520 



THE NEEYOUS SYSTEM. 



These enlargements of the spinal medulla are associated with the outgrowth of the 

 limbs. In the earlier developmental stages of the spinal medulla they are not present, 

 and they take form only as the limbs become developed. In different animals their size 

 corresponds with the degree of development of the limbs. Thus, in the long-armed orang 

 and gibbon the cervical swelling stands out with a remarkable degree of prominence. 



Development of the Spinal Medulla. The early stages of the process by 

 which the originally simple epithelial neural tube becomes converted into the 

 central nervous system have already been considered. It remains to be explained 

 how the features specially distinctive of the spinal medulla are produced. 



In the early stages of the development of the spinal medulla (Fig. 463), the 

 neuroblasts are found to be scattered in the intermediate of the three bands of 



Funiculus posterior 



Sensory 

 ganglion 



Marqinal 

 lo-ye?-- 





Floor 



plaCe 



Commissural fibre 



-Anterior nerve root 



FIG. 463. DIAGRAM OF TRANSVERSE SECTION OF THE LEFT HALF OF EARLY NEURAL TUBE. 



which the thick side wall of the neural tube is composed the mantle layer. 

 These primitive nerve-cells soon congregate in much larger numbers in the ventral 

 part of the basal lamina (Fig. 464), so that the mantle layer expands there intc 

 a broad excrescence, which is the rudiment of the columna anterior or anterioi 

 cornu of gray matter. This anterior column contains the efferent or motor nerve 

 cells, the axons of which emerge as the anterior root of a spinal nerve. At this 

 stage the rest of the mantle layer consists of a thin stratum of neuroblasts (Fig. 463^ 

 mainly intercalated cells, which receive the sensory impressions entering th> 

 spinal medulla through the radix posterior, and transmit impulses into axon 

 passing (a) to the motor nuclei, (&) to the other side of the spinal medull; 

 through the floor-plate (Fig. 463), or (c) into the superficial stratum (periphera 

 layer) of the spinal medulla where they bend upwards or downwards as constituen 

 elements of the funiculi (or white columns). As development proceeds (Fig. 462 

 the substantia grisea (gray substance) formed of these intercalated cells become 

 much more abundant and forms a broad blunt boss (Figs. 464, B and C), which :' 

 the rudiment of the columna posterior (O.T. posterior cornu). 



The surfaces of these gray columns become coated with a layer of white sul 



