Chapter XIII.

1. The external parts of the body are arranged in this manner; and, as I have said, are for the most part named and known from habit. But the internal parts are not so well known, and those of the human body are the least known. So that in order to explain them we must compare them with the same parts of those animals which are most nearly allied.

2. First of all, the brain is placed in the fore-part of the head, and it occupies the same position in all animals that have this part, which belongs to all sanguineous and cephalopodous animals. In proportion to his size, man has the largest brain of all animals, and the moistest. Two membranes enclose the brain: that outside the skull is the strongest; the inner membrane is slighter than the outer one. In all animals the brain is in two portions. The cerebellum is placed upon the brain at its lowest extremity. It is different from the brain both to the touch and in appearance.

3. The back of the head is empty and hollow in all animals in proportion to their size, for some have a large head, but the part lying under the face is less in those animals which have round faces; others have a small head and large jaws, as the whole tribe of Lophuri. In all animals the brain is without blood, nor does it contain any veins, and it is naturally cold to the touch. The greater number of animals have a small cavity in the centre of the brain. And round this a membrane filled with veins: this membrane is like skin, and encloses the brain. Above the brain is the smoothest and weakest bone in the head—it is called sinciput.

4. Three passages lead from the eye to the brain; the largest and the middle-sized to the cerebellum, the least to the brain itself. The least is that which is nearest the nostril; the greater are parallel, and do not meet; but the middle-sized passages meet: this is most evident in fishes, and these passages are nearer to the brain than the larger, but the least separate from each other, and do not meet.

5. Within the neck is the œsophagus, which also derives its additional name, the isthmus, from its length and narrowness, and the trachea. The trachea lies in front of the œsophagus in all animals which possess this part, that is, all animals which breathe from the lungs. The trachea is cartilaginous in its nature, and contains but little blood: it is surrounded with many smooth rings of cartilage, and it lies upon the upper part towards the mouth, opposite the passage from the nostril to the mouth, wherefore, also, if any liquid is drawn into it in drinking, it passes out of the mouth through the nostrils.

6. Between the passages is the epiglottis, which can be folded over the passage which extends from the trachea to the mouth; by the epiglottis the passage of the tongue is closed, at the other extremity the trachea reaches to the middle of the lungs, and afterwards divides to each side of the lungs. For the lung is double in all animals which possess this part, though the division is not so marked in viviparous animals, and least of all in man. The human lungs are anomalous, neither being divided into many lobes, as in other animals, nor being smooth.

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.