THE NERVOUS SYSTEM 



391 



The most extensive cellular migration leads to the formation of the myen- 

 teric and submucous plexuses (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. 



346) 



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 



CORPUS CALLOSUM 



TaXNCEPHALON 



THIRD VENTRICLE 



OPTIC NERVE- 



LATERAL VEirmiCLE 



ISTERNA SUPERIOR 



NTORIUM 

 REBaj-l 



SUR+ INF. COLLICUU 



CEREBELLUM 



CHORIOID PLEXUS 



CI STERNA 



SUBDURAL CAVITY 

 ARACHNOI 



dura mater 

 :entral canal 

 subarachnoidal space 

 subdural cavity 

 filum terminaie 



Fig. 347. — Diagram showing the relations of the meninges to the central nervous 

 system, as shown in median longitudinal section and in cross section. (Redrawn after 

 Rasmussen's "Principal Nervous Pathways," The Macmillan Co.) 



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 organs. There it becomes the highly vascular 

 meninx primitiva. Such connective-tissue membranes or meninges 



