I. THE HEART.

(The original arrangement of this section has been modified.)

The heart is situated in the middle line above the central portion of the shoulder-girdle and M. sternohyoideus, and below the oesophagus; the apex is directed backwards and lies between the lobes of the liver, the base is directed forwards and lies a short distance behind the larynx. The heart is surrounded by the pericardium.

A. The Pericardium [is a very thin membranous sac, which completely encloses the heart; it is attached, at some distance, to the large vessels passing to and from the heart, and has inserted into it some fibres of the M. obliquus abdominis internus (see page 70). A fold of the pericardium extends from the dorsal surface to the truncus arteriosus, and through it courses the vena cardiaca.

The pericardium is a connective-tissue membrane, and is lined on either side with endothelium, which is in part ciliated (Leydig); the endothelium of the inner surface is continued on to the heart and so forms a closed lymph-sac. The pericardium is more or less pigmented, and contains both vessels and nerves; it may contain fat-cells].

B. The Heart.

[The heart is a hollow muscular organ, composed of: (1) two auricles, forming the wider anterior portion; (2) the ventricle, placed behind the auricles; (3) the sinus venosus, situated dorsally; and (4) the truncus arteriosus, lying ventrally. Examined while still living, the auricles are seen to be much darker in colour than the ventricle, due to the blood being seen through their thinner walls; between the auricles and the ventricle is a distinct groove, the auriculo-ventricular groove. The whole organ is more or less pigmented, especially the ventricle (R. temporaria has much less pigment).]

a. The Sinus venosus (Sinus venarum cavarum), (Fig. 133 SV.) is a thin-walled sac lying on the dorsal surface of the heart, and very slightly to the right side (Hoffmann). It is somewhat triangular in form, receiving an anterior caval vein at each anterior angle (V.), and the posterior caval vein (IC.) at the posterior angle. The ventral wall has a transverse opening (SV′.), by which it communicates with the right auricle; the opening possesses two valves, an anterior and a posterior, and is placed close to the auricular septum.

b. The auricles [are not always separated, as the septum which usually brings about the separation is in some cases incomplete, and in rare instances appears to be absent (Hoffmann)]; according to Ecker, the auricular septum is sometimes placed so far to the left side that it appears to be absent (Fig. 135). The septum is attached to the walls of the auricles so as to form two distinct cavities, which communicate at the auriculo-ventricular opening, where the septum has a free concave border (Fig. 134). The right auricle is much larger than the left, and receives the systemic venous blood from the sinus venosus by the opening (Figs. 133 and 134 SV.) already described. The left auricle receives the blood from the lungs by the pulmonary vein (Fig. 133 PV.), which opens into the auricle near the septum (Fig. 134 PV.): [the openings from the sinus venosus and from the pulmonary vein may be so closely approximated as to be separated by the septum only. The septum is much thinner than the auricular walls]. Both auricles open into the ventricle by the auriculo-ventricular opening (Fig. 134).

c. The ventricle (Figs. 132, 134, and 136 V.), examined while still living, is seen to be bluntly conical in shape and darker on the left side than on the right; this is due to the ventricular wall of the right side, near the truncus arteriosus, being somewhat thinner. The inner wall is not smooth but possesses muscular ridges (Trabeculae). At its base the ventricle communicates with the auricles by the auriculo-ventricular opening (Figs. 134 and 136), and with the truncus arteriosus by a separate opening (Fig. 134 A.). The ventricle is lined by a layer of endocardial endothelium.

The auriculo-ventricular opening (Fig. 134 A.) is a large aperture guarded by two valves, a dorsal and a ventral: each valve is formed by a reduplication of the endothelium, and contains a small amount of connective-tissue; its free margin is bound down by a number (about twelve) of fibrous cords, the chordae tendineae, which are attached by their posterior extremities to the trabeculae.

d. The truncus arteriosus (Figs. 132 TA., 134 A.) arises at the base of the ventricle, on the right side of its ventral surface, then passes forwards and to the left, across the auricles, to reach their anterior border near the median line, where it divides to form two vessels.

Internally it is incompletely divided into two compartments by a spiral valve (Fig. 134); the valve is attached to the dorsal surface and is free ventrally: when the vessel is fully dilated (artificially), the valve extends two-thirds of its diameter. The opening of the vessel into the ventricle is guarded by two semilunar valves (Fig. 134 A.), the free margins of which are bound to the inner surface of the tube by chordae tendineae. The distal extremity has a semi-lunar valve in the left compartment (Fig. 134), which is attached by a delicate band to the spiral valve (Ecker).

C. The minute structure of the Heart.

[The heart consists chiefly of muscle, but possesses also nerve-fibres, nerve-cells, endothelium, and connective-tissue; the truncus arteriosus has, in addition, blood-vessels.

a. The muscular structure of the heart.

(1) The arrangement of the muscle-fibres varies in the different portions of the heart. That of the ventricle will be easily understood by reference to Fig. 137. A layer of circular fibres (CM) extends round the whole ventricle, and from it more or less radial or transverse bands (TM, also Fig. 138 TM) pass inwards; these, in the apical portion of the ventricle, seem to extend almost transversely across; towards the base they are directed onwards and forwards towards the auricles. Of these bands some, near the apex, can be traced directly to the opposite side, others join the wall more anteriorly, and still others join bands of muscular fibres (LM), which may be described as longitudinal, and form the Musculi papillares. In the anterior portion of the ventricle the same arrangement holds good, but is modified to form a central cavity; still more anteriorly, at the junction of auricles and ventricle, the trabecular structure is much less apparent, the fibres being closer and mainly circular.

In the auricles the muscular fibres are chiefly arranged in circular rings which cross one another; on the inner surface, however, traces of an arrangement similar to that in the ventricle may be made out. In the heart, from which the sections in Fig. 137 were drawn, transverse bands (cut longitudinally), 0.33 mm. in length, were numerous; their central ends terminated in bands, the fibres of which were cut transversely. The auricular septum (Fig. 141 M) also possesses muscular fibres which cross one another in its substance.

The fibres of the truncus arteriosus resemble those of an ordinary artery; those of the sinus venosus are arranged in rings, mostly transverse, but others oblique, and a few longitudinal.]

(2) The structure of the cardiac muscle-fibres (Figs. 138 and 141 a).

[The cardiac muscle of the frog consists of spindle-shaped cells, which as Pohl-Pincus pointed out, are of two kinds; the main difference between the two being in the size of the nucleus, which is much broader in one than in the other.

α. Cells with a broad nucleus (Fig. 138 B): the nucleus is lenticular or egg-shaped, 4 to 7 µ broad, 8 to 14 or 18 µ long; the ratio of length to breadth being 1 : 2.

β. Cells with a narrow nucleus (Fig. 138 A): the nucleus is rod-shaped, 2·5 to 3·5 µ broad and 25 to 43 µ in length. The ratio of length to breadth varying from 1 : 8 to 1 : 16. These cells are more frequent in the right half of the base of the ventricle than the former variety. In the trabeculae the second variety is more numerous (Pohl-Pincus).

The muscle-fibre may be simple (Fig. 141 a M) or possess branches (Fig. 138); the fibres are all more or less spindle-shaped, and striated transversely and longitudinally, but possess no sarcolemma. The protoplasmic contents of the cell are finely granular; the nucleus much more coarsely granular and possessing one or more distinctly marked nucleoli. The striations are due to the presence of a network, which has been carefully described by Messrs. B. Melland and C. F. Marshall‍63, and is similar to that of ordinary voluntary muscle (see histology of muscle).]

b. The nerves of the heart.

The nervous supply of the heart is derived from the sympathetic system and from the cardiac branches of the pneumogastric nerve; the course of which has already been traced (page 175) to the roots of the anterior caval veins, where they form a simple plexus by means of a connecting link (Fig. 139). The plexus possesses nerve-cells and sends off two nerves into the auricular septum; the one (d) lies dorsally and is shorter and thicker than the other (v), which lies ventrally in the septum. They course backwards to the posterior border of the septum, and there distribute fine branches to the base of the ventricle and surrounding parts.

[In the whole of their course in the heart they have numerous nerve-cells, either imbedded between their fibres or attached to the nerves and their branches.

The nerve-fibres are both medullated and non-medullated. The nerve-cells (Figs. 139, 140, 141) are usually oval, and are unipolar or rarely bipolar; sometimes two cells exist in the same envelope, forming the ‘twin-cells’ of Dogiel (Fig. 141 b II). According to the recent observations of Lavdowsky and Dogiel each cell receives one medullated fibre and gives off a fine non-medullated fibre; the two run together for a short distance, after which the non-medullated fibre separates to supply muscle-fibres.

Two large clusters of nerve-cells have received special names.

Remak’s ganglion is a large group of nerve-cells in the wall of the sinus venosus.

Bidder’s ganglion is formed, collectively, by groups of cells in the auriculo-ventricular groove, the chief groups being attached to the two cardiac nerves.

No nerve-cells have been discovered behind that portion of the ventricle near the auriculo-ventricular groove, nor has a direct connection between the cells or their processes with the fibres of the pneumogastric nerve been made out.

c and d. [The endothelium and connective-tissue of the heart.

Nerve-fibres have been described as existing in all parts of the ventricle and being connected with the muscle-fibres (Openchowsky and others); this cannot, as yet, be accepted as proved.

The truncus arteriosus also possesses nerve-fibres and nerve-cells (Pagliani, Löwit, and others), but their arrangement seems to be very irregular. Löwit did not succeed in finding the nerve-cells in all cases examined; and Engelmann denies their occurrence.]

The connective-tissue of the heart exists only in very small quantity. A fine layer on the outer surface of the ventricle, especially towards the auriculo-ventricular groove, can easily be seen (Fig. 138 CT); the various valves, the auricular septum, and more particularly the truncus arteriosus, all contain connective-tissue. In the latter part the spiral valve is wholly formed of this tissue.

The whole of the inner surface is lined with endothelium (endocardium), which covers the various trabeculae; the external surface of the heart is likewise covered with an epicardial layer of endothelium (Fig. 138 E).]

e. The blood-supply to the heart.

With the exception of the truncus arteriosus, no part of the heart possesses blood-vessels (Hyrtl). [The truncus arteriosus (Fig. 142 b) has a rich anastomosis, which receives its blood by the arteria bulbi (a), a branch of the carotid arch. Two veins connect this anastomosis with the systemic veins, and with the hepatic portal veins: the former, vena bulbi anterior (v), passes over the auricles to open into the left innominate vein (vi); the latter, the vena bulbi posterior (vc) or vena cardiaca, as it is more usually named, passes from the truncus arteriosus in a fold of pericardium, and then courses backwards to open into the anterior-abdominal vein.]

II. THE ARTERIES.

(The original arrangement of this section has been modified.)

The Truncus arteriosus divides to form two vessels (Figs. 132, 134); each of these is divided by two partitions into three compartments, which after a short course together separate to form three distinct vessels, the carotid, systemic, and pulmo-cutaneous arches.

1. The carotid arch (Canalis carotico-lingualis, Brücke; Ductus caroticus), (Fig. 143 I) is the most anterior of the three arches: it winds round the side of the oesophagus as far as the carotid gland, where it divides to form two branches; the right carotid arch gives off a small branch to the Truncus arteriosus, the Arteria bulbi (see above).

The carotid gland (Fig. 144 c′) is a dilated portion of the vessel. The interior contains bands or trabeculae of muscle, which give its walls a more or less spongy structure; the muscle-fibres are not transversely striated (Hoffmann).

a. The lingual artery (Arteria lingualis, Arteria hyoidea-lingualis, Arteria hyoidea), (Figs. 143 l, 144 L) arises by two roots from the inner surface of the carotid gland, and courses forwards and inwards, supplying branches to the thyroid gland and the Musculi geniohyoidei, to the ventral surface of the hyoid; another continues forwards on the outer border of the M. hyoglossus to supply branches to the hyoid and tongue.

b. The carotid artery (Arteria carotis communis), (Figs. 143 ca, 144 c″) arises by several roots from the outer wall of the carotid gland; and passes over the Musculus petrohyoideus I to the anterior end of the oesophagus, round which it courses towards the vertebral column, and then runs forwards to the base of the skull. The artery then courses forwards, lying immediately above the mucous membrane and underneath the transverse arm of the parasphenoid, which it crosses about midway between its back and external end; [at this point the artery gives off two palatine arteries, then ascends in the orbit to the origin of the M. pterygoideus, and divides into two branches, the internal carotid artery and the ophthalmic artery]. The branches are:‍—

(1) The pharyngeal artery (Arteria pharyngea ascendens), (Fig. 145 p) which runs towards the opening of the Eustachian tube, distributes branches to the pharynx, and anastomoses with the Arteria pharyngeo-maxillaris of the cutaneous artery, and with the Ramus inframaxillaris of the occipital artery.

(2) [The posterior palatine artery (Arteria palatina posterior, Virchow) immediately divides into a number of small twigs to supply the mucous membrane of the hinder and outer parts of the gums.]

(3) The anterior palatine artery (Arteria palatina anterior, Virchow; Arteria palatina, Ecker), (Fig. 145 p′) runs forwards, between the Musc. levator bulbi and the mucous membrane, with the Ramus palatinus of the trigeminal nerve. [On reaching the palatine bone, it curves outwards to the outer anterior angle of the orbit, where it gives off a branch backwards along the upper jaw to the hinder portion of the orbit.] In its course it supplies twigs to the surrounding tissues, but chiefly to the mucous membrane and Harder’s gland.

(4) The internal carotid artery (Art. cerebralis, Virchow); see Arteries of Brain, p. 162.

(5) The ophthalmic artery (Art. ophthalmica), (Fig. 145 o) [is larger than the internal carotid artery at this point; it crosses the origins of the M. rectus externus and of the M. retrahens bulbi and reaches the optic nerve; the artery then runs forwards along the under surface of this nerve and of the eyeball, lying close to the sclerotic coat as far as a little beyond the equator, where it pierces the sclerotic coat so obliquely that the choroid coat is only reached at the ciliary processes. In its course the artery supplies:‍—

α. Muscular branches, given off at the point where the artery reaches the eyeball.

β. Two arteries (AA. ciliares) to the choroid, given off at the same point as the foregoing.

γ. Two arteries to the iris.

δ. The Art. hyaloidea, the terminal portion of the ophthalmic artery. (For further description of these vessels, see Eye.)].

2. The systemic arch (Ductus aorticus), (Figs. 132, 143, and 145 II) is the middle arch of the three; it arises from the middle canal of the Ductus arteriosus, and winds obliquely round the oesophagus between the MM. petrohyoidei I and II towards the vertebral column, which it reaches at about the level of the sixth vertebra. The right systemic arch is continued as the dorsal aorta, the left arch communicates with it merely by a small opening, and is then continued as the coeliaco-mesenteric artery.

Branches of the systemic arch:‍—

a. The laryngeal artery (Arteria laryngea), (Fig. 146lg) arises from the inner border of the systemic arch before it reaches the oesophagus; it passes forwards and inwards to be distributed in the larynx and pharynx. According to Brücke this artery possesses a valve at its point of origin.

b. The oesophageal arteries (Arteriae oesophageae) pass from the upper part of the arch to the dorsal surface of the oesophagus.

α. The Rami spinales, which pass through the intervertebral foramina to the spinal cord (see Arteries of Spinal Cord, p. 162).

c. The occipito-vertebral artery (Ecker), (Art. oc­ci­pi­to-ver­te­bra­lis), (Fig. 147 o, v) [arises from the systemic arch immediately in front of the transverse process of the second vertebra, and ascends immediately in contact with the body of the first vertebra, which it separates from the MM. inter­trans­ver­sarii capitis and the sympathetic cord]. Immediately under the most anterior portion of the M. longissimus dorsi the artery divides into two branches, the vertebral and occipital arteries:‍—

(1) The vertebral artery (Arteria vertebralis or supra-vertebralis) (Fig. 147 v) courses backwards lying on the transverse processes of the vertebrae and on the MM. intertransversarii, under cover of the M. longissimus dorsi. In this course it is close to the oblique processes, and may even be partially covered by these; it extends beyond the sacrum, and gives off:‍—

β. The Rami dorsales; these supply the muscles of the back and send branches to the skin, which accompany the cutaneous branches of the spinal nerves through the Saccus lymphaticus cranio-dorsalis.

γ. The Rami intercostales are distributed in part to the MM. intertransversarii; other twigs pass ventrally to the periganglionic glands; the longest branches accompany the anterior divisions of the spinal nerves to the muscles and skin of the belly (Fig. 145).

(2) The occipital artery (Arteria occipitalis), (Figs. 147 and 148 o), the anterior branch of the occipito-vertebral artery, pierces the M. longissimus dorsi; then courses forwards under cover of the Fascia dorsalis and upon the M. temporalis, and divides, behind the eyeball [at the anterior border of the M. temporalis, Virchow], into two branches:‍—

α. The Ramus orbito-nasalis (Fig. 148 on) accompanies the ophthalmic nerve along the wall of the cranium, and passes through the sphenethmoid, with the nasal branch of the ophthalmic nerve, to the nose, where it divides into two branches supplying the mucous membrane of the nose. In its course through the orbit the artery supplies branches to the Harderian gland and neighbouring parts, and anastomoses with the ascending branch of the Arteria palatina.

β. The Ramus maxillaris communis (Art. temporalis, Virchow), (Fig. 148 m) runs downwards and outwards along the anterior border of the M. temporalis, and passes under the anterior arm of the squamosal bone to course backwards on the upper jaw. At this point the Ramus maxillaris superior (m′) is given off. The main artery continues its backward course to the tympanic membrane, where it divides into its three terminal branches:‍—

[(1) The Ramus maxillaris superior (AA. maxillares superiores, Virchow), usually not a single artery but a number of small twigs, which supply the space between the eye and the upper jaw.

(2) A small ascending branch to the tympanic membrane (Virchow).

(3) A Ramus auricularis (Virchow), which forms a rich anastomosis on the hinder wall of the tympanic cavity, and supplies a twig to the tympanic membrane. This passes from the upper border of the membrane to the point of attachment of the Columella auris (extrastapedial), round which it forms a circular anastomosis. The Ramus auricularis also anastomoses with the cutaneous artery.

(4) A branch passing inwards to the fat-body, and lying immediately in front of the deltoid muscle (Virchow).

(5) A small branch running on to the inner surface of the angle of the jaw and then forwards (Virchow).

(6) A larger vessel, the Ramus maxillaris inferior (Fig. 148 m″), which arises from the Ramus maxillaris superior under cover of the tympanic membrane, and accompanies the Ramus mandibularis of the trigeminal nerve through the M. masseter, or between this muscle and the M. temporalis to the mandible, and then courses forwards to the chin. A large cutaneous branch is given off half-way along the floor of the mouth. The artery gives off many small twigs to supply the neighbouring structures.]

3. The pulmo-cutaneous arch (Ductus pulmo-cutaneus), (Figs. 132 143, and 145 III) is the hindmost (fifth embryonic) of the three persistent arches; it passes upwards and forwards on the oesophagus, and at the level of the carotid gland divides into two branches, the pulmonary artery and the cutaneous artery.

a. The pulmonary artery (Arteria pulmonalis) runs backwards to the root of the lung, along the outer surface of which it is continued in a sinuous course. It supplies the lung.

b. The cutaneous artery (Arteria cutanea magna), (Fig. 149 c.m.) [passes along the deeper surface of the M. petrohyoideus III until near the prootic bone; in this course it travels forwards, outwards, and upwards, crosses the outer border of the muscles, and appears between the M. levator anguli scapulae and the M. sternocleidomastoideus. At this point the artery curves sharply round to reach the angle of the jaw, the hinder border of the M. depressor maxillae inferioris. The curve so formed and the descending limb are covered by the M. depressor maxillae inferioris. The artery then passes backwards to form a rich anastomosis in the skin almost as far as the posterior end of the trunk. It supplies the following branches:‍—

(1) The Ramus dorsalis (Virchow) arises immediately in front of the curve of the main artery; it ascends behind the M. temporalis, and passes along the anterior border of the M. depressor maxillae to the skin, where it curves backwards. It supplies small twigs forwards as far as the eyelids.

(2) The Ramus auricularis (Virchow) also arises in front of the curve in the cutaneous artery; sometimes it arises from the Ramus dorsalis. The artery winds around the M. petrohyoideus III near its insertion, and so reaches the posterior wall of the tympanic cavity, where it anastomoses with the R. auricularis of the Ramus maxillaris communis.]

(3) The Arteria pharyngo-maxillaris (Fig. 148 p.m.) is a small branch running forwards and outwards to supply the mucous membrane of the larynx, Eustachian tubes, gums, the lower jaw, and the skin under the mouth. It forms a rich anastomosis with branches of the Ramus maxillaris inferior of the occipital artery, and with others from the pharyngeal branch of the carotid artery; a large branch passes forwards to supply the skin of the throat and floor of the mouth.

(4) The Arteria cutanea pectoris (Fig. 149 c.p.) is distributed to the skin of the breast.