Subsection 3. Uniform parts.

The containing parts, being more solid in substance, are either homogeneous or heterogeneous, uniform or non-uniform. Aristotle divides them in this way in his History of Animals, book 1, chapter 1, as does Laurentius in book 1, chapter 20. Uniform, or homogeneous, parts are those which, when divided, yield parts of the same nature, as water divides into water. Some of these derive from seed; others are fleshy.

Parts derived from seed are those generated directly from it: bones, cartilage, ligaments, membranes, nerves, arteries, veins, skin, fibres or strands, and fat.

Bones are dry and hard, generated from the thickest part of the seed to strengthen and support the other parts. Some say there are 304 in the human body, others 307 or 313. They contain no nerves and therefore have no sensation.

Cartilage is a flexible substance, softer than bone and harder than the other tissues, which supports the parts used in movement. Ligaments bind the bones together and other parts to the bones, with the tendons that assist them. The function of membranes is to cover the other parts.

Nerves, or sinews, have membranes on the outside and are full of marrow within. They arise from the brain and carry the animal spirits responsible for sensation and movement. Some are harder, some softer. The softer ones serve the senses, and there are seven pairs. The first are the optic nerves, by which we see; the second move the eyes; the third enable the tongue to taste; the fourth serve taste in the palate; the fifth belong to the ears; the sixth pair is very extensive and runs through almost all the internal organs; the seventh moves the tongue.

The harder nerves serve to move the inner parts and arise from the spinal cord. There are thirty pairs of these: seven in the neck, twelve in the chest, and so on.

Arteries are long and hollow, with a double covering, and convey the vital spirits. To observe these more clearly, they say, the anatomist Vesalius used to dissect living people. Arteries arise from the left side of the heart, and there are two principal ones from which the rest branch: the aorta and the venosa. The aorta is the root of all the others that serve the whole body; the other goes to the lungs to draw in air and cool the heart.

Veins are hollow and round like pipes. Arising from the liver and carrying blood and natural spirits, they nourish all the parts. There are two principal veins, the portal vein and the vena cava, from which the rest branch. The portal vein comes from the concave surface of the liver and receives the mesenteric veins, through which it takes chyle from the stomach and intestines and conveys it to the liver.

The other carries blood from the liver to nourish all the parts distributed throughout the body. The branches of the portal vein are the mesenteric and haemorrhoidal veins. The branches of the vena cava are internal or external: internally, the seminal or renal veins; externally, those in the head, arms, feet and so on, which have various names.

Fibres are white, solid strands distributed throughout a bodily part. They run straight, obliquely or across, each direction having its own use. Fat is a uniform part, moist and bloodless, made from the thickest and oiliest matter of the blood. The skin covers the other parts and has a cuticle, or little skin, beneath it. Flesh is soft and reddish, formed from congealed blood, and so on.