DEVELOPMENT OF THE HEART AND VESSELS. 



325 



the ninth week it appears, growing from above and behind downwards and forwards, 

 and at length comes to meet and coalesce below with the rising edge of the inter- 



Fig. 244. SHOWS THE POSITION AND Fig. 224. 



FORM OF TDE HEART IN THE HCMAN 

 EMBKTO FROM THE FOURTH TO THE SIXTH 

 WEEK. 



A, upper half of the body of a human 

 embryo said to be four weeks old (from 

 Kolliker after Coste) ; B and C, anterior 

 and posterior views of the heart of a human 

 embryo of six weeks (from Kolliker after 

 Ecker) ; a, frontal lappet ; 6, mouth ; c, 

 below the lower jaw and in front of the 

 second and third branchial arches ; d, 

 upper limb; e, liver; /, intestine cut 

 short ; 1 , superia vena cava ; 1', left 

 superior cava or brachio-cephalic connected 

 with the coronary vein ; 1 ", opening of the 

 inferior vena cava; 2, 2', right and left 

 auricles ; 3, 3', right and left ventricles; 

 4, aortic bulb. 



ventricular septum. The interauricular sep- 

 tum, however, remains incomplete during 



intrauterine life, and leaves an opening in the middle, which forms the foramen ovale. 

 The farther steps in the separation of the auricles are connected with the changes 

 which take place at the entrances of the great veins. There are now three large vessels 

 terminating in the auricular extremity of the heart ; of these two correspond with 

 the superior and the inferior vena cava, and the third is the great coronary vein. 

 At first, after the interauricular septum is partly formed above, the inferior cava 

 opens directly into the left auricle, which is the smaller of the two ; but about the 

 twelfth week a septum, the valve of the foramen ovale, which afterwards forms the 

 floor of the fossa ovalis, rises up on the left side of the entrance of the vein, which 

 thus comes to open into the right auricle ; whilst at the same time the separation of 

 the two auricles is also rendered more complete by the gradual advance of the valve 

 over the foramen ovale, leaving, however, the passage open until after birth. 



Another valvular fold is developed at an early period on the right and anterior 

 border of the orifice of the inferior cava, between it and the auriculo-ventricular 

 orifice ; this is the Eustachian valve. It appears to continue the opening of the 

 inferior cava towards the upper margin of the foramen ovale, and directs the blood 

 of the vein through that passage. 



The left auricle has at first no connection with the pulmonary veins. The manner 

 in which this connection is afterwards established has not yet been ascertained. 



Originally the heart is composed of a mass of nucleated cells, similar in character 

 to those which primarily constitute the other organs of the body. Muscular tissue is 

 subsequently formed from these cells ; but the rhythmic contractions commence and 

 proceed for some time, whilst the heart is yet composed of cells, and before the mus- 

 cular fibres have been developed. 



The great vessels. At first the bulbus arteriosus is divided into two arches, which 

 pass upwards and outwards one on each side, then turn downwards and form a right 

 and left root of the aorta, which are at first separate, but afterwards unite behind 

 the heart and in front of the vertebral column to form the single stem of the descend- 

 ing aorta. The distance soon elongates between those arches and the arterial bulb, 

 and four other pairs of arches appear in series from above downwards, passing 

 outwards from the vessel which ascends to the first arch, and opening into that 

 which descends from it. Thus there are on each side five arches, an internal or 

 anterior trunk uniting the origins of the arches, and an external or posterior trunk 

 uniting their terminations, and continued into one of the roots of the aorta. These 

 vascular arches are placed each in one of the branchial processes of the dorsal plates 

 (p. 64), but it is to be noted that the whole five arches do not co-exist; for the highest 



