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 lobes ; 

 C, cerebellum ; Mo, medulla 

 oblongata. Sp, spinal nerves. 



