44. Muscular tissue possesses a slight degree of cohesion, a high degree of flexibility and extensibility, but no degree of elasticity; for although muscle, considered as a compound of muscular substance and membrane, be highly elastic, yet this property is probably altogether owing to the membranous matter in which it is enveloped. Its peculiar and distinctive property is vital, not physical, and consists in the power of diminishing its length, or of contracting or shortening itself on the application of a stimulus. This property, which is termed contractility, is the great, if not the sole source of motion in the body. Without doubt, elasticity and gravity, under the generating and controlling powder of contractility, aid in accomplishing various kinds of motion. Thus membranes, tendons, ligaments, cartilages, and bones, by their physical and mechanical properties, modify, economize, facilitate, concentrate and direct the motive power generated by the pure muscular substance; but still the only real source of motion in the body is muscular tissue, and the only mode in which motion is generated is by contractility. This will be more fully understood hereafter. [Illustration: Fig. XXXI. Ultimate fibres of nerve, very highly magnified; showing the strings of globules of which they consist.] 45. The last primary tissue, termed the NERVOUS, is equally distinct in nature and peculiar in property. It consists of a soft and pulpy matter, of a brownish white colour (fig. XXX.). According to some, the nervous, like the muscular pulp, is composed of minute globules, arranged in the same manner like a string of pearls (fig. XXXI.); according to others, it consists of solid elongated threads, of a cylindrical form, differing in thickness from that of a hair to the finest fibre of silk. The pulp, whatever its form of aggregation, is inclosed in a sheath of delicate cellular tissue. This external or containing membrane is called the neurilema, or the nerve-coat; the internal or contained substance, the proper nervous matter, is termed the nerve-string. The nerve-string, enveloped in its nerve-coat, constitutes the nervous filament. As in the muscle, so in the nerve, many filaments unite to form a fibre, many fibres to form a fasciculus, and many fasciculi to form the large cord termed a nerve. Moreover, as in the muscle, so in the nerve, the filament, the fibre, the fasciculus, the nervous cord itself, are each enveloped in its own distinct sheath of cellular membrane; but the arrangement of the nervous fibres differs from that of the muscular in this, that though the nervous fibres are placed in juxtaposition, yet they do not, like the muscular, maintain through their entire course a parallel disposition, but cross and penetrate each other, so as to form an intimate interlacement (fig. XXXII.). [Illustration: Fig. XXXII.