434 HISTORY OF THE HUMAN BODY 



all longitudinal and parallel, the well-known form of a fluted 

 column is produced, with a dorsal, lateral and ventral column 

 upon each side [posterior, lateral, anterior, BNA~\. The 

 shape of the mass of gray matter as seen in section varies from 

 that of a symmetrical triangle in lower forms to that of a figure 

 like a double crescent in the higher; in all cases it retains 

 the primitive position, bordering the lumen of the tube, the 

 original external surface. 



All parts of the body are in constant communication with the 

 central nervous system through the medium of the peripheral 

 nerves, which are in structure essentially the same as the white 

 matter of the brain and cord, as seen in the various commis- 

 sures of the former and in the columns of the latter, save that 

 here there is added a connective tissue element, which not only 

 forms an external sheath for each entire nerve (perineurium), 

 but also a delicate wrapping about each nerve fiber (neuri- 

 lemma). These nerves issue in pairs from both brain and 

 cord, and, although in form and character the transition from 

 one group of nerves to the other is a gradual one, the two 

 groups are distinguished for convenience as cranial and spinal. 



The latter, which are the less modified, issue from the cord 

 at approximately equal intervals and are metameric in arrange- 

 ment, a pair corresponding to each metamere or body somite, 

 as expressed in the muscles or the skeletal parts. This meta- 

 meric arrangement, which is often expressed with great clear- 

 ness in the trunk, is not distinct in the head, and the cranial 

 nerves, although showing indications of a former metameric 

 order, cannot be satisfactorily resolved into their separate ele- 

 ments. 



According to their use nerves are sharply divided into two 

 groups, sensory and motor. The first are distributed chiefly 

 to the external surface and are the media by which the central 

 organ receives intelligence concerning external stimuli. These 

 terminate in many cases in special sense organs arranged to 

 receive certain definite stimuli, but are distributed also over 

 the general surface, where they respond to simple contact. 

 Other sensory nerves supply certain internal parts, as the 



