1008 VASCULAR SVSTE Mh 
and it is conceivable that they may be induced in more than 
one way.! 
VASCULAR SYSTEM, the comprehensive term for the vessels 
conducting the blood and the lymph, and composed of (A) the 
HEART (p. 413), ARTERIES (p. 22) and VEINS, and (B) the 
Lymphatic Vessels. The walls of the blood-vessels have three 
layers, of which the outermost consists wholly of connective tissue, 
the middle one is made of annular unstriped muscular fibres, inter- 
woven with elastic bands, and is the thickest, while the inmost is a 
thin endothelial lining, forming the valves which are so arranged 
as to hinder the reflux of the blood. In considering the Vascular 
System it is convenient to divide it as follows :— 
A. Blood-System, consisting of— — 
I. The Pulmonary Circulation, or that of the LUNGS (p. 522), The 
right and left pulmonary Arteries arise with a short common stem, 
guarded at its base by three valves, from the right ventricle of the 
Heart, and each accompanied by its BRONCHUS (p. 58) enters the 
Lung of its own side, there breaking up into capillary vessels, which 
again combine and convey the arterialized blood into the right and 
left pulmonary veins respectively. These ultimately, as the vena 
pulmonalis conmunis, enter the left atrium. 
Il. The Systemic Circulation, or that of the body, divisible into 
Arterial and Venous. 
i. Arterial. The left ventricle sends all its blood into the 
truncus aortx, the base of which is guarded by 3 valvulx semilunares. 
The trunk is very short, sending off the right and left coronary 
arteries to nourish the heart itself, and then the left arteria anonyma 
or brachio-cephalica, while the rest of it, considerably thicker, divides 
into the right a. anonyma and the arcus ascendens aortx. 'The latter, 
situated between the TRACHEA and the right lung, runs headward 
and over the right bronchus, reaching the ventral surface of the 
as coming under this term or ‘‘ Erythrochroism” (Stejneger, tom. cit. p. 8); but 
it seems to me due to a cause of quite another kind than that which produces 
the change from normal yellow or orange, let alone green, to some shade of 
scarlet, and should therefore bear another name. Real ‘‘ Erythrism” is not 
common in species of the Holarctic Fauna. The Crosspriu (p. 114) is partly 
subject to it, and then has been regarded as specifically distinct under the name 
of Loxia rubrifasciata, and the Green Woodpecker, Gecinws viridis, has heen 
known to exhibit it (Zool. 1853, p. 3800; 1854, p. 4250); but perhaps the 
most abnormal case on record, if it belong to this category, is that of the 
pink-headed Pastor roscus described by Dr. A. B. Meyer (Proc. Zool. Soc. 1890, 
p- 590). 
1 For its bearings, more or less direct, on Variation Mr. Keeler’s Essay on the 
Evolution of the Colors of North American Land Birds (published as No. III. of 
the ‘‘ Occasional Papers of the Californian Academy of Science.” San Francisco: 
1893) should also be consulted. 
