ioo8 VASCULAR SYSTEM 



and it is conceivable that tliey 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 Avhich 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 communis, enter the left atrium. 



II. 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 amise, the base of which is guarded by 3 valvule 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 aortse. 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 " Erythrocliroism " (Stejneger, torn. 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 Crossbill (p. 114) is partly 

 subject to it, and then has been regarded as specifically distinct under the name 

 of Loxia ruhrifasciata, and the Green Woodpecker, Gecinus viridis, has l)een 

 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). 



^ 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. Ill, of 

 the " Occasional Papers of the Californian Academy of Science." San Francisco : 

 1893) should also be consulted. 



