104 



GENERAL ANATOMY. 



however, is not complete during foetal life, but leaves an aperture (foramen 

 ovale) by which the two auricles communicate. 



The heart is at first composed of a mass of foetal cells, but its rhythmic con- 

 tractions can be observed even in this condition before the development of any 

 muscular fibres, and even, according to some authors, before it is in connection 

 with any vessels. 



The vessels which are in communication with the foetal heart are as follows: 

 In its earliest state the circulation is external to the embryo. This primitive 

 circulation appears about the fifteenth day, and lasts till the fifth week. It con- 

 sists of two arteries, the first aortic arches, which unite into a single artery, 

 running down in front of the primitive vertebrae and in the walls of the intes- 

 tinal cavity, and joining in a single artery, which again divides into two 

 primitive aortse or vertebral arteries, and these give off five or six omphalo-mes enteric 

 arteries, which ramify in the germinal area, forming with their parent trunks 

 a close network, terminating in veins, which converge towards a venous trunk, 

 the terminal sinus. This vessel surrounds the vascular portion of the germinal 

 area, but does not extend up to the anterior end of the embryo. It terminates 

 on either side in a vein called omphalo-mesenteric. The two omphalo-mesen- 

 teric veins open by a single trunk into the auricular extremity of the heart. 

 This primitive circulation extends gradually from the germinal area over the 

 whole of the umbilical vesicle, and disappears as the latter becomes atrophied. 

 In a more advanced state of the embryo, the position of this first pair of aortic 

 arches, corresponds to the first pharyngeal arch. Next in succession, other 

 pairs of arches are formed behind the first 1 (Fig. 65). The total number is five, 

 but the whole five pairs do not exist together, for the first two have disappeared 

 before the others are formed. These two have no representatives in the per- 

 manent structures. The third pair gives origin to the carotids, the fourth pair 

 forms the innominata and subclavian on the right, the arch of the aorta and 

 subclavian on the left. The fifth forms on the left side the pulmonary artery, 

 the ductus arteriosus and the descending portion of the thoracic aorta. Its 

 right branch disappears. 



The ascending portion of the arch of the aorta, and the root of the pulmo- 

 nary artery, are at first blended together in the common dilatation (aortic 

 sinus), which has been above spoken of as connected with the ventricular end 

 of the rudimentary heart (Fig. 6-i, 1). The septum which divides this common 

 artery into two begins to appear very early, even before the interventricular 

 septum. The formation of the permanent vessels is shown by the following 

 diagram : 



Fig. 65. 





Diagram of the formation of the aortic arches and the large arteries. I. II. III. IV. V. First, second, 

 third, fourth and fifth aortic arches. A. Common trunk from which the first pair spring; the place Avhere 

 the succeeding pair* lire formed is indicated by dotted lines. B. Common trunk, with four arches nnd a trace 

 of the fifth. C. Common trunk with the tliree last pairs, the first two having been obliterated. D. The per- 

 sistent arteries, those which have disappeared l)t-ing indicated by dotted lines. 1. Common arterial trunk. 

 2. Thoracic norta. 3. Right branch of the common trunk, which is only temporary. 4. Left branch, per- 

 manent. 5. Axillary artery, ft. Yo-rti>l>ral. 7. 8. Subclnvian. 9. Common carotid. 10. External; and 

 11, Internal carotid. 12. Aorta. 13. Pulmonary artery. 14, 15. RL'ht nnd loft pulmonary arteries. 



1 The position of the first four of these aortio arches is behind the corresponding pharynx-pal 

 arches, and th;vt of the. fifth behind the fourth pharyngeal cleft. 



