GENERAL MORPHOLOGY OF THE NERVOUS SYSTEM. 13 



is very soft and somewhat gray in color, while the outer part is 

 more firm and white. The gray matter is composed chiefly of 

 cells, the white matter chiefly of fibers. The whiteness is due to 

 the sheaths covering the fibers. The distinction between cellular 

 and fibrous parts is not apparent in the cord of such an animal as 

 Petromyzon whose nerve fibers are not provided with such 

 sheaths. The gray matter has in cross section somewhat the form 

 of the letter H, the central canal being found in the crossbar of the 

 H. In either half of the cord a part of the gray matter extends 

 dorsal to the central canal and a part extends ventral to it. These 

 parts are called respectively the dorsal and ventral horns of the 

 gray matter. In mammals and man a smaller projection of the 

 gray matter laterally is spoken of as the lateral horn. 



Each lateral half of the cord has connected with it the roots 

 of the peripheral nerves. In all vertebrates there is in each 

 typical segment of the body a dorsal root connected with the dorsal 

 surface of the cord and a ventral root connected with the ventral 

 surface. In the lowest vertebrates (Amphioxus and Cyclostomes) 

 the dorsal and ventral roots of the same side alternate with one 

 another along the cord. This is due to the fact that the dorsal 

 nerve is destined to go in larger part to the skin while the ventral 

 nerves go to the muscles of the trunk. Since the muscles are 

 arranged in simple transverse segments following one another, 

 the dorsal nerve is placed in the interval between two segments 

 where it passes in the intermuscular septum of connective tissue 

 to the skin. The ventral nerve, on the other hand, is situated 

 opposite the middle of the muscle segment and is distributed 

 directly to the muscle. Since part of the muscle lies above the spinal 

 cord the ventral root divides into a dorsal and a ventral ramus. 

 The dorsal root, after emerging from the spinal column becomes 

 thickened by a collection of ganglion cells. This thickening is 

 called the spinal ganglion. Beyond the ganglion the nerve divides 

 into dorsal and ventral rami, which go to the skin. 



In true fishes and in all higher vertebrates there has come about 

 a shifting of parts such that the dorsal and ventral roots of a 

 given segment come to lie nearly in the same transverse plane of 

 the body and the two roots unite at or just beyond the ganglion 



