882 THE VASCULAR SYSTEM. 
of the adult heart ; the left end becomes the lower part of the oblique vein of Marshall, 
the rest of this vein representing the left duct of Cuvier. 
Thus, when the development of the heart is completed, all the large veins which reach 
the heart, with the exception of the pulmonary veins, open into the right auricle, and 
into that part of the chamber formed by the absorption of the right end of the sinus 
yenosus. Indications of the primitive separation of the auricle from the sinus venosus 
are still recognisable in the adult, as the suleus terminalis on the exterior, the corre- 
sponding crista terminalis in the interior, and the Eustachian and Thebesian valves ; 
the Eustachian valve is a remnant of the upper part of the valvular fold which was 
situated at the right margin of the slit-like aperture of communication between the 
sinus venosus and the auricle, whilst the valve of Thebesius represents the lower part of 
the right venous valve. 
The valves which guard the auriculo-ventricular orifices are downgrowths from the 
lower end of the auricular canal. The valves of the pulmonary and aortic apertures are 
preceded by four endocardial thickenings at the lower end of the aortic bulb—anterior, 
posterior, and two lateral. As the septum of the bulb descends it fuses with the middle 
parts of the lateral thickenings ; thus, when the septum of the bulb has descended below 
the lower orifice of the bulb, dividing it into aortic and pulmonary apertures, three 
endocardial thickenings are found in each aperture, one anterior and two posterior in the 
pulmonary aperture, and the reverse in the aortic orifice. From these thickenings the 
semilunar valves of the aortic and pulmonary apertures are developed, and they retain 
their original positions until after the sixth month of foetal life ; ultimately, however, they 
are twisted round, so that in the adult the pulmonary valves are placed two in front and 
one behind, and the aortic one in front and two behind. 
THe Aortic ARCHES—FORMATION OF THE CHIEF ARTERIES. 
The aortic arches at the head end of the embryo connect the aortic bulb and the 
ventral aortze in front of the bulb, with the corresponding parts of the primitive 
dorsal aortee. The arches, ten in number, are arranged in pairs. There are, therefore, 
five arches on each side, and they are distinguished from before backwards by their 
numerical designation. 
The first is formed during the development of the head fold by the simultaneous 
bending of the primitive aorta; it lies at the side of the bucco-pharyngeal area, and 
subsequently in the substance of the mandibular arch. The remaining aortic arches are 
formed quite differently, and grow dorsally through the substance of the remaining 
visceral arches, as these are formed in regular succession in the side wall of the pharyn- 
geal portion of the foregut. The second springs directly from the anterior ventral aorta, 
and passes through the hyoid arch to the dorsal aorta, but the third, fourth, and fifth 
spring by a common trunk from the apex of the aortic bulb. Subsequently, however, and 
as the neck grows forwards, the anterior ventral aorta is elongated, and the third and 
fourth arches arise separately from it. At this later period, therefore, four vessels, two 
on each side, spring from the aortic bulb, viz. the two fifth arches and the two 
anterior ventral aortee from which the anterior four pairs of arches arise. When the 
septum of the aortic bulb is developed, the bulb is divided into two parts, one (the 
pulmonary artery) connecting the fifth arches with the right ventricle, and the other 
(the ascending aorta) connecting the ventral aortic stems, and through them the 
first, second, third, and fourth pairs of arches, with the left ventricle. Each cephalic 
aortic arch is connected with the arch immediately behind it by a dorsal root, and in the 
vases of the first three arches by a ventral root also; but the ventral root of the fourth 
arch connects it with the aortic bulb. The dorsal and ventral roots of the arches are 
simply portions of the primitive ventral and dorsal aortz, which are so named merely 
for descriptive purposes. From the dorsal roots a series of segmental branches are 
given off, which pass dorsally, between the rudiments of the transverse processes of the 
cervical vertebrae, to supply the spinal cord and its membranes and the muscles and 
fascize of the back. 
The five pairs of arches do not all persist in their entirety, but remains of each are 
found even in the adult. The first and second pairs disappear almost entirely ; from the 
ventral ends of the first arches, however, the superficial temporal, the internal maxillary, 
and the lingual arteries are formed ; whilst from the ventral ends of the second arches the 
ascending pharyngeal, posterior auricular, and occipital arteries are derived. 
On each side the ventral roots of the first and second arches persist as the stem of the 
corresponding external carotid artery. The internal carotid is formed by the third arch 
——S i ee eee 
