CHAP. IL] THE CONTRACTILE TISSUES. 121 



round the muscle fibres themselves. From this plexus are given 

 off a number of nerve fibres, running singly, each of which joining 

 a muscle fibre ends in an end-plate. In forming these plexuses 

 the individual nerve fibres divide repeatedly, the division always 

 taking place at a node of Ranvier, so that what is a single nerve 

 fibre as the nerve enters the muscle may give rise to several nerve 

 fibres ending in several muscle fibres. The nerve fibre joins the 

 muscle fibre at about its middle or somewhat nearer one end, and 

 occasionally two nerve fibres may join one muscle fibre and form 

 two end-plates. The general distribution of the bundles of nerve 

 fibres and single nerve fibres is such that some portion of the 

 muscle is left free from nerve fibres ; thus at the lower and at the 

 upper end of the sartorius of the frog there is a portion of muscle 

 quite free from nerve fibres. 



A single nerve fibre, running by itself, has outside the neuri- 

 lemma an additional delicate sheath of fine connective tissue 

 known as Henles sheath, which appears to be a continuation of the 

 connective tissue forming the sheath of the nerve branch from 

 which the fibre sprang, or uniting the fibres together in the 

 branch. 



The actual ending of the nerve fibre in the muscle fibre differs 

 in different classes of animals. 



In mammals and some other animals the single nerve fibre 

 joins the muscle fibre in a swelling or projection having a more or 

 less oval base, and appearing when seen sideways as a low conical 

 or rounded eminence. At the summit of this eminence the nerve 

 fibre loses both its sheath of Henle and its neurilemma, one or 

 other or both (for on this point observers do not agree) becoming 

 continuous with the sarcolemma of the muscle fibre. At the 

 summit of the eminence, where the sheaths fuse, the fibre, now 

 consisting only of axis-cylinder and medulla, loses its medulla 

 abruptly (in the muscles of the tongue the nerve fibre in many 

 cases loses its medulla at some considerable distance before it 

 joins the muscle fibre to form the end-plate), while the axis, 

 cylinder branches out in all directions, the somewhat varicose 

 branches, which sometimes anastomose, forming a low conical mass, 

 which when viewed from above has an arborescent or labyrinthine 

 appearance. On the branches of this arborescence may lie one 

 or more somewhat granular oval nuclei. The arborescence itself 

 has, like the axis-cylinder of which it is a development, a very 

 faintly granular or cloudy appearance, but lying between it and 

 the actual muscle substance is a disc or bed of somewhat coarsely 

 granular material, called the sole of the end-plate, on which the 

 ramified arborescent axis-cylinder rests, more or less overlapping 

 it at the edge, but with which it appears not to be actually 

 continuous. Lying in the midst of this ' sole ' are a number of 

 clear oval transparent nuclei. 



The end-plate then beneath the sarcolemma consists of two 



