THE NERVOUS SYSTEM 



561 



vessels and viscera. Connexions with the nerves from which the sympa- 

 thetic ganglia arise persist as the rami communicantes. The metameric 

 ganglia become secondarily connected by a sympathetic cord which 

 runs parallel with the dorsal aorta. The prevertebral plexuses, cardiac, 

 coeliac, and hypogastric, arise by the more extensive migration of cells 

 from the ganglia of the vagus nerve. The most extensive cellular migra- 

 tion leads to the formation of the myenteric and submucous plexuses 



PLEXUS OF AUERBACH/^ 



.LONGITUDINAL 

 MUSCLES 



-PERITONEUM 



Fig. 463. — A stereogram of a portion of the small intestine, showing the arrangement 

 of sympathetic neurons in the plexuses of Meissner and Auerbach. Motor cells are 

 shown in black, sensory cells with white nuclei. (Redrawn after Kahn.) 



(the plexuses of Auerbach and of Meissner). But, however remote from 

 their source such sympathetic cells may be, they retain fibrous connection 

 with the rest of the nervous system. (Fig. 463) 



The whole sympathetic system is well established in a three months 

 human embryo. 



MENINGES 



The spinal cord of Amphioxus is surrounded by loose connective 

 tissue. In cyclostomes this tissue shows the beginnings of differentiation 

 into compact outer and inner layers with loose tissue between, the three 

 representing possibly the three meningeal layers of higher vertebrates. 

 In fishes the cranium and the vertebrae are lined by compact periosteum 

 or perichondrium, between which and the brain or cord the connective 

 tissue is loose, except where the connective tissue comes in contact with 

 the central nervous system. There it becomes the highly vascular 

 meninx primitiva. Such connective tissue membranes or meninges 

 surrounding the central nervous system (brain and cord) serve both for 

 protection and for nourishment. 



