298 



PHYSIOLOGY 



are formed from the most superficial layer of the invaginated epiblast, and 

 are spoken of as spongioblasts. The deeper layer of cells, which are to give 

 rise to the permanent nerve-cells, and are therefore known as neuroblasts, 

 rapidly divide and form a thick layer surrounding the internal layer of 

 spongioblasts, through which pass the peripheral processes of the latter. 

 When first formed these cells have no processes. Later on each neuroblast 

 acquires a pear shape, the stalk of the pear having a somewhat bulbous 

 extremity (Fig. 142). The stalk continually elongates, and the elongated 

 process may leave the spinal cord altogether and grow outwards to any part 



of the body of the embryo, or may pass 

 to other parts of the central nervous sys- 

 tem. This long process of the developing 

 nerve-cell is known as the axon. Some 

 time after its formation other processes 

 grow out from the cell, which soon branch 

 and end in the immediate neighbourhood 

 of the cell. The axons of the cells near the 

 ventral part of the neural tube grow out 

 to the different muscles of the body, 

 where they end in close connection with 

 the muscular fibres by an arborisation 

 1, which forms the end-plate. They provide 

 an efferent path for impulses running 

 from the central nervous system to the 

 musculature of the body. The afferent 

 channel is formed in a somewhat different 

 manner. Even before the neural groove 

 has closed in, a thickening of the epiblast 

 is seen immediately external to the 

 groove on each side. This thickening 



becomes divided into a series of collections of cells lying immediately under 

 the epiblast on the lateral and dorsal surface of the neural canal. The 

 cells, which are at first round or oval, send off two processes in opposite 

 directions so that they become bi-polar (Fig. 142). One process passes into 

 the central nervous system, where it divides, some of its branches being 

 distributed in the nervous system at the same level, while others run a con- 

 siderable distance towards the head immediately outside the tube of nerve- 

 cells. The other process grows downwards, along with the processes from 

 the ventral cells of the tube, towards the periphery of the body, where it ends 

 in close connection with the surface in the various sense-organs of the skin 

 and muscles. These collections of bi-polar cells form the posterior root 

 ganglia. In fishes they retain their primitive character throughout life, 

 but in mammals the bi-polar cell is to be found only in the spiral and vesti- 

 bular ganglia which give origin to the fibres of the eighth nerve. In all 

 the other ganglia the shape of the cell becomes modified by an approxima- 

 tion of the points of attachment of the two processes until finally the cell 



FIG. 141. Neuroblasts from the spinal 

 cord of a chick embryo. (CAJAL.) 



A, three neuroblasts stained to show 

 neuro- fibrils ; a, a bi-polar cell. 



B, a neuroblast showing the ' incre- 

 mental cone ' c. 



