296 



THE BODY AT WORK 



large enough to be seen with the naked eye and traversed by 

 capillary bloodvessels. The axon of this nerve-cell its single 

 nerve-fibre ramifies to supply a separate branch to each of 

 the 2,000,000 chambers of the electric organ, and each branch 

 breaks up into a bunch of twigs within the chamber. Contrast 

 with such a giant cell as this one of the granules of the 

 retina or cerebellum, the smallest cells to be found in the 

 body, yet each a perfect neurone, exquisitely elaborate in 

 form. 



As types for description we may take one of the motor cells of 

 the spinal cord and a granule of the cerebellum. Every nerve- 

 fibre which supplies a group of Voluntary muscle-fibres is a 



FIG. 19. A NERVE-FIBRE CONSISTING OP A, THE UNDIVIDED, FIBRILLATED AXON OF A NERVE- 

 CELL, WITH ITS VARIOUS WRAPPINGS. 



In segment 1 the wrappings comprise B, a tube of phosphatic fat (myelin), interrupted at H, 

 a node of Ranvier ; C, a delicate membrane (sarcolemma) ; D, connective tissue ; E, the 

 rind of the axon ; F, a tubular space containing lymph, between the axon and its sheath 

 of myelin ; G, nucleus of an enwrapping cell. At I the myelin is seen to be divided into 

 overlapping conical rings. 2, The medullated nerve-fibre, running an isolated course, is 

 merely enclosed in a tube of connective tissue containing lymph. 3, As a " grey " or 

 " non-medullated " fibre, the axon has lost its myelin sheath. 



thread drawn out from a large cell-body which lies in the grey 

 matter of the spinal cord or of the axis of the brain. The fibres 

 pass out in the anterior root of a spinal nerve or in a cranial 

 nerve. The cell-body may have a diameter of as much as 100 /u, 

 (1 1* = 0-001 millimetre). In shape it is like a very irregular 

 starfish, owing to its being continued into several, usually four 

 or five, thick tapering branching limbs or processes, known as 

 dendrites, in addition to its slender thread-like axon. From 

 its origin in a cell-body to itsdestination in a muscle it may 

 be a few inches, or it may be a yard away the axon is an 

 unbroken thread. A short distance from the cell-body it 

 enters a tubular sheath, which protects and insulates it, re- 

 calling the covering of gutta-percha in which the wires of a 



