. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. The Dominating Factor 697 sensory neurons are located in the cord most of them are in dorsal root ganglia as in higher vertebrates. The neurites of these sensory cells run short distances in the dorsal part of the cord giving off collaterals into the gray matter. In general the cord is for local reflexes but there are a few very large motor fibers of Muller which run from the brain to the posterior end of the cord. The cartilaginous fishes show definite ad
. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. The Dominating Factor 697 sensory neurons are located in the cord most of them are in dorsal root ganglia as in higher vertebrates. The neurites of these sensory cells run short distances in the dorsal part of the cord giving off collaterals into the gray matter. In general the cord is for local reflexes but there are a few very large motor fibers of Muller which run from the brain to the posterior end of the cord. The cartilaginous fishes show definite advances toward the plan of higher vertebrates. For the first time myelinization of the fibers occurs, forming true white matter. Further the gray matter shows an arrangement into dorsal and ventral horns, although the dorsal horns are usually com- bined into a single broad region so that the gray matter is shaped like an inverted T rather than like an H as in mammals (Fig. 624). Instead of being arranged in compact dorsal funiculi, the sensory neurites form small bundles scattered through the dorsal gray matter. Also these sensory fibers are usually short, not extending into the brain region as they do in mam- mals. Compared with cyclostomes there has been an increase in the number of tracts set up between brain and cord. There are ascending fiber groups, probably corresponding to the dorsal spinocerebellar tract, as well as several descending tracts from the medulla and mid-brain. Dorsal Root Dorsal Horn. Ventral Root Fig. 624. Cross section of the spinal cord of a cartilaginous fish. (From Kappers, Huber, and Crosby, The Comparative Anatomy of the Nervous System of Vertebrates, including Man, copyright 1936, by permission of The Macmillan Company, publishers.) The amphibians are the first to have brachial and lumbar enlargements, which make their appearance as the result of the development of the legs. As large dorsal funiculi are laid down, the gray matter assumes the butter-
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