298 



PHYSIOLOGY 



ff. 



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 r 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 

 i 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 pro- 

 cesses in opposite directions so that they become bi-polar (Fig. 142). 

 One process passes into the central nervous system, where it divides, some 

 "I 11- branches being distributed in the nervous system at the same level, 

 win!.' others run a considerable distance towards the head immediately out- 

 side the tube of nerve cells. The other process grows downwards, along 

 with tin- 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 urL'aii* of t lie 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 vestibular 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 approximation of the points of attachment of the two processes until 



FIG. 141. Neuroblasts from the spinal 

 cord of a chick embryo. (CAJAL.) 



A, three neuroblasts stained to show 

 neurofibrils ; a. a bi-polar cell. 



B. a neuroblast showing the ' incre- 

 mental cone ' r. 



