Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0101clau Year: 1884 82 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. in the latter case may, by the fusion of the brain and ventral cord, be carried to a still further extent, so that in many cases (numerous Arthropods) only a sub-oesophageal ganglion is present. In Molluscs, animals in which segments are not de- veloped, the subcesophageal ganglion is represented by the pedal ganglion, and there is in addition a third pair of ganglia constituting the visceral ganglia (fig. 55). In Vertebrates, the ne
Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0101clau Year: 1884 82 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. in the latter case may, by the fusion of the brain and ventral cord, be carried to a still further extent, so that in many cases (numerous Arthropods) only a sub-oesophageal ganglion is present. In Molluscs, animals in which segments are not de- veloped, the subcesophageal ganglion is represented by the pedal ganglion, and there is in addition a third pair of ganglia constituting the visceral ganglia (fig. 55). In Vertebrates, the nervous centres are arranged as a cord, lying on the dorsal side of the skeletal axis, and known as the spinal cord, the segmentation of which is indicated by the regular repetition of the spinal nerves. This cord, which is traversed by a central canal, is anteriorly widened and (except in Amphioxus) differentiated into a complicated ganglionic apparatus, the brain (fig. 79). The so-called sympathetic or visceral nervous system appears in the higher animals (Vertebrata, Arthropoda, Hiru- dinea, etc.) as a comparatively indepen- dent part of the nervous system. It consists of ganglia and plexuses of nerves which stand in connection with the central nervous system, but are not under the direct control of the will of the animal. They innervate the organs of digestion, circulation, respiration, and generation, and they can carry on their functions for a longer or shorter time after destruction of the sensory and motor centres. In the Vertebrata (fig. 80), the system of visceral nerves consists of a double chain of ganglia, placed on each side of the vertebral column and con- nected with the spinal nerves and the spinal-like cranial nerves, by connecting branches, the rami communicantes. The ganglia correspond in number with the above- mentioned spinal and cranial nerves, and they send nerves to the FIG. 79.—Brain and spinal cord of a pigeon. //, cerebral hemispheres ; Cb, optic
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