20 



NERVOUS SYSTEM AND SENSE-ORGANS 



Front 



connected with the brain and spinal cord, to which it is sub- 

 ordinate, and its nerves branch out in the organs of digestion, 

 circulation, &c. A few further details have already been given 

 with regard to the nervous system of Man (see vol. i, p. 49). 



It was stated at the commencement of this section that the 

 essential elements of the nervous system, i.e. the neurons, are 

 derived from the ectoderm or outer cell -layer. Considering 

 that brain and spinal cord are far removed from the surface, 

 while the body is traversed in all directions by nerves, it seems 



very difficult to believe 



./. such a statement, but the 



study of development 

 shows that there is no 

 doubt at all about the 

 matter. 



At a comparatively 

 early stage in the de- 

 velopment of an embryo 

 part of the ectoderm 

 covering the upper sur- 

 face thickens into a nerve- 

 plate, which sinks below 

 the surface, and at the 

 same time folds up to 

 constitute the nerve-tube. 

 The details for the Lance- 

 let have already been given (vol. iii, p. 345), but in that animal 

 the nerve-plate sinks below the surface before it is completely 

 folded into a tube, while in average cases the two processes go 

 on simultaneously, as will be gathered from fig. 1024. 



The walls of the nerve-tube thicken, and by a process of 

 unequal growth the spinal cord and the various regions of the 

 brain come into existence. The rest of the nervous system 

 grows out from the nerve-tube, e.g. the spinal nerves grow out 

 from the spinal cord to the parts of the body which they supply. 

 It therefore follows that these and the other nerves, as well as 

 the sympathetic system, are really ingrowths from the ectoderm 

 or outer cell-layer, although in the adult they are far removed 

 from the surface. 



THE BRAIN OF VERTEBRATES. At first sight the brains of Fishes, 



Back 



Fig. 1024. Development of Central Nervous System in a 

 Vertebrate Embryo, diagrammatic 



A, Upper side of embryo, showing folding-up of the nerve-plate; 

 B, c, and D, stages in folding-up of nerve-plate, as seen in cross- 

 sections through upper part of A, taken at levels i-i, 2-2, and 3-3; 

 n.g., nerve-groove; n.f.\ nerve-folds; ./., nerve-tube. 



