Some of the common malformations of the body appear to reproduce a condition which is only seen in fishes. The deformity known as " hare-lip " is a case in point. The name, however, is a misnomer, for the cleft in the upper lip of the hare is situated below the septum of the nose and separates the lip into a right and left half. The cleft, however, which is apt to appear in the human upper lip occurs to the right or to the left of the middle line ; the cleft may be double, so that the lip is divided into a middle part attached to the septum of the nose and two larger lateral parts continuous with the cheeks. The explanation is found in the manner in which the upper lip is formed in the foetus ; it is produced by the fusion of three parts. If these fail to unite, the condition of double hare lip is produced, leaving a cleft or groove between the nostrils and the mouth. Now the only adult vertebrate animals in which such a condition is seen to persist is the group of fishes to which the sharks and rays belong. In " hare-lip," then, we appear to see the reproduction of a condition known to occur only in gill-breathing vertebrates.
Cleft palate is often combined with hare lip, but it represents an arrest at a later stage of evolution. A complete palate, such as is seen in man, occurs only in mammals. The palate has been evolved in connexion with mastication; it separates the respiratory passage of the nose from the mouth, so that the animal, as it masticates a mouthful of food, may continue to breathe freely. While the parts of the upper lip are united before the end of the second month of development, the right and left outgrowths which meet along the roof of the mouth and form the palate are not completely joined until the end of the third month. In cases of cleft palate the process of fusion does not take place. Another use of the palate is seen at birth. It it is cleft, the child has a difficulty in sucking and swallowing, for the milk regurgitates through the nose. In amphibia, reptiles and birds, the three parts representing the upper lip unite, but an open cleft remains between the mouth and the nose. In a child suffering from cleft palate we see reproduced a reptilian condition, and have at the same time an illustration of the uses of the palate.
Organs are often arrested in their development on the point of passing from one stage to another. Children are occasionally born with malformed hearts, which illustrate the truth of this statement. In gill-breathing animals, the chief function of the heart is to pump the blood to the gills ; after leaving the gills the oxygenated blood enters the arteries and is circulated to the tissues of the body. When gills are replaced by lungs, the heart becomes changed in form and in function. Its main duty is then to pump blood to the body, only a part of the heart—the right ventricle—being set aside to supply blood to the lungs. Now, at the time when the gill clefts appear in the neck of the human embryo, the heart is like that of a fish. It is composed of four chambers, the one leading to the other. The first of these is named the sinus venosus ; it receives the blood from the body and passes it on to the next chamber, the auricle. It in turn sends the blood to the chief chamber—the ventricle. There is a fourth chamber—the bulbus cordis—situated between the ventricle and the aorta. The aorta carries the blood to the gill arches. When the gill arches begin to disappear in the human embryo and the rudiments of the lungs are formed, we see the heart, still beating, for it starts to beat very early, begin to undergo a series of changes. The first chamber— the sinus—becomes incorporated in the right side of the auricle, while the fourth chamber, or bulbus, sinks within and forms part of the ventricle. While the sinus and bulbus are being included, developmental changes are seen to be taking place in the auricle and ventricle, which lead to a division of each of these into right and left cavities. The right chambers receive the venous blood and pass it onwards to the lungs and placenta, while the left receive the blood from the lungs and placenta and supply it to the body. The commonest malformation of the human heart is due to an arrest of development during the passage from the gill breathing to the lung breathing stage. In the malformed hearts of children one usually finds that the fourth chamber, or bulbus, has been incompletely incorporated in the right ventricle, and that it is so small that the blood can scarcely pass into the pulmonary artery and thus reach the lungs. Indeed, such children would die from suffocation at birth were it not that the septum between the ventricles is incomplete, so that the impure blood can pass from the right ventricle to the left, and be pumped with the pure blood to the body. Some of it, by an indirect channel, reaches the lungs, where it is oxygenated, and thus the child is prevented from dying of asphyxia. In some cases, there is only a common ventricle as in the fishes and amphibia, the septum, or partition which separates the original ventricle into right and left chambers, having failed to form. The rapid transformation of the heart of the embryo from a simple pump as in fishes into the double-chambered heart of the mammal is very wonderful, and yet not more so than the transformation which produces a butterfly from a caterpillar.
We see a rigid economy being practised in the human embryo, as the branchial or gill stage is replaced by a pulmonary system. There are altogether six pairs of gill arches ; into each of these the aorta sends a vessel, or stem. When the branchial arches disappear, the aortic branches within some of them are utilized for other purposes. Those in the third pair of arches form part of the two internal carotid arteries; the vessel of the fourth arch on the left side forms the bend of the aorta, while the sixth one of the left side has a most remarkable history (see Fig. 2). Part of it becomes the stem of the pulmonary artery—the great vessel to the lungs—while the other part remains open only during fcetal life. This latter part, which is known as the ductus arteriosus, allows the blood to pass from the pulmonary artery to the aorta (Fig. 2). The need for such a channel in the foetus will be very apparent to the reader, when he remembers that the lungs come into use only at birth. Before then the placenta serves the purposes of respiration. The presence of a communication between the pulmonary artery and aorta allows the right ventricle to pump the impure blood into the aorta and thus to the placenta, instead of to the lungs. At birth the ductus begins to close and is soon occluded, so that the blood can no longer enter the aorta but must pass to the lungs (Fig. 2). We see, then, a part of the artery of a gill arch being employed to secure a placental respiration in the foetus, and then suddenly closed up to bring about a pulmonary respiration in the newly born child.