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 



I part of an organ, may persist and be transmitted through an 



< immense period of time, when its functional activity is to a 



1 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, e.g. certain Coelenterates, 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 

 I portion of the vertebral column. The spinal cord no longer 

 extends throughout the whole length of the vertebral canal, its 

 posterior tapering extremity [i.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 



