DEVELOPMENT OF NERVOUS SYSTEM 



757 



occur singly or in groups in such plexuses as those of Auerbach and Meissner, within the tunics 

 of the walls of the ahmentary canal. Scattered along between these proximal, intermediate, 

 and distal groups there are to be found small straggling ganglia, many of which contain so few 

 cell-bodies that they are indistinguishable with the unaided eye. All these sympathetic neu- 

 rones, however, are always either directly or indirectly anatomically associated with and 



Fig. 598.^ — Diagrams of Oral Portion of Human Neural Tube Showing the Three 



Primary Brain Vesicles and Some of the Secondary Vesicles Derived from Them. 

 A, diagram of dorsal view of early stage. B, lateral view at about the third week. C, lateral 



view at about the eighth week. After His, modified, m, mamillary vesicle; i, infundibular 



recess; o, olfactory vesicle. 



Anterior primary veeicle 

 ^ opHc vesicle 



"middle pritnary vesicle ' 



'Posterior primary Tesicle ' 



Auditory vesicle ' 



Middle primary vesicle ^ 

 Anterior pnmary vesn 

 Tdencephalon^ 



ep iphjisis 

 iicle_ 



spinal cord 



- -cerebellum 

 ~ ; 'Posterior pninary 

 vesicle 



medulla 



oUongata 



olfadeiy vesicle 

 optic vesicle' 



Pontine flfexure 



cervical flexure 



Fig. 599. — Diagrammatic Sagittal Section of a Vertebrate Brain. (After Huxley.) 

 4, fourth ventricle; s, cerebral aqueduct; 3, third ventricle. 

 Corpora quadrigemina Mid-brain Epiphysis 



Lateral ventricle 



Medulla oblongata 

 (hind -brain) 



Pons Varoli 

 (,hind-brainj 



Cerebral hemisphere 

 Corpus striatum 



Cerebral peduncle Thalamus | Hypophysis 



Hypothalamus 



Foramen of Monro 



Fig. 600.— Diagrammatic Horizontal Section of a Vertebrate Brain. 

 4, fourth ventricle; 3, third ventricle. 



Metencephalon Thalamus 



MeduUa oblongata' 

 Cerebellum 



(Afte Huxley.) 



Lateral ventricle 



Lamina terminalis 



Corpus striatum 



Mid-brain Epiphysis Foramen of Monro 



largely under the control of the neurones of the central system through central visceral efferent 

 fibres passing to them by way of the rami communicantes or by way of the peripheral distri- 

 bution of the spinal nerves. 



The ganglia of the sensory portions of all those cranial nerves attached to the inferior of 

 the main divisions of the brain and all the sympathetic ganglia of the head have an origin 

 similar to that of the spinal and sympathetic gangha in the remainder of the body. 



The behavior of the walls of the three primary vesicles, into which the oral end of the neural 

 tube is converted, is much mare complex than in case of the spinal cord. Their walls do not 



