THE NERVOUS SYSTEM 379 



smaller particle, named the " nucleolus". The surface of the nerve-cell is 

 sometimes smooth, and gives off one or two fine filamentous processes; 

 in some instances, however, many such processes shoot from it. These 

 divide and subdivide as they recede from the cell, and either join with 

 or enter into very close relation with the processes from other cells. In 

 most, if not in all cases, one of the processes is larger and longer than 

 the rest. If traced for some distance it may be seen to become a true 

 nerve -fibre, which may terminate in a muscle or in a gland, or in one 

 of the organs of sense ear, eye, nose, &c. or may serve to bring two 

 nerve-cells into connection with each other. 



The Nerve-fibres. Nerve-fibres are processes or outrunners from the 

 cells. At the point where a fibre springs from a cell it is exceedingly 

 fragile and delicate, but as it travels away from the cell it gradually 

 acquires a protective covering or sheath, which calls to mind that employed 

 to insulate a telegraph wire. It loses it, however, again as it approximates 

 its destination, becoming reduced to a very attenuated thread. If examined 

 in the middle of its course, say, for example, in the sciatic nerve, each 

 fibre will be found to consist of a central core or axis cylinder, believed 

 to be the path along which all nervous impressions and impulses are 

 propagated. Covering this is a layer of white substance of a fatty nature, 

 named the medullary sheath, or white substance of 

 Schwann, and outside this again is a delicate but firm 

 and resistant membrane, which is the neurilemma; such 

 nerve-fibres are named " medullated fibres". The central 

 core or cylinder axis often runs for long distances without 

 division, but at times gives off collateral branches, and 

 may even divide into a set of branches like a bouquet. 

 Medullated fibres vary in diameter from 1- 1500th to 

 l-12,000th of an inch. The fibres are bound together of Nero 



into bundles by connective tissue, and these bundles are A, Nerve-Fibre, 

 associated into groups, the whole having a strong invest- J NeuriiemLf 6 ^ 11 * 

 ment of connective tissue and constituting a nerve. 



The results of injury to the central parts of the nervous system are 

 so disastrous that they are everywhere protected from mechanical violence 

 with the greatest care. The spinal cord, cerebellum, and brain are con- 

 tained in a strong case of bone particularly well adapted to preserve these 

 soft parts from blows or pressure. The head bones are composed of an 

 outer and an inner layer of compact bone, between which is a layer of 

 loose cancellous or spongy bone, the whole requiring great force to cause 

 fracture, whilst the solid bodies and projecting spines of the backbone, 

 together with the successive layers of thick skin, fibrous tissue, and muscles 



