THE PERIPHERAL NERVOUS SYSTEM. 261 



the third ventricle, or cavity of the fore-brain, through a pair of 

 narrow apertures, the foramina of Monro. 



4. The Peripheral Nervous System. 



a. General Account. The nerves, both cranial and spinal, 

 which compose the peripheral nervous system are entirely of 

 epiblastic origin, and develop in a manner closely similar to that 

 already described in the frog. 



The nerves fall under two categories : — 



(i) The ganglionated nerves. These arise directly from the 

 inner surface of the epiblast, as a pair of longitudinal neural 

 ridges, along the margins of the neural plate. They appear before 

 the neural tube is closed (Fig. 117, ma), and by the folding of 

 its walls to complete the tube they get carried on to its dorsal 

 surface, where they form a pair of bands (Fig. 118, NA), projecting 



Fig. 117.— Transverse section across the body of a Chick Embryo at the twenty - 

 fourth hour of incubation. (Cf. Fig. 110.) x 200. 



CH, notoohord. E. epiblast. H, hypoblast. M, mesoblast. MA. commencing 

 neural ridge. WGr, neural groove. NP, neural plate. 



outwards from the angles between the external epiblast and the 

 walls of the neural tube. On the completion of the neural tube 

 by fusion of its lips, the neural ridges separate from the surface 

 epiblast, but remain in close contact with the dorsal surface of 

 the tube (Fig. 119, nb). 



I The neural ridges are at first continuous structures, from 

 hich the nerve ganglia arise as paired outgrowths ; these grow 

 ipidly, extending outwards and downwards, and acquire their 

 ^rmanent roots of attachment by outgrowth of nerve fibres 

 om the ganglion cells into the brain or spinal cord. 

 To this category belong the fifth, the seventh and eighth, the 

 sensory roots of the ninth and tenth, with perhaps one or two 

 of the other cranial nerves; and the dorsal or sensory roots 

 of the spinal nerves. 



