CHAPTER VII.

Classification of Muscle-Parts of a Muscle--Microscopical appearances and structure of Muscle, Fibres, and Fibrillæ-Connexion of Fibres with their Tendons -Vascular Supply -- Nerves and Lymphatics of Muscles-Structure of unstriated Muscle and of the muscular Fibre of the HeartPhysical properties and chemical Composition of Muscle-The uses of muscular Tissue. MUSCLE.

CLASSIFICATION OF MUSCLE.-In speaking of muscle tissue we may classify it either histologically, and divide it into the striated, and unstriated, or physiologically, when it may be divided into the quick-contracting, and slow-contracting-the quick-contracting being represented by the striated, and the slow-contracting by the unstriated variety. Muscle is frequently classed under the heads voluntary and involuntary, the striated variety representing the voluntary and the unstriated. the involuntary; but this classification is misleading, as we have the notable instance of the muscle of the heart, which is striated but involuntary, and the equally important ex

ception of the ciliary muscle of the eye, which is unstriated but voluntary in its action. In birds the ciliary muscle is of the striated variety.

STRUCTURE OF MUSCLE.-The fibres of striated muscles are aggregated together to form distinct masses or muscles, which vary much in size and shape. They terminate at each end in a tendon,-either abruptly or by a process of gradual continuity,—by which they are attached to the bones. The muscular fibres, which are made up of minute fibrils, are collected into bundles called fasciculi. The fibrils run parallel with one another in the fibres, and the fibres are parallel in the fasciculi. The fasciculi also are, as a rule, parallel with one another, although some converge towards the terminal tendon; but in the ordinary skeletal muscles they do not anastomose. Every muscle is invested externally by a continuous sheath of more or less dense areolar tissue, which sends in processes between the fasciculi or bundles of fibres, by means of which they are associated together to form the muscle (P. V. Fig. 18). Finer prolongations of the sheath, entering

the fasciculi, pass between the individual fibres, but they do not completely surround them. Besides connecting the fibres and fasciculi together, the prolongations of the sheath afford support to the nerves and vessels which ramify in the muscle substance.

MICROSCOPICAL APPEARANCES AND STRUCTURE OF THE FIBRES.-When a specimen of skeletal muscle is examined under the microscope the fibres are seen to be of a yellowish red colour, and possess well-marked transverse striations and less well-marked longitudinal ones (P. V. Fig. 21). Each fibre is invested by a yellowish membrane called the sarcolemma, which possesses the physical and chemical properties of yellow elastic tissue, and is sufficiently tough to allow of the included muscle substance being ruptured without being itself injured. Situate on the surface of the muscle fibre, but placed beneath the sarcolemma, are some clear oval nuclei, around each of which is a certain amount of granular protoplasmic material. These play an important part in taking up nutrient material from the adjacent bloodvessels, and preside over the growth and nu

trition of the muscle. Kölliker has set down the diameter of muscle fibres as varying from 7th tooth of an inch. The fibres rarely exceed one inch and a half in length. The fact that in examining striated muscle fibres under the microscope, longitudinal as well as transverse striations were observed, tells us that each fibre is again split up into fibrillæ, a bundle of fibrillæ surrounded by the sarcolemma constituting a muscle fibre.

STRUCTURE OF MUSCLE FIBRILS.-The study of the structure of the ultimate muscle fibrils is surrounded with great difficulty, and their exact structure has not yet been accurately determined. When examined under a high power, they appear to be made up of a number of dark quadrangular particles with a light intervening substance-these constitute the sarcous elements of Bowman. The two kinds of particles possess different refractive powers; the dark particles, according to Brücke, being doubly refractive, while the light substance only refracts light singly. According to Engelmann the fibres of muscle when fresh are transparent; and the appearance of superimposed disks are due to a kind

of coagulation which takes place, the advent of which is hastened by the addition of any reagent. The alternate arrangement of the dark and light particles gives the muscle its transversely striated appearance. The light particles in each fibre are crossed by little septa, described by Krause and called Krause's membranes. As these septa cross adjacent fibrils at the same level they run across the whole fibre, and appear to be firmly attached to the under surface of the sarcolemma. On either side of each membrane is a thin layer of light single refractile substance, to which the name lateral disk has been given. In the lateral disks, on either side of Krause's membrane, is a row of dark granules. Hensen has described a light band as crossing the dark particles, this has been termed the middle disk of Hensen; its precise nature has not yet been clearly made out. Schäfer takes another view of the structure of muscle; he describes a muscle fibre as being composed of a number of rodlike particles, which he calls muscle rods, placed side by side in the long axis of the muscle, with a certain amount of cement or