NERVE-FIBKES. 



509 



, 

 'cylinder 





Myelin 



Primitive 

 sheath 



orain and spinal medulla, with the exception of the olfactory and optic, are formed 

 3f medullated fibres provided with a primitive sheath; whilst the entire mass of 

 the white substance of the brain and spinal medulla, and also the optic nerves, 

 4v . a are formed of medullated fibres devoid of a primitive sheath. 



It is important to note that the distinction between the 

 medullated and non- medullated fibres is not one which exists 

 throughout all stages of development. As will be presently pointed 

 out, every fibre is the prolongation of a cell, and in the first instance 

 it is not provided with a medullary sheath. Indeed, it is not 

 until about the fifth month of foetal life that those fibres which 

 are to form the white substance of the cerebro- spinal axis begin to 

 acquire their coating of myelin. Further, this coating appears in 

 the fibres of different fasciculi or tracts at different periods, and a 

 knowledge of this fact has enabled anatomists to follow out the 

 connexions of the tracts of fibres which compose the white matter 

 of the brain and spinal medulla. 



Every nerve-fibre is directly continuous by one extremity with a 

 nerve-cell, whilst its opposite extremity breaks up into a number of 

 ramifications, all of which end in relation to another nerve-cell, or 

 in relation to certain tissues of the body, as, for example, muscle- 

 fibres or the epithelial cells of the epidermis. The length of nerve- 

 fibres, therefore, varies very greatly. Some fibres are short and 

 merely bring two neighbouring nerve-cells into relation with each 

 other ; others travel long distances. Thus, a fibre arising from one 

 of the motor cells of the lower end of the spinal medulla may, after 

 leaving the spinal medulla, extend to the most outlying muscle in 

 the sole of the foot, before it reaches 

 its destination. But even when a fibre 

 does not leave the central axis, a great 

 length may be attained, and cells situated 

 in the uppermost part of the brain give 

 origin to fibres which pass down to the 

 lower end of the spinal medulla. 



It has already been explained that 

 fibres which form the nerves may be 

 classified into two sets, afferent and 

 efferent. Afferent nerve -fibres conduct 

 impressions from the peripheral organs into the central 

 nervous system ; and as a change of consciousness, or, 

 in other words, a sensation is a frequent result, these 

 fibres are often called sensory. Efferent nerve -fibres 

 carry impulses out from the brain and spinal medulla to 

 peripheral organs. The majority of these fibres go to 

 muscles and are termed motor ; others, however, go to 

 glands and are called secretory ; whilst some are in- 

 hibitory and serve to carry impulses which restrain or 

 check movement or secretion. 



The dendrites, or protoplasmic processes of the nerve- 

 cell, are thicker than the axon, and present a rough- 

 edged irregular contour. They divide into numerous 

 branches, and these gradually, as they pass from the 

 cell-body, become more and more attenuated until finally 

 they appear to end in free extremities. The branching 

 of the dendritic processes sometimes attains a marvellous degree of complexity 

 'Fig. 449), but it is commonly supposed that there is no anastomosis between 

 the dendrites of neighbouring cells, or between the dendrites of the same cell. 



It is commonly believed that the neuroblast passes through stages analogous to 

 those shown in the diagram (Fig. -451) ; that just as a seed gives off a root which 

 strikes downward, and leaves which grow upward, so the neuroblast sprouts out an 



FIG. 450. 

 NERVE-FIBRE 



MENTAL STAGES EXHIBITED BY A 

 PYRAMIDAL CELL OF THE BRAIN. 



a, Neuroblast with rudimentary 

 axon, but no dendrites ; b and c, 

 The dendrites beginning to sprout 

 out ; d and e, Further develop- 

 ment of the dendrites and appear- 

 ance of collateral branches on 

 the axon. 



