THE NERVOUS SYSTEM. 



169 



two cords with a ventral chain of ganglia one pair, appar- 

 ently a single ganglion, for each segment. In the lower 

 Arthropods, such as Crustacea, Centipedes, and larval In- 

 sects, the arrangement is substan- 

 tially the same. In higher Insects 

 and Crustacea, many of the gan- 

 glia are fused together in the head 

 and thorax, indicating a concen- 

 tration of organs for sensation and 

 locomotion. 



In Vertebrates, the nervous 

 system is more highly developed, 

 more complex, and more concen- 

 trated than in the lower forms. 

 In fact, there are some parts, as the 

 brain, to which we find nothing 

 homologous in the Invertebrates ; 

 and while the actions of the lat- 

 ter are mainly, if not wholly, au- 

 tomatic, those of backboned ani- 

 mals are largely voluntary. Its 

 position, moreover, is peculiar, 

 the great mass of the nervous 

 matter being accumulated on the 

 dorsal side, and enclosed by the 

 neural arches of the skeleton. 



The brain and spinal cord lie in 

 the cavity of the skull and spinal 

 column, wrapped in three mem- 

 branes. Each consists of gray 

 and white nervous matter; but in 

 the brain the gray is on the out- 

 side, and the white within ; while 

 the white of the spinal cord is external, and the gray in- 

 ternal. Both are double, a deep fissure running from the 



Pie. 137 Human Brain and Spinal 

 Cord, about one tenth natural 

 size ; a, great longitudinal fissure ; 

 6, anterior lobe ; c, middle lobe ; 

 d, medulla oblongata ; e, cerebel- 

 lum ; /, first spinal nerve ; g, bra- 

 cbial plexus of nerves supplying 

 the arms ; h, dorsal nerves ; ?, 

 lumbar nerves ; k, sacral plexus 

 of nerves for the limbs ; I, cauda 

 equina: the figures indicate the 

 twelve pairs of cranial nerves, of 

 which 1 is olfactory, 2 optic, and 

 8 auditory. 



