494 TEXT-BOOK OF PHYSIOLOGY. 



the special sense-organs, are excited to activity by impressions made on their 

 peripheral terminations by agencies in the external world. The nerve 

 impulses thus generated are transmitted in part only as far as the spinal 

 cord and medulla oblongata while the remainder ascend to nerve-cells in 

 localized areas of the cerebral cortex where they evoke sensations. These 

 sensations by their grouping and combinations become the primary elements 

 of intelligence. The afferent nerves 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 voli- 

 tional efforts. The nerve impulses thus developed and discharged from 

 localized areas in the cerebral cortex are transmitted by way of the med- 

 ulla and spinal cord 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 resistance offered by the external world 

 they become capable of modifying it in accordance with the mental states. 

 The efferent nerves thus become a means of communication 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 function of these separate parts 

 it will be found convenient, and conducive 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 gms., and extends from the atlas to the second 

 lumbar vertebra, beyond which it is continued as a narrow thread, the 

 {Hum terminate. (Fig. 229.) It is divided by the anterior and posterior 

 longitudinal fissures into halves, and is therefore bilaterally symmetric. A 

 transverse 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. 



Structure of the Gray Matter. The gray matter is arranged in the 

 form of two crescents, united in the median line by a transverse band or 

 commissure forming a figure resembling the letter H. Though varying 

 in shape in different regions of the cord, the gray matter in all situations 

 presents on either side an anterior or ventral and a posterior or dorsal horn. 

 Between the two horns there is a portion termed the intermediate gray sub- 

 stance. The commissure presents in its center a narrow canal which extends 

 throughout the entire length of the cord. This canal is lined by cylindric 

 epithelium and surrounded by gelatinous material. (Fig. 230.) 



The anterior horn is short and broad and entirely surrounded by white 



