Be A ee ak 2 ihc: 
878 THE VASCULAR SYSTEM. 
apex of the bulbus arteriosus. After the completion of the cephalic aortic arches the 
portions of the anterior ventral aorta which le between their lower extremities are 
known as the ventral roots of the cephalic aortic arches. 
The primitive dorsal aorta may be conveniently divided into two parts. The anterior 
part extends from the dorsal end of the first cephalic aortic arch to the root of the fore- 
limb. It remains separate from its fellow of the opposite side, and forms the-dorsal roots 
of the cephalic aortic arches. It takes part in the formation of the great vessels of the 
head and neck and, on the left side, of part of the aorta of the adult. 
The remaining portion extends from the root of the fore-limb to the pelvic region ; it 
passes inwards, and unites with its fellow of 
the opposite side beneath the vertebral column 
to form the greater part of the permanent 
systemic aorta. 
The primary caudal arch connects the 
primitive dorsal with the posterior ventral 
inner side of the Wolffian duct. After a time 
it is replaced by a secondary caudal: arch 
which lies at the outer side of the Wolfhan 
duct, and this subsequently becomes the 
common and internal iliac arteries and the 
root of the hypogastric artery, the external 
iliac being merely an offset from it to the 
hind-limb. 
The greater part of the primitive posterior 
ventral aorta disappears early, and its secondary 
continuation to the placenta becomes the 
remainder of the hypogastric artery, which 
passes from the internal iliac artery, by the 
side of the bladder and along the ventral wall 
of the abdomen, to the umbilicus, whence it 
is continued along the umbilical cord to the 
placenta. 
Having considered thus briefly the main 
parts of the primitive aortic vessels, and 
having noted, shortly, the fate of each portion 
in the subsequent phases of development, we 
may now turn to a more detailed consideration 
of the metamorphoses which occur in those 
parts of primitive vessels, viz. the anterior 
ventral aortee, the cephalic aortic arches, and 
the anterior parts of the primitive dorsal aorte, 
where the transformations are most striking 
and most complicated, and which result in the 
formation of the heart, the aorta in part, the 
pulmonary artery and its primary branches, 
the chief arterial trunks of the head and neck, 
and the first part of the main artery of the 
right upper extremity. 
Fic. 600.—DEVELOPMENT OF THE Heart. DEVELOPMENT OF THE HEART, OF THE 
Diagram showing the changes of form and external FIRST PART OF THE AORTA, AND OF 
appearances at different stages. Modified from THE PULMONARY ARTERY. 
His’s models. III.B and IV.B are side views ; 
the other figures represent the heart as seen Of the three parts into which each anterior 
com fae ont : ventral aorta is divisible the middle is situated 
A, Auricle; A.B, Aortic bulb; A.C, Auricular canal ; 
Ege , i splanchnic mesoder he dorsal wall 
A.P, Auricular appendix ; L.V, Left ventricle ; in the sp! anchnic mesoderm et the d ae 
0.A.C, Opening of auricular canal; P.V.A, Primi- of the pericardium. As deve opment proceeds 
tive ventral aorta; R.V, Right ventricle; $.V, the middle parts of both anterior ventral aorte 
Sinus venosus; V, Ventricle; V.V, Vitelline enlarge and project into the cavity of the peri- 
are cardium, whilst at the same time they grow 
inwards, their inner walls come in contact, fuse together, and disappear, and so a simple 
single median tube is formed. This is the primitive heart, which is completed in front 
aorta. As it passes ventrally it lies on the 
