NERVOUS SYSTEMS OF BACKBONED ANIMALS 19 



tralization is at a maximum. The Common Cuttle- Fish (Sepia 

 officinalis, fig. 1023) possesses a nerve-ring of which the ganglia 

 are exceedingly large and closely connected. In one respect the 

 nerve-ring is less complex than that of the Garden-Snail, for it 

 here includes two only of the three ganglia of the nerve-loop, 

 which is long, distinct, and, like the body, not twisted. The 

 nerve-ring of Cephalopods is more or less enclosed in a gristly 

 case, serving as a sort of skull. 



NERVOUS SYSTEMS OF BACKBONED ANIMALS 



(VERTEBRATA) 



The nervous system attains its maximum complexity in back- 

 boned animals, especially in the highest Mammals. The chief 

 part of the central organs consists of a tube, which is placed 

 near the upper side of the body, and in all but the lowest 

 members of the group is sheltered within a skull and backbone. 

 The front end of this nerve-tube is usually swollen into a brain, 

 which is the chief organ of correlation and adjustment, while 

 the rest of it is known as the spinal cord or spinal marrow. 

 The central structures also include a visceral, or, as it is here 

 usually called, a sympathetic nervous system, which where best 

 developed consists of a couple of cords running longitudinally near 

 the under side of the backbone, and swelling at intervals into 

 sympathetic ganglia. Besides these there are outlying ganglia 

 of similar nature in close connection with some of the internal 

 organs, and connected with the cords just mentioned. 



The body of a Vertebrate is undoubtedly made up of rings 

 or segments, and although this is not at first sight apparent, the 

 serial arrangement of certain structures shows it to be the case. 

 We find, for example, that a regular succession of spinal nerves 

 is given off from the spinal cord, one pair to each segment. 

 From the brain arise from 10 to 12 pairs of cranial nerves, the 

 number of which, however, does not tell us how many segments 

 are fused to form the head. The number would be a guide if 

 cranial nerves were precisely equivalent to spinal nerves, but 

 this does not appear to be the case. While on the one hand 

 some of them are complex, and equivalent to more than one 

 pair of spinal nerves, others are only comparable to bits of 

 such nerves, so to speak. The sympathetic system is closely 



