THE EVOLUTION OF THE NERVOUS SYSTEM 787 



tion in the nervous system of the lower as compared with the higher 

 animals, but there is a very distinct morphological difference: in the lower 

 or invertebrate animals the ganglion nerve chain is ventral to the alimen- 

 tary canal, whereas in the higher or vertebrate, the spinal cord, which 

 takes the place of the ganglia, is dorsal to the alimentary canal. In both 

 groups the head ganglia are dorsal to the alimentary canal, but in the 

 vertebrates these become much more definite in structure, and constitute 

 the brain. This morphological difference between vertebrates and inverte- 

 brates is probably not so fundamental as at first sight it may appear to 

 be, for, as Gaskell has shown, it is possible that the alimentary canal of 

 the invertebrates is really homologous with the central canal of the 

 spinal cord and the ventricles of the brain of the vertebrates. Accord- 

 ing to this observer, what has really happened in the latter group of 

 animals is that the ganglia have grown up so as to surround the alimen- 

 tary canal and so constitute a continuous structure, a new alimentary 

 canal being meanwhile provided by the enclosure of a space as a result 

 of ventral downgrowth of the body walls. Although this view has not 

 been generally accepted by biologists, there is no inherent reason why it 

 should not be accepted. It is no more to be wondered at than the well- 

 known fact that a new respiratory system becomes developed in the 

 passage from aquatic to land amphibians. 



The fibers of the sensory neurons in vertebrates are collected together 

 to form the posterior roots of the spinal cord, and the cell bodies of these 

 neurons are located not on the surface, as in invertebrates, but in the 

 posterior root ganglia, the cells being connected to the fibers by T-shaped 

 junctions. The olfactory nerve is the only one in the higher vertebrates 

 which retains its primitive condition. 



In the vertebrate animals the spinal member in the integration of the 

 central nervous system is the motor neuron, the fibers being collected in 

 the anterior roots. Toward the cell of this neuron impulses are transmitted, 

 not only from the segment in which it is itself located, but by way of as- 

 sociation neurons from other segments or from far distant parts of the 

 central nervous system. In other words, this motor neuron may transmit 

 impulses which cause the muscles to perform local independent move- 

 ments, which are coordinated with those of adjacent segments and which 

 may be of widely varying types. The motor neuron has therefore very 

 appropriately been called the final common path, and it will be one of our 

 main objects later to show the conditions under which several different 

 competing influences may obtain possession of this path. 



