146 



DEVELOPMENT OF THE PRINCIPAL ARTERIES. 



into it from the compact outer layer of muscle, and unite with one another to form 

 a spongework of muscular trabeculaa, while the endothelium of the cavity becomes 

 depressed between and over these trabeculas, and lines all the spaces between them, 

 which thus communicate with the cavity of the ventricle. The ventricles are therefore 

 now in the same condition in which they are found permanently in many of the 

 lower vertebrates (e.g. frog). 



Ultimately the compact outer layer of muscle becomes greatly increased in thick- 

 ness, and the spongework of trabeculse occupies a relatively much smaller portion of 

 the cavity, being developed in part into the columnas carneae of the adult heart. 



Peculiarities of the foetal heart. Besides the peculiarities of structure,, 

 which have been above described, the foetal heart differs in position, in relative size, 

 and in the thickness of its several parts, from the organ after birth. Thus it is at 

 first placed immediately under the head, but subsequently, with the development of 

 the neck, it gradually assumes a position farther back. In early foetal life it is 

 much larger in proportion to the size of the body than at a later period, and at birth 

 it is still proportionally large. The walls of both ventricles are of equal thickness 

 during foetal life, a peculiarity which is evidently connected with the fact that in 

 consequence of the communication of the pulmonary artery, through the ductus 

 arteriosus, with the aorta, the blood pressure which they have to overcome is the 

 same. 



DEVELOPMENT OF THE PRINCIPAL ABTEEIES. 



From the point of insertion of the aortic bulb into the ventral wall of the foregut, 

 first one, and then in succession four other arterial arches, become formed, and pass 

 on either side, one into each visceral arch. Half encircling this part of the 



Fig. 176, DIAGRAMMATIC OUT- 

 LINES OP THE HEART AND PRI- 

 MITIVE VESSELS OP THE EMBRYO 

 CHICK AS SEEN PROM BELOW AND 

 ENLARGED. (A. T. ) 



A, soon after the first establish- 

 ment of the circulation ; B, C, at 

 a somewhat later period ; 1,1, 

 the veins returning from the vas- 

 cular area ; 2, 3, 4, the heart, 

 now in the form of a notched 

 tube ; 5, 5 (upper), the two primi- 

 tive aortic arches ; 5, 5 (lower), 

 the primitive double aorta ; A, the 

 single or united aorta ; 5', 5', the 

 continuation of the double aorta? 

 beyond the oridn of the large 

 omphalo-meseuteric arteries, 6, 6. 

 The division above 4 is repre- 

 sented as carried rather too far 

 down. 



alimentary canal, they are continued above it into two descending or primitive aortce. 

 These two vessels run down the trunk on either side of the notochord, yielding, as 

 they descend, lateral offsets to the body walls and to the yolk sac. Finally they give 

 off, at ^ the lower or posterior extremity, two large vessels, which accompany the 

 allantois, and furnish blood to the foetal part of the placenta (umbilical or allantoic 

 arteries). 



The primitive aortae do not long remain double. As was first shown by means 

 of sections by Allen Thomson, they unite in the middle line, the union beginning 

 in the dorsal region and extending forwards and backwards ; in the latter direction 



