BOOK THE THIRD.
Chapter I.
1. We have treated of the other internal parts of animals, their number, their nature and varieties. It now remains for us to speak of the organs of generation. In females these are always internal; but there is much difference in males, for some sanguineous animals have no testicles at all, in others they are internal; and in some animals with internal testicles, they are placed near the kidneys, in others near the abdomen; in other animals they are external. The penis of these last is sometimes united to the abdomen, in others it is loose as well as the testicles; but in promingent and retromingent animals it is suspended from the abdomen in a different manner. Neither fish nor any other animal with gills, nor the whole class of serpents, have testicles; neither has any apodal animal which is not internally viviparous.
2. Birds have testicles, but they are internal and near the loins, and so have oviparous quadrupeds, as the lizard, tortoise, and crocodile, and among viviparous animals, the hedgehog. In some viviparous animals they are situated internally upon the abdomen, as the dolphin among apodal creatures, and the elephant among quadrupeds. In other animals the testicles are external. It has been previously observed, that the manner and position of their junction with the abdomen is various, for in some they are joined on and do not hang down, as in swine, in others they hang down as in man.
3. It has also been observed that neither fishes nor serpents have testicles, but they have two passages hanging down on each side of the spine from the diaphragm, and these unite in one passage above the anus, by above, we mean nearer the spinal column. At the season of coition these passages are full of semen, which exudes on pressure; the differences among these may be seen by dissection, and in another place they will each be considered more particularly.
4. All oviparous animals, whether bipeds or quadrupeds, have their testicles placed in the loins below the diaphragm, some of a white colour, others ochreous, but in all surrounded with small veins; from each of these a passage is produced, which afterwards become united in one, and, as in fish, open near the anus. This is the penis, which is inconspicuous in small animals; but in the larger, as the goose and such like, it becomes more conspicuous immediately after coition.
5. And these passages, both in fish and other animals, are joined to the loins below the stomach and between the entrails and the great vein, from which passages proceed to each of the kidneys; and, as in fish, the semen may be seen entering them at the period of coition, when these passages become very conspicuous, but when this season is passed the passages again become invisible. So also the testicles of birds are either small or entirely invisible when not excited, but when urged by desire they become very large; this is so remarkable in pigeons and partridges, that some persons have supposed that they had no testicles during winter.
6. In some of those animals in which the testicles are placed forwards, they are internal and upon the abdomen, as in the dolphin; in others they are externally conspicuous upon the extremity of the abdomen. These animals are similar in other respects, but differ in this, for in some the testicles are uncovered, and others that have external testes they are placed in a scrotum.
7. This is the nature of the testicles of all viviparous animals with feet: from the aorta, passages like veins proceed to the head of each testicle, and two others from the kidneys, these last are full of blood, but those from the aorta contain no blood. From the head of each testicle to the testicle itself, there proceeds a thicker and more muscular passage, which is in each testicle reflected back to the head of the testicle, and from this point they again unite upon the penis towards the fore-part of it.