 caliber varies greatly at different points. The greatest diameter (ca. 5 cm.) is in 

 the atlas, where it contains the dens of the axis in addition to the spinal cord, and 

 provision must be made for extensive movement. It is very much smaller in the 

 axis (ca. 2.5 cm. wide, 3 cm. high). It widens considerably at the junction of the 

 cervical and thoracic regions to accommodate the cervical enlargement of the spinal 

 cord. Beyond this it diminishes, and is smaller in the middle of the back than at 

 any preceding point; this is correlated with the small size of the spinal cord and the 

 very limited movement of the spine. At the middle of the lumbar region it again 

 widens considerably to contain the lum])ar enlargcMuent of the spinal cord. The 

 caliber diminishes very rapidly from the second sacral segment backward, and the 

 canal ceases to be complete at the fourth coccygeal vertebra. 



The articular processes are very large and wide apart in the neck, greatly 

 reduced and much closer together in the back, larger and interlocking in the lumbar 

 region. 



The transverse processes are large and outstanding in the neck, where they 

 form the outer boundary of a ventral groove occupied by the longus colli muscle. 

 In the back they are short and stout, and are characterized by the facets for the 

 tubercles of the ribs. On the first thoracic vertebra this facet is large, deeply con- 



I 



