STBUCTUEE OF THE SPINAL COED. 



67 



also undergo an increase in size at places where many root-fibers leave the 

 cord. Thus, in animals with legs there may be observed an enlargement of 

 the spinal cord for both anterior and posterior extremities, the Intu- 

 mescentia cervicalis and lumbalis, respectively. The difference in this 

 respect between lizards and the snake-like 'T)lind worms" is clearly visible. 

 The enlargements are especially noticeable in a section of the spinal cord of 

 the turtle, because these armored animals have very large nerves to supply 

 the extremities, while the thoracic nerves are very small. 



The columns of large ganglion-cells contain the centers of innervation 





Fig. 32. — Projection upon a plane of the enumerated ganglion-cells of a 

 frog's spinal cord. The figures at the right indicate the number of the corre- 

 sponding spinal nerve. Note the enormous increase of the number of cells for 

 arms and legs. (After Birge.) 



for the individual muscles. These are arranged in groups. For the mam- 

 malian spinal cord the significance of a few of these groups is already 

 known. It is known, for example, that the ganglion-cells which lie nearest 

 the median line innervate the muscles of the back; and that a certain 

 group of laterally located cells in the cervical cord provides innervation for 

 the musculature of the thumb. This was found out by a careful study of 



