Fig. 428.
1. Muscular fibre broken across, the fragments connected by the connective tissue membrane × 100; 2. Fibre broken up into irregular distinct bands: a few blood corpuscles distributed about × 200; 3. A fasciculus of muscular fibre from leg of pig × 600.
In [Plate XIX]., [Fig. 11], the muscular fibre taken from the tongue of a lamb shows the continuity of the upper portion with the connective tissue membrane. In [Fig. 12], a branching-out bundle of muscular fibre, taken from the upper lip of the rat, is seen to end in stellate connective cells. The delicate homogeneous sheath that binds the fibres together is termed sarcolemma. This is readily seen in prepared muscle of the frog and water-beetle, less plainly in man. Each muscle is provided with a sheath of connective tissue; this surrounds it, binds the fasciculi together, and supports the blood-vessels; it is called the perimysium, and sends fine prolongations in between the fibres, termed endomysium. The intervals seen on high amplification between the dark striæ are called Kruse’s membrane. On breaking up the striated structure it is resolvable into fibrillæ and furthermore into discs.
Fig. 429.
1. Vertical section of epidermis; 2. Pigment cells from a lower layer of cutis.
Among mammalia the pig furnishes the best examples of muscle fibrillæ; among insects the water-beetle and the thorax of the housefly. A power of 600 or 800 diameters is required to separate the fibrillæ. Blood-vessels are well supplied with striated muscle, but none of their minuter branches penetrate the sarcolemma. The involuntary or non-striated variety of muscular fibre exists in all parts of the body where movements occur independently of the will, also in the ciliary muscle and the iris of the eye, as well as in the middle coats of the arteries. Non-striated fibres are pale in colour, prismatic in shape, and easily flattened by pressure. In size, they vary from 1⁄7000th to 1⁄3500th of an inch in diameter, and are marked at short intervals by oblong corpuscles.
The Integument or Skin consists of epidermis or cuticle, dermis, corium or cutis vera, sweat-glands, nails, hairs, sebaceous glands, and numerous nerves and vessels. The epidermis forms a protective covering over the whole surface of the body, and is moulded on to the surface of the corium beneath, covering the ridges, depressions and papillæ. It is made up of three principal layers: the horny layer or stratium corneum, the most superficial, this consists of layers of flattened cells, which are without a nucleus; the stratum lucidum, composed of layers of nucleated cells, more or less indistinct in section; the rete mucosum or malpighian layer; is composed in its upper part of layers of “prickle cells” and its inferior of a single stratum of columnar cells. Pigment is principally found in the lowest layer, [Fig. 429].
The gradations of colour in the skin are due to the granular contents of the pigment cells. This is seen on steeping sections cut from the skin of a negro in chlorine; the colour is discharged. In [Plate XIX]., No. 13, the pigment cells of the choroid coat of eye are shown. Here the pigment is darker in colour, and its function is the absorption of light and the prevention of disturbing effects occasioned by circles of dispersion.
