THE I VOLUTION OF Till \i RVOl i.M 



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

 animals, bu1 there is a very distinct morphological difference: in the lower 

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

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

 lakes the place of the ganglia, is dorsal to the alimentary canal. In b 

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

 vertebrates these become much more definite in structure, and constitute 

 the brain. This morphological difference between vertebrates and inverte- 

 brates is probably no1 so fundamental as a1 first sighl it may appear to 

 bo, for, as Gaskell has shown, ii is possible thai the alimentary canal of 



the invertebrates is really homologous with tl intra] canal of the 



spinal curd 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 thai the ganglia have grown up so as to Burround the alimen- 

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

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

 of ventral downgrowth of the body walls. Although this view'has nol 

 Keen generally accepted by biologists, their is no inherent reason why it 

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

 known fact thai a new respiratory system 1" mes developed in the 



passage from aquatic to land amphibians. 



The fibers of the sensory neurons in vertebrates are collected tog 

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

 neurons are located not on the surface, as in invertebrates, l>ut in t In- 

 posterior root ganglia, the cells being connected to the filters by T-sha] 

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

 winch retains its primitive condition. 



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

 central nervous Bystem 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 segmenl in which it is itself located, hut by way of 

 SOCiation neurons from other BCgmentS 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 mi 



merits, which are coordinated with those of adjl nd which 



may lie of widely varying types. The motor neuron has therefore ■ 

 appropriately been called the final common /><»//'. ami it will be one of our 

 main objects later to show the conditions under which several diffei 

 competing influences may obtain possession of this path. 



