526 TEXT-BOOK OF PHYSIOLOGY 



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 and 

 ending with the cerebrum. 



The Relation of the Spinal Nerves to the Spinal Cord. The spinal 

 nerves present near the spinal cord two divisions which from their connection 

 with the anterior or ventral and the posterior or dorsal surfaces are known 

 as the ventral and dorsal roots. 



The ventral roots are the axons of various groups of nerve-cells situated 

 in the anterior horns of the gray matter. From their origin these axons 

 pass almost horizontally forward through the anterior column in three 

 distinct bundles. After emerging from the cord they curve downward and 

 backward to join the dorsal roots. 



The dorsal roots are the centrally directed axons of nerve-cells in the 

 ^taaJLganglia. After entering the cord they divide into two main groups, 

 a lateral and a mesial. A portion of the lateral group enters the posterior 

 horn directly through the caput cornu; the other portion turns upward and 

 runs through Lissauer's tract and ultimately enters the posterior horn. The 

 mesial group passes into the postero-external column (Burdach), where 

 the fibers divide into descending and ascending branches. The former 

 probably constitute the comma tract, the terminal branches of which sur- 

 round cells in the gray matter; the latter (ascending) cross the column 

 obliquely and enter the postero-internal column (Goll), in which they pass 

 upward to terminate around the cells of the nucleus gracilis of the same side. 

 As these root fibers pass up and down the cord, collateral branches are 

 given off which enter the gray matter at successive levels and come into 

 physiologic relation with the cells of Clark's vesicular column on the same and 

 opposite sides and with the cells of the anterior horn. 



The peripherally directed axons of the nerve-cells in the spinal nerve 

 ganglia become associated with the axons of the ventral roots and together 

 they pass as a spinal nerve to peripheral organs. 



Classification of Spinal -nerve Fibers. The ventral root axons have 

 been shown by histologic and physiologic methods of investigation to be dis- 

 tributed to skeletal muscles, glands, blood-vessels, and viscera. The dorsal 

 root axons have been shown by the same methods to be distributed to skin, 

 mucous membranes, and muscles. Hence the ventral and dorsal root fibers 

 may be classified in accordance with their distribution, and the character- 

 istic modes of activity to which they give rise into several groups as shown 

 in the tabulation on pages 99, 100. 



Though both the efferent and afferent fibers of the spinal nerves are 

 directly connected with nerve-cells in the spinal cord, they are also indirectly 

 connected by efferent and afferent nerve-tracts with the cerebral cortex. 



Experimentally, it has been determined that the anterior or ventral 



