THE NERVOUS SYSTEM. 



169 



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

 parently a single one, 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 w T e 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 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 

 membranes. Both consist 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 



Fig. 137. — Human Brain and Spinal 

 Cord, one fifth natural size : a, 

 great longitudinal fissure ; b, an- 

 terior lobe ; c, middle lobe ; d, 

 medulla oblongata; e, cerebel- 

 lum ; /, first spinal nerve ; g, 

 brachial plexus of nerves supply- 

 ing the arms ; h, dorsal nerves ; i, 

 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. 



