534 TEXT-BOOK OF PHYSIOLOGY 



The nerve-trunks constituting this part of the nerve system may be 

 divided into two groups, as follows: 



1. The first group comprises nerves in connection with the special sense- 



organs, e.g., skin, eye, ear, nose, tongue, as well as nerves in connection 

 with the general or organic sense-organs, e.g., mucous membranes, 

 viscera, etc., which are connected primarily with nerve-cells in the spinal 

 cord and medulla oblongata, and secondarily with nerve-cells in local- 

 ized areas of the cerebral cortex. 



2. The second group comprises nerves which terminate mainly in the 



muscle apparatus and which constitute the continuation of nerve paths 

 which have their origin in nerve-cells of localized areas of the cerebral 

 cortex. 



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. 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 me- 

 dulla 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, after a brief pre- 

 sentation of the relation of the spinal nerves to the spinal cord, to consider 

 them in the order of their complexity, beginning with the spinal cord. 



THE SPINAL CORD 



The narrow elongated portion of the central nerve system contained 

 within the spinal canal is named, from its situation and appearance, the 

 spinal cord. 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, ihefilum terminate. It is divided 

 by the anterior or ventral, and posterior or dorsal longitudinal fissures 

 into halves, and is therefore bilaterally symmetric. A transverse section 



