. Animal life as affected by the natural conditions of existence. Animal ecology. BESPIEATION IN AIE. 179 membrane of the lungs into the blood; this oxygenated blood is then carried to all the organs, of which the living portions take up the oxygen from it, precisely as the corresponding parts of insects take it up diiectly from the air by means of the trachete. In all animals that breathe thus through the lungs, there is a strongly marked contrast'" in the blood contained in different parts of the vascular system. That which is carried back from the lungs to the heart is rich in oxygen a
. Animal life as affected by the natural conditions of existence. Animal ecology. BESPIEATION IN AIE. 179 membrane of the lungs into the blood; this oxygenated blood is then carried to all the organs, of which the living portions take up the oxygen from it, precisely as the corresponding parts of insects take it up diiectly from the air by means of the trachete. In all animals that breathe thus through the lungs, there is a strongly marked contrast'" in the blood contained in different parts of the vascular system. That which is carried back from the lungs to the heart is rich in oxygen and known as arterial blood, and that which circulates in the vessels which convey it from the organs to the heart, or from the heart to the lungs, is poor in oxygen, and is called venous blood. We need not, however, in this place, investigate more closely the relations of the vascular system to the respiratory organs,. Fio. 51.—Section of the lung of the embryo of the Pig, showing the spongy texture. nor the physiological distinctions which are based on the dif- ferent organs of respiration and their structure. On the other hand, it is essential that we should in the first instance deter- mine which of the constituents of the air are advantageous or injurious to animal life. Air contains, when it is pure, almost 21 per cent, of oxygen, with about 79 per cent, of nitrogen, and a variable trace of carbonic acid, besides water which it holds in solution in the form of vapour in a quantity varying according to the tempera- ture. All the other kinds of gas which are occasionally present in the atmosphere are of no importance. They are either irre- spirable or actually injurious, while the above-mentioned mix- ture is the normal one, and thus is the most favourable for animal life. Certainly we must make this statement with. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these
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