THE VASCULAR SYSTEM. 89 
capillaries with the heart. The control of the muscular action of 
the heart and arteries through the vagus and sympathetic nerves is 
an important element in maintaining pressure and tone in the 
vascular system (cf. p. 62). 
Many of the peculiar features of the mammalian circulation 
which at first sight do not appear to be general, but are so in 
reality, depend on the circumstance that the complete partition 
of the organs is a final stage of a general progressive development, 
observable in air-breathing vertebrates, in which the lungs and their 
vascular connections becom? per- 
fected for pulmonary respiration. 
On the other hand, the vascular 
system as it appears in the embryo, 
more especially its aortic portion, 
is arranged according to the type 
of branchial respiration as found 
in fishes. In this condition the 
blood is sent forward from the 
heart through a ventral aorta. The 
latter is connected with a series of 
paired branchial aortic arches, 
traversing the rudimentary gill struc- 
tures, and thus passing upward 

aroun the sides f Fic. 47. Planofthe branchial aortic 
PRIMITIVE th v d it d I Q arches. ‘ne adult mammalian vessels 
€ rimitive la- are indicated in black (systemic) or 
AORTIC ARCHES. P Pp shaded (pulmonary). 1-6, primary 
rymx. The dorsal arches; ao., aorta; a.p., pulmonary 
. 7 . artery; c.e., external carotid; c.i., in- 
aorta is formed by the junction of the ternal carotid; d.a., ductus arteriosus 
(Botalli); i., innominate artery; s.d., 
branchial aortic arches, and passes _ right subclavian; s.s., left subclavian. 
backward as a main distributing erie eat oe 
vessel on the ventral side of the axial support. The heart itself 
is formed primarily on a two-chambered plan, similar to that in 
fishes, where all the blood is received by single atrium and is 
delivered forward to the gills by a single ventricle. 
The definitive condition of the chief arterial vessels is arrived 
at by an extensive modification of the branchial plan. As indicated 
in the accompanying diagram (Fig. 47) the arch condition is retained 
by the aorta and by the pulmonary artery, and it is interesting to 
note also that the primary connections of these vessels, represented 
in the embryo by an open canal, the ductus arteriosus (Botallt), is 
