736 



NEUROLOGY 



The fibers of the posterior roots are developed from the cells of the spinal ganglia. 

 Before the neural groove is closed to form the neural tube a ridge of ectodermal 

 cells, the ganglion ridge or neural crest (Fig. 644), appears along the prominent: 

 margin of each neural fold. When the folds meet in the middle line the two gan- 

 glion ridges fuse and form a wedge-shaped area along the line of closure of the tube. 

 The cells of this area proliferate rapidly opposite the primitive segments and then 

 migrate in a lateral and ventral direction to the sides of the neural tube, where they 

 ultimately form a series of oval-shaped masses, the future spinal ganglia. These 

 ganglia are arranged symmetrically on the two sides of the neural tube and, except 

 in the region of the tail, are equal in number to the primitive segments. The cells 

 of the ganglia, like the cells of the mantle layer, are of two kinds, viz., spongio- 

 blasts and neuroblasts. The spongioblasts develop into the neuroglial cells of the 

 ganglia. The neuroblasts are at first round or oval in shape, but soon assume 

 the form of spindles the extremities of which gradually elongate into central and 



peripheral processes. The central 

 processes grow medialward and, be- 

 coming connected with the neural 

 tube, constitute the fibers of the 

 posterior nerve roots, while the per- 

 ipheral processes grow lateralward to 

 mingle with the fibers of the anterior 

 root in the spinal nerve. As de- 

 velopment proceeds the original 

 bipolar form of the cells changes; 

 the two processes become approxi- 

 mated until they ultimately arise 

 from a single stem in a T-shaped 

 manner. Only in the ganglia of the 

 acoustic nerve is the bipolar form 

 retained. More recent observers hold, 

 however, that the T-form is derived 

 from the branching of a single pro- 

 cess which grows out from the cell. 



The anterior or ventral and the pos- 

 terior or dorsal nerve roots join imme- 

 diately beyond the spinal ganglion to form the spinal nerve, which then divides into 

 anterior, posterior, arid visceral divisions. The anterior and posterior divisions 

 proceed directly to their areas of distribution without further association with 

 ganglion cells (Fig. 645). The visceral divisions are distributed to the thoracic, 

 abdominal, and pelvic viscera, to reach which they pass through the sympathetic 

 trunk, and many of the fibers form arborizations around the ganglion cells of this 

 trunk. Visceral branches are not given off from all the spinal nerves; they form 

 two groups, viz., (a) thoracico-lumbar, from the first or second thoracic, to the 

 second or third lumbar nerves; and (6) pelvic, from the second and third, or 

 third and fourth sacral nerves. 



The Brain. The brain is developed from the anterior end of the neural tube, 

 which at an early period becomes expanded into three vesicles, the primary cerebral 

 vesicles (Fig. 18). These are marked off from each other by intervening con- 

 strictions, and are named the fore-brain or prosencephalon, the mid-brain or 

 mesencephalon, and the hind-brain or rhombencephalon the last being continuous 

 with the medulla spinalis. As the result of unequal growth of these different 

 parts three flexures are formed and the embryonic brain becomes bent on itself 

 in a somewhat zigzag fashion; the two earliest flexures are concave ventrally 

 and are associated with corresponding flexures of the whole head. The first flexure 





FIG. 644. Two stages in the development of the neural 

 crest in the human embryo. (Lenhossfik.) 



