362 
NERVOUS SYSTEM OF VERTEBRATA. 
united into one continuous mass ; and this mass, constituting 
the Brain and Spinal Cord , is inclosed within the bony 
Fig. 190. —Nervous System of Octopus (Poulp). 
casing formed by the skull and vertebral column, in such a 
manner as to be protected by it from injuries to which it 
would otherwise be continually liable (§§ 72, 73). We have 
seen that among the Invertebrated classes the nervous system 
has no such peculiar defence, but lies among the other organs, 
sharing with them the protection afforded by the general hard 
envelope of the body. But in the Yertebrata, its development 
