160 STRUCTURE OF PLAIN MUSCLE. [BOOK i. 



mutual pressure appear polygonal. In the centre of some of these 

 sections of fibres the nucleus may be seen, but it will of course be 

 absent from those fibres in the which plane of section has passed 

 either above or below the nucleus. When a thin sheet of plain 

 muscle is spread out or teased out under the microscope, the 

 bands may also be recognized, and at the torn ends of some of the 

 bands the individual fibres may be seen projecting after the 

 fashion of a palisade. 



Blood vessels and lymphatics are carried by the connective 

 tissue, and form capillary networks and lymphatic plexuses round 

 the smaller bands. 



90. The arrangement of the nerves in unstriated muscle differs 

 from that in striated muscle. Whereas in striated muscle me- 

 dullated fibres coming direct from the anterior roots of spinal 

 nerves predominate, in plain muscle non-medullated fibres are most 

 abundant ; in fact the nerves going to plain muscles are not only 

 small but are almost exclusively composed of non-medullated fibres 

 and come to the muscle from the so-called sympathetic system. 

 Passing into the connective tissue between the bundles the nerves 

 divide and, joining again, form a plexus around the bundles ; 

 that is to say, a small twig consisting of a few, or perhaps only 

 one axis- cylinder, coming from one branch will run alongside of or 

 join a similar small twig coming from another branch ; the indivi- 

 dual axis-cylinders however do not themselves coalesce. From such 

 primary plexuses, in which a few medullated fibres are present 

 among the non-medullated fibres, are given off still finer, 'inter- 

 mediate' plexuses, consisting exclusively of non-medullated fibres; 

 these embrace the smaller bundles of muscular fibres. The 

 branches of these plexuses may consist of a single axis-cylinder, or 

 may even be filaments corresponding to several or to a few only 

 of the fibrillse of which an axis- cylinder is supposed to be 

 composed. From these intermediate plexuses are given off single 

 fibrillae or very small bundles of fibrilla3, which running in the 

 cement substance between the individual fibres form a fine net- 

 work around the individual fibres, which network differs from the 

 plexuses just spoken of inasmuch as some of the filaments com- 

 posing it appear to coalesce. The ultimate ending of this network 

 has not yet been conclusively traced ; but it seems probable 

 that fibrils from the network terminate in small knobs or swellings 

 lying on the substance of the muscular fibres, somewhat after the 

 fashion of minute end-plates. 



A similar termination of nerves in a plexus or network is met 

 with in other tissues, and is not confined to non-medullated fibres. 

 A medullated fibre may end in a plexus, and when it does so loses 

 first its medulla and subsequently its neurilemma, the plexus 

 becoming ultimately like that formed by a non-medullated fibre and 

 consisting of attenuated axis-cylinders with thickenings, and some- 

 times with nuclei, at the nodal points. 



