Taree Mee 7 ames ee oP ee 
DEVELOPMENT OF THE BLOOD VASCULAR SYSTEM. ‘ 875 
vessels which accompany the hepatic veins and their tributaries. Their efferent 
vessels pass mainly to the thoracic duct, but some of the upper ones on the right 
side end in the right lymphatic duct, and a few join the bronchial glands. 
| The bronchial “glands (lymphoglandulze bronchiales) lie along the sides of the 
bronchi, both within and outside ine lung. They receive afferents from the 
“superficial and deep parts of the lungs, from the visceral pleura, and from the pos- 
terior mediastinal glands. Their efferents pass, on the left side to the thoracic 
duct, and on the right side to the right lymphatic duct. 
! The deep lymphatic vessels of the thorax have been included in the description 
of the tributaries of the deep glands. As in the abdomen and pelvis, the main 
deep lymphatic vessels accompany the blood-vessels of the region. 
DEVELOPMENT OF THE BLOOD VASCULAR SYSTEM. 
THE PERICARDIUM, THE PRIMITIVE AORTA, AND THE HEART. 
A general account of the development of the primitive vascular system and of ee 
establishment of the foetal circulation has been given in a previous chapter (see p. 
et seq.), and it is there pointed out that the earliest blood-vessels of the developing ovum 
appear in the vascular area of the yolk sac, 7.e. outside the body of the embryo altogether. 
Almost simultaneously, however, two longitudin: al vessels appear in the embryo. itself. 
They are formed in the splanchnic mesoderm of the pericardial area, and are easily 
distinguishable before that area is carried downwards to form the ventral wall of the 
foregut during the evolution of the headfold. 
The two longitudinal vessels are the rudiments of the primitive heart and of the 
principal blood-vessels. The changes which take place in them, and which result in the 
formation of the fully-developed heart and vessels, will be more easily understood after the 
development of the pericardial sac, together with the alterations it undergoes both as 
regards position and relations, have been carefully studied. 
Development of the Pericardium and the Primitive Aorte.—The pericardial 
area is recognisable as soon as the mesoderm has extended over the embryonic area of the 
developing ovum. It is somewhat semilunar in shape, it lies at the extreme anterior end 
of the embryonic region, and it limits the bucco-pharyngeal area or membrane in front and 
at the sides (Figs. 15 and 18). The mesoderm of the pericardial area is continuous laterally 
with the general mesoderm of the embryonic area, but in those mammals in which a 
pro-amnion is formed it is separated in front from the extra-embryonic mesoderm by the 
pro-amniotic area, whilst in other mammals also it remains quite separate from the extra- 
embryonic mesoderm in front though it is more closely related to it. 
With the formation of the coelom the mesoderm of the pericardial area is separated 
into an upper or somatic and a lower or splanchnic layer, and it is in the latter that 
the two small tubes which constitute the first blood-vessels of the body of the embryo 
appear. The two tubes, or primitive aortee, which run longitudinally and parallel to 
one another, apparently end at first blindly both in front and behind, but as development 
proceeds they extend backwards, one on each side of the bucco- pharyngeal membrane, 
beneath the protovertebral somites, to the caudal region, behind which they pass on to 
the walls of the yolk sac to join the blood-vessels of the vascular area ; before ending 
they give off branches to the allantois. In the human embryo the yolk sac is relatively 
small and unimportant, and accordingly the branches which go to the allantois, or rather 
to the chorion along the body stalk, appear to form the more direct posterior continuations 
of the primitive vessels. The anterior end of each primitive trunk grows outwards along 
the margin of the pericardial area to its lateral extremity, and is continued on to the 
yolk sac, where it also joins the vessels of the vascular area. 
During the formation and evolution of the headfold the pericardial area increases in 
size, its cavity enlarges, and both it and the bucco-pharyngeal area are reversed in oe 
(Figs. 16 and 25). Both these areas are carried forw ards somewhat with the headfold, 
which it is to be remembered the primitive foregut is included, but when the He diold iS 
completely formed the pericardial area lies ventral to the foregut, and its primitive upper 
somatic surface is now its lower or anterior surface ; the original lower or splanchnic surface 
similarly is superior or dorsal, whilst what was, at first, the anterior border of the peri- 
cardial area is converted into ae posterior end of the reversed area, and it forms the anterior 
limit of the umbilical orifice. In brief, the relative positions of its several parts are 
is, 
