mammal shows distinct indications of division into its several parts; indeed, these parts are apparent even while the two tubes are still distinct, as the accompanying sketches of the rabbit's heart clearly show (fig. 159). The heart does not, however,

jj, jugular veins; ao, descending aorta; ph, pharynx; hp, epiblast of body-wall ih, endothelial lining of the still divided heart; ah, outer wall of the heart; p, pericardial cœlom; df, df', visceral mesoblast (somatopleure); e', prolongation of the hypoblast of the foregut and the anterior wall of the pericardial cavity into the partition between the two halves of the heart; bl, bilaminar portion of blastoderm forming pro-amnion; ect, ent, its two layers (epiblast and hypoblast).

long retain its symmetrical position. It soon becomes bent upon itself, so as to assume the shape of an S, the anterior part of the tube bending over to the right and the posterior to the left (fig. 164, B). At the same time the posterior, or sino-auricular

Fig. 163. SECTION THROUGH THE REGION OF THE HEART IN A RABBIT EMBRYO OF 10 DAYS, AFTER THE TWO TUBES HAVE UNITED INTO A SINGLE MEDIAN ORGAN. (Kölliker.)

ao, descending aorta; ba, bulbus aorta; ah, its external wall; mp, posterior mesocardium, uniting the heart to the ventral wall of the pharynx, ph, and here separating the pleuropericardial, coelom, p, into two halves, which are, however, united on the ventral side of the heart; ent, hypoblast of yolk sac; df, its mesoblast; df', mesoblast of pharynx; ect, epiblast.

Fig. 164.-OUTLINES OF THE ANTERIOR HALF OF THE EMBRYO CHICK VIEWED FROM BELOW, SHOWING THE (After Remak.) 20

A, embryo of about 28 to 30 hours; B. of about 36 to 40 hours; a, anterior cerebral vesicle; b, proto-vertebral segments; c, cephalic fold; 1, 1, vitelline or omphalo-mesenteric veins entering the heart posteriorly; 2, their union in the posterior part of the heart; 3, the middle part of the tube corresponding to the ventricle; (in B) the arterial bulb.

end of the heart, gradually comes to lie behind or dorsal to the ventricular part, which arches transversely from left to right, where it turns sharply upward (towards the head), and terminates in the bulb. The tube is divided by slight constrictions into successive portions, viz.: (1) the part formed by the junction of the principal veins, sinus venosus; (2) the auricular part; (3) the ventricular part; and (4) the aortic bulb.

mn.

b.a.

The sinus venosus may be described as consisting of two lateral enlargements or horns, and of a transverse part connecting these horns. The veins which it at this time receives are the umbilical, the vitelline, and the ducts of Cuvier (formed

2.

V.C.S.
S.7.

u.v.

v'.

au.

d.

87.

.7.
b.d.

аи.

Fig. 165.-CONDITION OF THE HEART IN THE HUMAN EMBRYO OF ABOUT FIFTEEN DAYS, RECONSTRUCTED FROM SERIAL SECTIONS. (His.)

A, from before, showing external appearance of heart; B, the same with the muscular substance of heart removed showing the endothelial tube; C, from behind.

mn, mandibular arch with maxillary process; hy, hyoidean arch; b.a, bulbus aorta; e, right ventricle; ', left ventricle; au, auricular part of heart; c.a., canalis auricularis; s. r, horn of sinus venosus with umbilical vein (u.v), superior vena cava (v.c.s), and vitelline vein entering it; d, diaphragm; m p, mesocardium posterius; l, liver; b.d, bile duct.

by the junction of the primitive jugular from the head and the cardinal from the trunk). The three veins are nearly symmetrical on the two sides, and enter the

Fig. 166.-HEART OF A SOMEWHAT MORE ADVANCED HUMAN EMBRYO. (His.)

r.v, right ventricle; l., left ventricle; b.a, bulbus aorta; r.au, right auricle; l.au, left auricle; v.c.s, vena cava superior; u.v, umbilical vein; v.v, vitelline vein; d, diaphragm.

corresponding horn of the sinus (fig. 168). The sinus is at first in free communication with the common auricular cavity, but the junction presently becomes narrowed, and the resulting aperture, which eventually acquires a slit-like character,

is found to open from the right horn of the sinus into the right part of the common auricle. The sinus now forms a transversely disposed sac, lying below and behind the common auricle, with a larger right and a smaller left horn (the latter being

Fig. 167.-HEART OF HUMAN EMBRYO SLIGHTLY MORE ADVANCED THAN THAT SHOWN IN FIG. 166. (His.) A, interior of auricle and ventricle displayed.

B, endothelial tube.

a. c, auricular canal; a. i, area interposita of His; m, posterior mesocardium; r.au, l.au, right and left auricles; l. v, left ventricle; r.v, right ventricle; b. a, bulbus aorta.

tapered off into the left duct of Cuvier); in this condition it has been termed by His saccus reuniens (fig. 169, B, and fig. 171). The umbilical and vitelline veins soon open into it by a common trunk, which becomes the upper end of the vena cava inferior.

-каш

umb.v. vi.v.

umb.v.

A, from before; B, from behind.

vi.v.

(Born.)

8.8, sinus venosus; l.v, left ventricle; r. v, right ventricle; b, bulbus aorta; ao', first aortic arch; ao", second aortic arch; r.au, right auricle; l.au, left auricle; umb.v, umbilical vein; vi.v, vitelline vein; v.c.s, vena cava superior.

The slit-like orifice of the sinus in the back of the right auricle is guarded by two valve-like folds of the endocardium, which project into the cavity of the auricle (right

(Born.)

p.a, pulmonary artery; s.r, 81, s.tr, right and left horns and transverse part of sinus respectively; r.8.c, l.s.c, right and left superior cavæ; pe, aperture of pulmonary vein; h. v, hepatic veins; d., ductus venosus; me, mesocardium posterius.

The other letters as in fig. 168.

Fig. 170.-SECTION THROUGH THE HEART OF A RABBIT EMBRYO AT THE STAGE SHOWN IN FIG. 169.

(Born.)

r.s, l.s, right and left horns of sinus receiving from above the respective superior vena cavæ; r.au, l.au, right and left auricles; r.v, l.v, right and left parts of the ventricle; r.v v, l.v.v, right and left valves guarding the orifice from the right horn of the sinus into the right auricle; au.v.c, one of the two endocardial cushions which are beginning to sub-divide the common auriculo-ventricular aperture. The dotted line encloses the extent of the endocardial thickening; s', first septum superior growing down between the auricles and prolonged below by a thickening of endocardium. Close to this septum in the left auricle is seen the opening of the pulmonary vein; s.inf, inferior septum of the ventricles. Fig. 171.-VIEW FROM BEHIND OF THE HEART OF A HUMAN EMBRYO OF ABOUT 4 WEEKS, MAGNIFIED. (His.) The two superior cave, right and left, and the inferior cava are seen opening separately into the sinus which is a transversely elongated sac communicating only by a narrow orifice with the right auricle.

and left venous valves) (fig. 170, r.v.v., l.v.v.). These pass above into a muscular fold of the auricular wall, which extends over the roof of the auricle heart parallel to the septum atriorum, and is known as the septum spurium (fig. 173, B). It disappears at length, probably by uniting with the septum atriorum. Subsequently the venous orifice opens out, and the right horn of the sinus, which is now seen to receive all the great veins except the left duct of Cuvier, becomes gradually incorporated with the cavity of the auricle. The transverse part of the sinus and its left horn are continuous with the left duct of Cuvier (fig. 171), and eventually the transverse part forms the coronary sinus. From the right venous valve the Eustachian valve is formed, and the development of the Thebesian valve is also connected with its lower end (Schmidt). The left venous valve disappears.

The transversely placed ventricular part of the heart receives at first at its left end the orifice of the common auricle, which opens into its posterior wall (fig.

Fig. 172.-DIAGRAM TO SHOW THE FORMATION OF THE SEPTUM OF THE VENTRICLES AND BULB, AND THE MODE OF DIVISION OF THE COMMON AURICULO-VENTRICULAR APERTURE. (Born.)

au.v.c (in A and B), auriculo-ventricular aperture, partially divided into two by endocardial cushions; r.au.v, l.au.v, right and left auriculo-ventricular apertures which have resulted from the division of the common aperture; r.v, l.v, right and left ventricles; b, bulbus aortæ, replaced in C, by p.a and a.o, pulmonary artery and aorta; s.b, septum bulbi; s. inf, septum inferius ventriculorum; o (in A), orifice between the two ventricles.

172, A, a.v.c.). At its right end it turns sharply upwards into the aortic bulb, into which it gradually tapers, although there is at a certain point a constriction of the endothelial tube, where the semilunar valves are subsequently formed (fretum Halleri). Soon the right and left halves of the ventricle are separated externally by a groove which extends from below, partially encircling the tube (fig. 169). If the interior of the heart is examined at this stage, it is seen that a muscular septum, corresponding internally to this groove, is growing upwards and backwards from the antero-inferior part of the tube, and is gradually separating it into two parts, which become the right and left ventricles respectively (fig. 170, s.inf). This septum (septum inferius of His) is placed obliquely to the long axis of the tube, and extends eventually nearly to the level of the auriculo-ventricular orifice, which has by this time become shifted along the posterior wall of the tube, so as to open into it about its middle instead of at the right end, as was previously the case (fig. 172, B). The septum of the ventricles remains incomplete for some time, a communication between the two ventricles being maintained above it. Eventually the septum inferius unites with prolongations, (1) from the endocardial cushions which divide the common auriculo-ventricular orifice into right and left

This muscular prolongation may, as Born suggests, be of use in assisting the action of the valves, and in preventing their being forced backwards into the sinus when the auricle contracts.