188 ANIMAL PHYSIOLOGY. 



which, in transverse section, is seen to project forwards and 

 backwards, constituting the anterior and posterior cornua. 



The spinal nerves are attached to the cord symmetrically, 

 and emerge from the spinal canal by the intervertebral fora- 

 mina, one on each side, between each pair of successive 

 vertebra?. There are thirty-one pairs of them, the upper- 

 most emerging between the atlas and skull, and the lowermost 

 behind the coccyx; and while the upper pairs pass directly 

 outwards to pierce the dura mater, the lower pairs, including 

 the large nerves for the lower limbs, come off, crowded 

 together, from the lower end of the cord, and descend to 

 their apertures of exit in a bundle, called cauda equina, in- 

 vested with a common bag of arachnoid. The first eight 

 spinal nerves are termed cervical, then follow twelve dorsal, 

 five lumbar, and five sacral, each one of which is named after 

 the vertebra above it; and lastly, there is a small coccygeal 

 pair. 



Each spinal nerve arises by an anterior and a posterior 

 root, which emerge from the cord in two vertical series of 

 fibres; and when the cord is cut across, the fibres of the 

 posterior roots, which enter at the level of the section, are 

 seen to pass in to the posterior cornu so compactly as to 

 divide a posterior column of the white matter of the cord 

 completely from the rest; while the fibres of the anterior 

 root pass to the anterior cornu in scattered bundles, which 

 make by no means so definite a separation between what are 

 termed the anterior and lateral columns. 



The posterior roots are larger than the anterior, and have 

 each a ganglion situated in the invertebral foramen, imme- 

 diately internal to the point where the anterior and posterior 

 roots unite; but the use of these spinal ganglia is unknown. 

 No sooner are the fibres of the roots united in one trunk 

 than they divide into an anterior and posterior division, 

 each containing fibres from both roots. The posterior divi- 

 sions supply the muscles which erect the back and head, and 

 also a tract of integument extending from the crown of the 

 head the whole length of the back. The anterior divisions 

 supply the whole of the rest of the body: those between the 

 ribs are termed intercostal, and pass separately round the 

 visceral cavity, between the intercostal muscles, giving off 



