1468 



A MANUAL OF ANATOA^ 



ventricle, as the others are, but merely an isolated portion of the great longi- 

 tudinal fissure. 



Meninges of the Encephalon. — The walls of all the cerebral vesicles are 

 invested by mesoderm, and this tissue becomes differentiated into the three 

 meninges — namely, the dura mater, arachnoid, and pia mater. 



Choroid Plexuses. — ^The choroid plexuses of the two lateral, third, and 

 fourth ventricles are developed as infoldings of the ependymal walls of the 

 ventricles. Vascular mesenchyme (mesoderm) dips in between the two layers 

 of each infolding, and in this manner plicss choroidecs are formed. These 

 choroidal folds give rise to the choroid plexuses which, as they project into 

 the ventricles, carry the ependymal walls, already infolded, before them. 



Development of the Peripheral Nervous System. 



The peripheral nerves are arranged in two groups — namely, spinal, which 

 are derived from the spinal cord, and cranial, which arise from the brain. 



The spinal nerves are composed of two kinds of fibres — efferent, centrifugal, 

 or motor, and afferent, centripetal, or sensory. 



A motor spinal nerve-flbre arises as the axon of a neuroblast or nerve-cell 

 in the mantle-layer of the neural tube. (See Development of the Spinal 

 Cord.) 



A sensory spinal nerve-flbre is developed from a cell of a spinal ganglion, 

 and these gangUa are developed from the corresponding neural crest. 



Neural Crest (Ectoderm) 



Neural Tube 



derm 



ce of Sen.sory Roots 



Fig, 613. — Development of the Neurai, or Ganglion Crest (Keibel 

 AND Mall) (after Von Lenhossek and Kollmann). 



Neural Crests. — The neural or ganglionic crests, right and left, are ridges 

 of ectodermic cells, which lie on either side of the neural tube. They are 

 derived from a single crest of ectoderm, which is formed by the fusion of 

 the ectoderm over each neural fold, this single crest being situated mesially 

 on the dorsal aspect of the neural tube, along the line of fusion of the neurail 

 folds to close the tube. Subsequently the mesial crest divides into right. 

 and left halves, which migrate to the lateral aspects of the neural tube. ^* 



Each neural crest becomes broken up into a number of segments, or ganglia, 

 there being four pairs for the head-region, and thirty-one pairs for the region 

 of the trunk. 



Spinal Ganglia. — The spinal ganglia are arranged in thirty-one pairs, right 

 and left. 



Each cell of a ganglion acquires two poles— afferent or centripetal, and 

 efferent or centrifugal, and, at this stage, it is consequently a bipolar cell. 



