NERVOUS SYSTEMS OF BACKBONED ANIMALS 19 
tralization is at a maximum.’ The Common Cuttle-Fish (Sepza 
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 
