34 THE STRUCTUllE OF 



another, and are invested by a sheath. These again, in 

 their course towards the centre, collect into larger and 

 larger bundles, the different elements of which are all 

 bound together into one white trunk or * nerve ' (fig. 6), 

 by means of a firm connective tissue envelope, which 

 sends thinner investing prolongations in amongst the 

 constituent cords. 



These ' nerves ' of various sizes frequently contain 

 within the same bundle both ingoing and outgoing fibres, 

 and are then known as ' mixed nerves '. Others contain 

 only 'sensory', or only 'motor' fibres. In their course 

 nerves often communicate freely one with another by 



Fig. 6.— Portion of the Tnink of a Nerve, consisting of many smaller Cords 

 wrapped up in a common Sheath (Quain after Sir C. Bell), a, the nerve ; b, a single 

 cord drawn out from the rest. Magnified several diameters. 



means of branches. Such communicating branches are 

 especially numerous in the course of the visceral nerves, 

 and, when ma ay occur amon(]^st some particular set of 

 cords, what is termed a ' plexus ' is formed (fig. 7). In these 

 plexuses the individual nerve fibres do not undergo 

 division. Some of them merely leave one bundle or cord 

 and pass to another, with the fibres of which they are 

 ultimately distributed, either to muscles or to nerve 

 centres. 



The smaller medullatcd nerve fibres unite, so far as we 

 know, only near their commencements, and the larger 

 motor fibres only undergo bifurcation near their termina- 

 tions in muscles or glands (fig. 4). The fibrils or ele- 

 mentary constituents of the fibres probably do not divide 



