44 



THE SKELETON OF THE HORSE 



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

 one. In old age the first is often fused -ndth the sacrum, and sometnnes with the second, ihe 

 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 vertebra, 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 and diminishes from the sixth or seventh 

 to the sixteenth thoracic, which is vertical and is termed the anticlinal or dia- 

 phragmatic 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 arches 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 should be noted that a line 

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

 the bodies. 



The vertebral canal corresponds in its curvature to that of the bodies. Its 

 caliber varies greatly at different points. The greatest diameter 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. It 

 widens considerably at the junction of the cervical and thoracic regions to accom- 

 modate 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 corre- 

 lated with the small size of the spinal cord and the very limited movement of the 

 spine here. Beyond the middle of the lumbar region it again enlarges considerably 

 to contain the lumbar enlargement of the spinal cord. The caliber diminishes very 

 rapidly from the second sacral segment backward, and the canal ceases to be com- 

 plete at the fourth coccygeal vertebra. 



The transverse and vertical diameters of the vertebral canal at various points are given 

 in the annexed table. The measurements were made on a horse of medium size; they represent 

 the maximum width and height in the middle of the vertebra and are expressed in centimeters. 



S5 



1.8 

 1.5 



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



