5l6 COMPARATIVE ANATOMY 



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, amphibia, 

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

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

 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. 429) 



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 as a result of 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. 429) 



The spinal cord of reptiles differs 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 color 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 neurones located in the gray matter. The intermediolateral 

 column, which throughout the vertebrate series contains the ganglion 

 cells of the motor nerves which supply visceral muscles, becomes more 

 distinctly demarked than in lower vertebrates. The gray matter crosses 

 the median plane of the cord as the gray coniinissure which surrounds 

 the central canal. 



A considerable increase in the amount of white matter in mammals 

 indicates a further dominance of the brain and a greater integration 

 of the body. The relative amount of white matter diminishes from the 

 medulla to the filum terminale in which the spinal cord ends. In the 

 region of the arms and legs, at the cervical and lumbar enlargements, both 

 gray and white matter increase in quantity. The division of the white 

 matter into funicuH, begun in cyclostomes, reaches its cUmax in man. 

 Dorsal, lateral, and ventral funicuU are separated from one another not 



