54 ANATOMY. 



fibres, from which non-medullated fibres arise. These terminal plates, 

 one to each fibre, are said to be found only near the middle, not at 

 the ends of a muscle ; they are compared to certain parts of the elec- 

 tric organs of fishes ; they are described as consisting of areolar 

 tissue, as partially embracing a fibre, and as being placed at the point 

 whence the non-medullated fibres are given off from the dark-bordered 

 ones. The sensory nerve-fibres of muscles end also in pale non-me- 

 dullated branches ; but they are finer, and are distributed on the sur- 

 faces of the muscle, or of its principal bundles. In the papillae of 

 the tongue and skin the sensory nerve-fibres sometimes really form 

 elongated loops ; but here, as in other situations, they may lose their 

 medullary substance and double contour, and perhaps even their 

 tubular envelope, so as to be reduced to the axis or central band 

 only, and then end amidst the tissues to which they are distributed, 

 either abruptly by swollen extremities, or after previously becoming 

 finer and finer, or even after subdividing into fine twigs. The reticu- 

 lar mode of termination of the nerve- fibres has been observed in the 

 retina of the eyeball, and in the submucous tissue of the intestines. 

 Special modes of termination in the organs of sense, and, in certain 

 bodies, the tactile and Pacinian corpuscles, in the skin, will be de- 

 scribed hereafter. 



The bloodvessels. The three kinds of bloodvessels, arteries, veins, and 

 capillaries, differ very much in their structure. 



The arteries, the strong yellowish or whjte cylindrical branching 

 tubes which proceed from the heart to all parts of the body, have thick 

 elastic walls ; so that they remain open when they are cut across . 

 These walls consist of three coats, Fig. 31, a, viz., of an external coat, 

 composed of areolar and elastic tissue ; of a middle or muscular coat, 

 the thickest, composed of unstriped muscular fibres, arranged circularly 

 around the vessel, mixed with a very few elastic fibres ; and of a thin 

 smooth internal coat, consisting chiefly of a fenestrated or striated 

 elastic membrane, Fig. 19, lined by the vestiges of a delicate epithe- 

 lium, Fig. 43, c. The inner coat is brittle, arid the middle one tender ; 

 the outer one is very tough ; so that a string tied tightly round an 

 artery cuts through the middle and inner coats but not the outer. The 

 smaller arteries have relatively more muscular tissue, and the larger 

 ones relatively more elastic tissue, in their walls. The outer and per- 

 haps the middle coats of the arteries are themselves vascular, being 

 supplied with nutrient bloodvessels, called the vasa vasorum. The 

 arteries are supplied with nerves derived chiefly from the sympathetic 

 system. In the limbs, all but the very finest arteries have a loose 

 sheath of areolar tissue, in which they can be moved. 



The veins, which, proceeding from all parts of the body, end in the 

 heart, are more yielding tubes and have thinner walls than the arte- 

 ries, so that they collapse when cut across. Their coats are also three 

 in number, Fig. 31, b, and similar in general structure to those of the 

 arteries ; but the middle coat contains fewer unstriped muscular fibres, 

 and the internal coat has no fenestrated layer, except in the veins of 

 the pia mater of the brain, though it has fine elastic fibres and vestiges 

 of a delicate epithelium, Fig. 43, b. In the largest veins, there are 



