THE SPINAL CORD. 443 



to certain localized areas in the cerebral cortex, where they are 

 translated into conscious sensations. These sensations, ^both 

 special and general, by their grouping and combinations are the 

 primary elements of intelligence. 



The second group comprises those nerves which terminate in the 

 muscle apparatus and which transmit nerve impulses, by way of 

 the medulla and spinal cord, from localized areas in the cerebral 

 cortex to the muscles of the face, trunk and extremities, which are 

 in consequence excited to activity. The muscle movements thus 

 become physical expressions of mental states, and if directed in a 

 definite manner to the overcoming of the resistances offered by 

 the external world become capable of modifying it in accordance 

 with the mental states. 



The first group of nerves, the afferent, especially those connected 

 with the special sense-organs, are excited to activity by impressions 

 made on their peripheral terminations by agencies in the external 

 world, and thus become a means of communication between the 

 physical and the mental worlds. 



The second group of nerves, the efferent, are excited to activity 

 by those molecular disturbances in their related nerve-cells which 

 accompany volitional efforts, and thus they become a means of com- 

 munication between the mental and the physical worlds. 



The central nerve system is thus composed of a number of 

 separate though closely related parts, to each of which a separate 

 function has been assigned. In the study of the structure and func- 

 tion of these separate parts it will be found convenient, and con- 

 ducive to clearness, to consider them in the order of their complexity, 

 beginning with the spinal cord and ending with the cerebrum. 



THE SPINAL CORD. 



The spinal cord is the narrow elongated portion of the central 

 nerve system contained within the spinal canal. It is cylindric in 

 shape though presenting an enlargement in both the lower cervical 

 and lower lumbar regions corresponding to the origins of the nerves 

 distributed to the upper and lower extremities. The cord varies 

 in length from, 40 to 45 cm., measures 12 mm. -in diameter, weighs 

 42 grams, and^xtends from the atlas to the second lumbar vertebra, 

 beyond which it is continued as a narrow thread, the filum terminate. 

 (Fig. 203.) It is divided by the anterior and posterior longitudinal 

 fissure into halves, and is therefore bilaterally symmetric. A trans- 

 verse section of the cord shows that it is composed of both white and 

 gray matter, the former covering the surface, the latter occupying 

 the center. 



