LESSON XXIV.

STRUCTURE OF THE HEART.

1. In a section through the wall of the auricle which has been stained with magenta and mounted in glycerine, notice the relative thickness of the epicardium, myocardium, and endocardium. Observe the blood-vessels and nerve-fibres under the epicardium, often embedded in fat; here and there a ganglion may be seen under this membrane. Notice also the elastic networks under both the pericardium and endocardium. Make a general sketch from this section.

2. Section through the wall of the ventricle, stained with logwood and mounted in Canada balsam. The muscular fibres are variously cut. In those cut longitudinally, notice the branching of the fibres and their union into a network. Notice also that although the fibres are cross-striated this is less distinct than in voluntary muscle, and the nuclei lie in the centre of each fibre. Transverse markings may also be seen passing across the fibres between the nuclei and indicating a division into cells. The endocardium is very thin, especially over the columnæ carneæ.

3. The lymphatics of the heart are easily injected with Berlin blue by sticking the nozzle of the injecting syringe into the muscular substance, in the interstices of which the lymphatics arise. These commencing lymphatics lead to efferent vessels which pass to the base of the heart under the epicardium.

4. Section through one of the valves of the heart, stained and mounted as preparation 2.

5. The epithelium which covers the epicardium, and that which lines the endocardium, may be studied in preparations of the fresh organ which have been treated with nitrate of silver and subsequently exposed to the light and hardened in alcohol.

The muscular substance of the heart (myocardium) is composed of transversely striated muscular fibres (fig. 139), which differ from those of voluntary muscle in the following particulars: their striations are less distinct; they have no sarcolemma; they branch and unite with neighbouring fibres, and their nuclei lie in the centre of the fibres. Moreover, the fibres are composed of a series of short cylindrical cells (fig. 140) joined together end to end, each corresponding to one of the nuclei. The lines of junction of these cells may sometimes be seen in longitudinal sections stained with hæmatoxylin or magenta; but they come much more distinctly into view in sections of the fresh tissue stained with nitrate of silver.

In the interstices of the muscular tissue there is a little areolar tissue in which run the very numerous blood-capillaries and the lacunar lymphatics.

FIG. 141.-SECTION OF THE EPICARDIUM OF THE RIGHT AURICLE.

a, serous epithelium in section; b, connective-tissue layer; c, elastic network; d, subserous areolar tissue; e, fat; f, section of a blood-vessel; g, a small ganglion; h, muscular fibres of the myocardium; i, intermuscular areolar tissue.

The myocardium is covered externally by a layer of serous membrane-the epicardium (cardiac pericardium, fig. 141) --composed, like other serous membranes, of connective tissue and elastic fibres, the

latter being most numerous in its deeper parts. Underneath the epicardium run the blood-vessels, nerves, and lymphatic vessels of the heart, embedded in areolar and adipose tissue; this tissue being continuous with that which lies between the muscular bundles.

The endocardium (fig. 142) has a structure not very unlike the pericardium. It is lined by a pavement-epithelium, like the epithelium of a serous membrane, and consists of connective tissue with elastic fibres in its deeper part, between which there may, in some parts, be

FIG. 143.-SECTION THROUGH ONE OF
THE FLAPS OF THE AORTIC VALVE,
AND PART OF THE CORRESPONDING

SINUS OF VALSALVA, WITH THE AD-
JOINING PART OF THE VENTRICULAR
WALL.

a, endocardium, prolonged over the valve; b,
sub-endocardial tissue; c, fibrous tissue of
the valve, thickened at c' near the free edge;
d, section of the lunula; e, section of the
fibrous ring; f, muscular fibres of the ven-
tricle attached to it; g, loose areolar tissue
at the base of the ventricle; s. V. sinus
Valsalvæ; 1, 2, 3, inner, middle, and outer
coats of the aorta.

found a few plain muscular fibres. Fat is sometimes met with under the endocardium,

In some animals, e.g. the sheep, and sometimes also in man, large beaded fibres are found under the endocardium. These are formed of large clear cells joined end to end, and generally containing in their centre two nuclei, whilst the peripheral part of the cell is formed of crossstriated muscular tissue; they are known as the fibres of Purkinje.

The valves of the heart are formed of folds of the endocardium strengthened by fibrous tissue (fig. 143). This tissue forms a thicken

ing near the free edge of the valve (c'). At the base of the auriculoventricular valves a little of the muscular tissue of the auricle may be found passing a short distance into the valve.

The nerves of the heart are seen in sections underneath the epicardium of both auricles and ventricles; in the former situation, they are connected at intervals with small ganglia (fig. 141, g). Their branches pass to the muscular substance, but their mode of termination has not been ascertained.

LESSON XXV.

THE TRACHEA AND LUNGS.

1. In sections of trachea, stained with logwood or borax-carmine, and mounted in Canada balsam, notice the ciliated epithelium, the basement-membrane (of some thickness in the human trachea), the lymphoid tissue of the mucous membrane, the elastic tissue external to this, and lastly the fibrous membrane containing the cartilages. In the mucous membrane and submucous areolar tissue look for sections of mucous glands, ducts of which may be seen opening on the surface. At the back of the trachea notice the plain muscular fibres transversely arranged; there may be larger mucous glands external to these.

2. In sections of lung similarly prepared, notice the sections of the alveoli collected into groups (infundibula). Find sections of bronchial tubes, some cut longitudinally and passing at their extremities into the infundibula, others cut across; the latter show the structure of the tubes best. In each tube notice the ciliated epithelium internally. Next to this the mucous membrane containing numerous elastic fibres and often thrown into folds; then the layer of circular muscular fibres, and outside this, loose fibrous tissue in which in larger bronchial tubes the pieces of cartilage may be seen embedded. Small mucous glands may also be observed in the fibrous tissue sending their ducts through the other layers to open on the inner surface. Notice always accompanying a section of a bronchial tube the section of a branch of the pulmonary artery.

In the sections of the alveoli observe the capillary vessels passing from one side to the other of the intervening septa; and in places where the thin wall of an alveolus is to be seen in the section, try and make out the network of blood-capillaries upon it. Notice within the alveoli nucleated corpuscles which very frequently contain dark particles in their protoplasm. They appear to be amoeboid cells which have migrated from the blood-vessels and have taken in inhaled particles of carbon. They seem to pass back into the lung tissue, for similar cells may be seen in this. Make a sketch of part of the wall of a bronchial tube and of one or two of the alveoli.

3. Mount in Canada balsam a section of lung in which the pulmonary vessels have been injected. Study the general arrangement of the vessels with a low power, and the network of capillaries of the alveoli with a high power. Observe that the veins run apart from the arteries. Sketch the capillary network of one or two adjoining alveoli.

The trachea or windpipe is a fibrous and muscular tube, the wall of which is rendered somewhat rigid by C-shaped hoops of cartilage which are embedded in the fibrous tissue. The muscular tissue, which is of the plain variety, forms a flat band, the fibres of which run transversely at the back of the tube. The trachea is lined by a mucous