EMBRYOLOGY AND COMPARATIVE ANATOMY OF BRAIN. 



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fibres passes to the central organ ; the other grows toward the 

 periphery as a sensory nerve. 



The ganglia which are called upon to play so important 

 a part in the peripheral nervous system first appear in the form 

 of a groove, the sides of which, later, close so as to form a ridge 

 (ganglionic ridge). This ridge lies on both sides of the central 

 organ, and at a very early period becomes separated into the 

 individual ganglia. From it, opposite the secondary fore-brain, 

 is developed the olfactory fossa ; 

 opposite the inter-, mid-, and hind- 

 brains, the complex of the trigem- 

 inus ganglia ; opposite the after- 

 bran we see the ganglia of the 

 acustico-facialis, of the glosso- 

 pharyngeus and of the vagus, 

 developed from this ganglionic 

 ridge. The auditory fossa, which 

 lies between them, apparently 

 originates in this ridge. (His, 

 oppos. Beard.) Further caudad, 

 the spinal ganglia replace the 

 above-named ganglia of the cra- 

 nial nerves, along the sides of the 

 spinal cord. The roots of all 

 the above-mentioned sensory era- 



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nial nerves and of the posterior 

 roots of the spinal cord issue, therefore, from the ganglia and 

 grow into the central organ. In the spinal cord there is a cor- 

 responding dorsal sensory root for each ventral motor root. In 

 the brain, however, this is not the case. The distribution of 

 the motor nuclei does not correspond closely to the divisions of 

 the complex of ganglia. The ganglionic ridge has a greater 

 n-umber of segments, and several motor nuclei are equivalent to 

 a single complex of ganglia. These are the main points in the 

 development of the human brain. 



FIG. 10. 



Transverse section through the spinal 

 cord of a human embryo of 4 weeks. 

 Ventrad we see the anterior roots de- 

 veloping from the cells of the cord. 

 Porsad (from an embryo of 4% weeks) 

 the posterior roots grow centrally from 

 the cells of the spinal ganglion. (Com- 

 bined from designs by His. ) 



