THE NERVOUS SYSTEM 
THROUGHOUT the animal kingdom the nervous system is more 
conservative in character than any other, and it thus offers a 
more limited field for the study of vestigial structures. The 
latter, however, as we shall see, are not altogether wanting ; 
indeed, they may be here of special interest, as they afford the 
best proof of the extreme tenacity with which an organ, or some 
part of an organ, may persist and be transmitted through an 
immense period of time, when its functional activity is to a 
marked degree reduced, or even no longer evident. 
The central nervous system of the Vertebrata, as is well 
known, arises from the so-called medullary folds of the outer 
germinal layer, and is thus essentially a modified derivative of 
the epiblast—the so-called “sensory layer.” The latter, in the 
lower animals, eg. certain Ccelenterates, in which there is no 
sharp differentiation into a central and a peripheral nervous 
system, remains superficial in position and is directly the medium 
of communication with the external world. This, combined with 
the fact that, in Vertebrates, the brain and spinal cord are among 
the first differentiated organs, is a distinct proof of the great age 
and physiological importance of the nervous system. 
THE SPINAL CORD 
When first differentiated, the nervous axis, as already men- 
tioned, corresponds in extent with the axial skeleton; but it 
soon appears to shorten, partly from inequality of growth, and 
partly in consequence of modification taking place in the posterior 
portion of the vertebral column. The spinal cord no longer 
extends throughout the whole length of the vertebral canal, its 
posterior tapering extremity [7.e. the portion caudad of the spinal 
nerve-roots, where the filum terminale begins] reaches no 
farther down [in Man] than to about the boundary between the 
