THE SPECIALIZATION OF THE NERVE CORDS. 45 



The Process of Specialization. — The axial or central nervous system under- 

 goes progressive evolution, or specialization, in a transverse and in a longitudinal 

 direction. 



The first process consists in the segregation of similar nerve fibers and cells 

 into concentric, overlying longitudinal zones or tracts, the most notable example 

 of this being the assembling of motor elements toward the haemal surface, and 

 of sensory ones toward the neural surface of the cords. 



The second is the transverse division of the cords into blocks, or neuromeres, 

 which then, singly or in groups, become the centers of some particular function. 

 The linear specialization of the neuron is due to the gradual elimination of the 

 heart, digestive and locomotor organs, from the anterior body metameres, and to 

 the increased size of the sensory and ingestive organs. These changes lead to a 

 great reduction in the number and volume of the motor nerve elements in the 

 anterior metameres, and to the location of functional centers in the neuromeres 

 according to a definite order, which follows that established in the corresponding 

 groups of metameres. See page 209. This order, which is initiated at a very 

 early period in the history of segmented animals, is as follows: olfactory; coordinat- 

 ing; visual; ingestive (i.e., masticatory, swallowing, and gustatory); auditory; 

 locomotory; respiratory (cardiac and branchial); digestive, and urogenital. 

 This process of cephalization progresses in a cephalo-caudal direction, the func- 

 tional centers becoming more and more sharply localized in the direction and 

 order named above. 



IV. The Differentiation of Peripheral Nerves. 



The primary system of transverse nerves forms the foundation of the peri- 

 pheral nervous system. The evolution of these nerves consists mainly in the 

 resolution of the primary network into special nerve bundles composed of fibers 

 having similar central and peripheral terminals. 



The principal stages of the process appear to be as follows : i . Each neuromere 

 is at first connected with several pairs (four?) of transverse nerves, all of which 

 may contain both motor and sensory elements. 2. The number of nerves for 

 each neuromere is ultimately reduced to two main pairs, an anterior and a posterior. 

 3. The roots of the anterior nerves gradually shift toward the neural surface of 

 the cord; the posterior ones retain a more hasmal position. The two series of 

 ner^'es thus formed, are called the neural and the haemal nerves. 4. The neural 

 nerves develop ganglia on their proximal ends, and in those regions of the body 

 where appendages are developed, supply only the appendages. The haemal 

 nerves are without ganglia and supply the remaining parts of the metamere. 5. 

 In the cephalothoracic, or head region, the neural and haemal nerves remain 

 separate (vertebrates and arthropods), while in the more posterior regions they 

 may unite, for a longer or shorter distance, forming single nerves with two sets of 

 roots, ganglionated neural roots, and non-ganglionated haemal roots. 6. Both 

 neural and haemal nerve roots contain motor and sensory elements, but at an 



