68 



ANATOMY OF THE CENTBAL NEEVOUS SYSTEM. 



the spinal cord of such animals as had suffered — through disease or experi- 

 ment — paralysis of a particular group of muscles, comparing the microscopic 

 appearances with those of the normal individual. How far such observa- 

 tions have been made in the human subject will be discussed later. 



The cell-groups in the lower vertebrates appear so similar to those in 

 the well-studied mammals that one is justified in assuming that there are 

 also in the lower vertebrates definitely circumscribed centers. 



The anterior gray horns do not, moreover, send all of their fibers 

 through the anterior roots. It has been demonstrated for representatives 

 of most classes of vertebrates that a very large bundle of fibres passes dorsally 

 and leaves the cord by the posterior roots (see Fig. 33). 



Since motor fibers reach the sympathetic nerve through the posterior 



Fig. 33. — Section through spinal cord of a chick, showing motor fibers to the 

 dorsal root (Fibr. mot. ad rad. dors.). (After Eetzius.) 



root-fibers which innervate the musculature of the viscera, it is probable 

 that in the above-mentioned fibers from the anterior horn we see the motor 

 nerves of the viscera. 



The two ventral cell-columns, belonging essentially to the motor sys- 

 tem, and the two dorsal enlargements of the gray matter which receive the 

 sensory roots produce upon a cross-section of the spinal cord of most verte- 

 brates a gray figure similar to an H. 



The central gray matter, in addition to the several elements already 

 enumerated, contains such innumerable dendrites and collaterals of neu- 

 raxons from the root-cells that it presents a perfect labyrinth of meshes, 

 into which enter the large tracts from the posterior roots, and these, pass- 

 ing through, ramify about the cells of the anterior horn. These sensory 



