7. In oviparous animals, such as birds and the oviparous quadrupeds, the parts are very widely separated, so that they appear to have two lungs; they are, however, only two divisions of the trachea extending to each side of the lungs; the trachea is also united with the great vein and with the part called the aorta. When the trachea is filled with air, it distributes the breath into the cavities of the lungs, which have cartilaginous interstices ending in a point; the passages of these interstices go through the whole lungs, always dividing from greater into less. 8. The heart is connected with the trachea by fatty and cartilaginous muscular bands. There is a cavity near the junction, and in some animals, when the trachea is filled with breath, this cavity is not always distinguishable, but in larger animals it is evident that the breath enters it. This then is the form of the trachea, which only inhales and exhales breath, and nothing else either dry or moist, or it suffers pain till that which has passed down is coughed up. 9. The œsophagus is joined to the mouth from above, near the trachea, being united both to the spine and the trachea by membranaceous ligaments. It passes through the diaphragm into the cavity of the stomach, is fleshy in its nature, and is extensible both in length and breadth. The human stomach is like that of a dog, not a great deal larger than the entrail, but like a wide bowel; after this there is an entrail simply rolled together, then an entrail of moderate width. The lower part of the abdomen is like that of a hog, for it is wide, and from this to the seat it is short and thick. 10. The omentum is united to the abdomen in the middle, and is in its nature a fatty membrane, as in other animals with a single stomach and teeth in both jaws. The mesenterium is over the bowels; it is membranaceous, broad, and fat; it is united to the great vein and the aorta: through it extend many numerous veins at its junction with the intestines, reaching from above downwards. This is the nature of the œsophagus, trachea, and the parts of the abdominal cavity. CHAPTER XIV. 1. The heart has three cavities: it lies above the lungs, near the division of the trachea. It has a fat and thick membrane, by which it is united to the great vein and the aorta, and it lies upon the aorta near the apex; and the apex is placed in the same situation in all animals which have a chest; and in all animals, whether they have or have not a chest, the apex of the heart is forwards, though it often escapes notice by the change of position in the parts when dissected. The gibbous portion of the heart is upwards; its apex is generally fleshy and thick, and there is a sinew in the cavities.