THE NERVOUS SYSTEM 369 



The spinal cord of cyclostomes is much flattened, with the cellular 

 matter distributed in a pair of lateral wings. Ventral to the central 

 lumen, a number of giant or Muller's fibers extend lengthwise and carry 

 from the brain impulses thought to be chiefly static. Dorsal to the lumen, 

 are sensory ganglion cells like those of Amphioxus. The outer fibrous or 

 marginal layer of the cord is divided into longitudinal bundles of fibers, 

 the funiculi. Medullary sheaths are lacking, so that sympathetic flbers 

 can not be distinguished from others. Relations of neuron in the cord 

 indicate that it is a reflex center and a pathway for intersegmental nervous 

 connexions. Increase in descending fiber tracts demonstrates the increas- 

 ing dominance of the brain. Polynuclear gland cells, possibly of endocrinal 

 function, occur in the caudal region of the cord. (Fig. 326) 



In elasmobranchs nerve fibers are myelinated, so that white and gray 

 matter show the same contrast as in the cord of higher vertebrates. 

 Dorsal and ventral columns of gray matter are differentiated, but the 

 dorsal columns merge together in the median plane. Somatic motor 

 cells of the ventral column are very large, as in other fishes, amphibians, 

 and reptiles; and the dendrites extend into the dorsal column. Sensory 

 ganglion cells, except the embryonic and transient Rohon-Beard ceUs, 

 have migrated into the spinal ganglia. Dorsal, lateral, and ventral 

 funiculi have relations similar to those of higher vertebrates. The enlarge- 

 ment of the lateral walls of the cord results in the formation of a deep 

 ventral fissure. The suggestions of the formation of a dorsal septum are, 

 however, slight. (Fig. 326) 



The spinal cord of Amphibia resembles in fundamental characters 

 that of elasmobranchs. The dorsal columns of gray matter become more 

 distinctly paired, so that the gray matter assumes in cross section the 

 form of a capital H characteristic of all higher animals. In the gray 

 matter the nerve cells retain their central position surrounded by a 

 network of fibers and their synaptic connexions. A dorsal septum has 

 developed in connexion with the increased thickness of the dorsal portion 

 of the lateral wall of the cord. In the cervical and lumbar regions the 

 diameter of the cord is considerably increased in correlation with the 

 enlargement of the appendages. (Fig. 326) 



The spinal cord of reptiles difi"ers in no essentials from that of mammals. 

 The increase in thickness of the marginal layer of longitudinal fibers 

 indicates an increased integration of the body. The fibers which ramify 

 through the gray matter are non-medullated, and their color is gray in 

 contrast with the white of the medullated fibers of the marginal zone. 



Within the gray matter of the cord, sensory and motor nerves of 

 reflex arcs usually effect their synaptic connexions by the intermediation 

 of association neurons located in the gray matter. The intermedio-lateral 

 column, which throughout the vertebrate series contains the ganglion 



