E. NERVOUS SYSTEM. 



The nervous system has the important function of placing the 

 organism in communication with its surroundings, stimuli received 

 by the sensory organs being transformed into nerve-impulses which 

 are conducted along the afferent or sensory nerve-tracks to the 

 central organ of the system. In the latter these stimuli are 

 transformed, or new ones are originated, and they travel along the 



cut or motor nerve-tracks to muscular elements, thus causing 

 their contraction, or to glands, causing them to secrete. The 

 intimate connection between muscle and nerve has already been 

 referred to. 



It was pointed out in the Introduction that the nervous system 

 arises from the ectoderm. The parts of it which first become 

 differentiated histologically are the nerve- cells (ganglion-cells), 

 from which nerve-fibres arise later and serve as the conductors 

 of nervous impulses. The most '."important constituent of the 

 nerve-fibre is a central neuraxis or axis-fibre, and in those nerve- 

 fibres which are spoken of as medulla ted this is surrounded by a 

 highly refractile, fat-like substance (myelin), which forms the 

 medullary sheath. In certain (non-medullated) nerve-fibres this 

 sheath is wanting, but the two kinds of fibres are not sharply 

 marked off from one another, either locally or genetically : a fibre 

 may be medullated in one part of its course, and non-medullated 

 in another. Externally each nerve-fibre is enclosed by a delicate 

 sheath, the neurilemma. 



Part of the ectodermic tissue which forms the nervous system 

 of the embryo does not become transformed into nervous tissue, 

 but gives rise to an epithelial layer (ependyme) and also to a 

 supporting, connecting, or isolating framework the neuroglia, 

 which plays a very important part in the central nervous system ; 

 externally, investing membranes as well as blood- and lymph- vessels 

 are formed from the mesoderm. As compared with the central 

 organs, the peripheral tracks are comparatively poorly supplied with 

 blood. 



The nervous system thus consists of central and peripheral 

 portions (Fig. 145). The central part (brain and spinal cord) is 



O 



