The _fourth_ part is an external membrane called _almochatim_ [Arabian term for cranial periosteum] which, when the skin is raised, appears to be continuous and covers the whole cranium. And nature made this membrane firstly so that the skin which is soft should not come into contact with the hard bone, secondly that the bone of the head should have sensation through it, and thirdly that the internal membrane of the head, called _dura mater_, should, by means of this membrane, be attached to the bone of the cranium by certain nerves and ligaments. These, issuing through the commissures of the bones, have thus their origin in the aforesaid internal membrane, while on emerging through the bone, they weave themselves into or rather compose the external membrane called _almochatim_. The _fifth_ part is the _skull_. This is a bone like a cap, inside the cavity of which is located the brain. In the skull are four bones sutured together. Nature made the skull not of one but of many pieces, firstly, so that if harm should fall on one part it might not spread to the others; secondly, so that by their joints or rather sutures [Italian _cusiture_=sewings], the humours of the brain might be the better exhaled; and thirdly, so that when there is need of applying medicines, these might the better penetrate to the parts within. Hence it is that four pieces of bone are sutured and joined together by nature in a denticulate fashion, so that they might be the firmer and stronger. Nor are they bound with ligaments as are the joints, for these would not have been so strong, and furthermore the bones of the head do not need to move. These sutures are five in number, three being true and two false. The true sutures are those which pass right through the bone, while the false do not. Of the true sutures one is in the anterior part and is called _coronal_; it is made like the letter C, and stretches from right to left of the head, the two wings of the C being directed towards the forehead. The second true suture extends along the length of the head, beginning from the coronal and reaching the back part of the head. It is like a shaft or rather arrow that goes backwards from the brow, wherefore it is called _sagittal_ ----(. The third true suture is in the posterior part and is called laudal, for it is made like a Λ, the letter called by the Greeks _lauda_. The sagittal suture extends from the coronal to the lauda ❯----(. The false sutures are two, one on each side. They are called _cortical_ because they do not penetrate. Now if we consider these five sutures we shall see that there are four bones articulated together. One is the forehead bone [frontal] which begins at the coronal and ends below at another suture, which itself begins as a branch of the coronal suture and proceeds by way of the eyebrow to the corresponding branch [of the other side] Ɑ.