THE NERVOUS SYSTEM 



381 



swellings, one extending from the fifth vertebra 

 of the neck to the fourth vertebra of the back, 

 and the other situated in the region of the loins. 

 These enlargements of the nervous mass are ren- 

 dered necessary in order to supply the great nerves 

 distributed to the fore and hind limbs respectively. 

 The spinal cord gives origin in the horse to forty- 

 two or forty-three pairs of nerves, each of which 

 arises by two roots, a superior and an inferior. 



The position and arrangement of the successive 

 pairs are shown in the accompanying diagram. 



If the cord be divided transversely the appear- 

 ance presented in fig. 173 will be seen. 



The shaded area is named the gray substance, 

 the light area the white substance, of the cord. 

 The white substance is composed almost exclusively 

 of nerve-fibres ; the gray substance, whilst contain- 

 ing many fibres, presents also a large number of 

 nerve-cells. The cord is seen to be divided into 

 symmetrical lateral halves by two fissures, the 

 superior fissure being narrow and deep, and the 

 inferior wide and more shallow. The gray sub- 

 stance of the cord somewhat resembles the letter 

 H, or a pair of inverted commas, placed back to 

 back and united by a cross bar. The extremities 



13 8 B. * 



Fig. 173. Sections of Spinal Cord 



1 Superior Root. 2 Inferior Root. 3 Ganglion. * Superior 

 Nerve. 5 Inferior Nerve. 6 Choroid Plexus. 7 Nerve Substance. 

 8 , 10 Inferior Cornua. 9 Superior Cornu. n Central Canal. 

 18 Superior Longitudinal Fissure. 13 Inferior Longitudinal Fissure. 



