. Text book of vertebrate zoology. Vertebrates; Anatomy, Comparative. 46 MORPHOLOGY OF THE ORG A AS OF VERTEBRATES. ism consists in alternate expansions and contractions of the cord and its contained canal.^ This early segmentation disap- pears with growth, but the same segments are later indicated by the roots of the spinal nerves. The spinal nerves are paired structures passing off from either side of the cord (Fig. 47), each nerve arising by two roots, one dorsal the other ventral in position. These roots differ markedly in structure and function. The dorsal roots are connected with the dor
. Text book of vertebrate zoology. Vertebrates; Anatomy, Comparative. 46 MORPHOLOGY OF THE ORG A AS OF VERTEBRATES. ism consists in alternate expansions and contractions of the cord and its contained canal.^ This early segmentation disap- pears with growth, but the same segments are later indicated by the roots of the spinal nerves. The spinal nerves are paired structures passing off from either side of the cord (Fig. 47), each nerve arising by two roots, one dorsal the other ventral in position. These roots differ markedly in structure and function. The dorsal roots are connected with the dorsal horn of the gray matter, and soon after leaving the cord each becomes enlarged into a ganglion composed of those ganglion cells which give rise to the fibres of which this root is composed. The ventral roots,. Fig. 47. Section through spinal cord showing the roots of a spinal nerve. d, dorsal root with its ganglion; g, gray matter of cord; v, ventral root; rt', white matter. on the other hand, are not ganglionated, but their fibres are connected with the ganglion cells of the gray matter of the ventral horns of the cord, from which they pass out into the root. Just beyond the ganglion of the dorsal root the two roots of a spinal nerve unite, and the fibres of each follow a common course. Experiment shows that the dorsal roots are sensory; , they carry impulses from the terminal sensory structures to the central nervous system. The ventral roots are motor in func- tion ; that is, the nervous impulses which they transmit come from the central nervous system, and are carried to peripheral portions (muscles, glands, etc.) which they cause to act. Since the dorsal roots convey stimuli from without to the 1 It must be understood that this metamerism is not necessarily primitive in its Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not
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