The main distinction between vertebrate and invertebrate animals lies in the fact that the former always possess a skull and a vertebral column, and with them a brain and spinal cord.
The brain and nervous system are constructed on a very similar plan in all vertebrate animals, although they reach their highest perfection in man. The weight of the human brain averages from 3 lbs. to 3.5 lbs.; it weighs absolutely more than that of any other animal except the elephant and the whale. The elephant"s brain weighs from 8 to 10 lbs.; that of the whale about 5 lbs.; and that of the horse only about 19 ounces. When we compare the relative sizes of these creatures, we see at once the immense superiority of man over them all. Birds have smaller brains than mammals, and reptiles smaller still, while fishes take the lowest place in this respect among the vertebrate animals.
The general plan of structure, as to the trunk and the organs lodged in it, is very similar in all mammals and birds. There is an upper chamber (the thorax), and a lower one (the abdomen), the two being separated by a partition known as the diaphragm. The heart and lungs are lodgedin the upper chamber, or thorax. The heart is the center of the circulator y system; the lungs do the work of carrying off carbonic acid, and supplying the blood with fresh oxygen from the air which is breathed in. The mouth and nostrils do the double work of taking in fresh air and breathing out carbonic acid gas.
In mammals and birds the heart always contains four chambers: the auricle and ventricle on the right side contain impure venous blood; the corresponding chambers on the left side contain the purified blood which has been sent back from the lungs.
The lung-breathing in all these animals supplies the blood with abundance of oxygen, and their bodies are warm because of the burning which this large amount of oxygen produces.
In birds the lungs themselves are smaller than in mammals, but breathing goes on not only through these organs, but through all parts of the body, so that air penetrates everywhere. Even the hollow bones are filled with air, and there are air cells in the cavity of the chest.
In reptiles the heart has only three chambers instead of four, and the lungs are small. Very little oxygen is taken in by these lungs; consequently there is not sufficient burning to keep up a warm temperature, and the body is cold.
The two auricles of the heart open into a common ventricle. The ventricle has to receive the venous blood from allparts of the body, as well as the purified blood sent back from the lungs. Instead, therefore, of sending out through the arteries pure blood, well supplied with oxygen, it sends out this mixture of the two.
Fishes differ from all other vertebrate animals in having gills instead of lungs. The heart has only two chambers, a single auricle and ventricle. The veins bring the blood to the auricle, and, after passing through the ventricle, it is sent out to the gills, and thence into all parts of the body. A very small amount of oxygen is taken in from the water in this way ; consequently the oxidation is not sufficient to warm the body. Fishes, like reptiles, are cold-bloodedanimals.
Lesson 60
