74 



protuberances D, D, by which the head is connected with the atlas, or 

 .upper, or first vertebra, or bone of the neck, and these are called the condy- 

 loid, cup-shaped, processes of the occipital bone. All the motions of the 

 head are partly and many of them wholly performed by this joint. 



Between them is a large hole, called the forameii magnum, or great 

 aperture, E, through which the continuation of the brain, called the spinal 

 cord or marrow, passes out of the skull. 



As an additional contrivance to support the great weight of the head, 

 are two other projections of the occipital bone, peculiar to animals whose 

 heads are set on in a slanting direction, and into which powerful muscles 

 are inserted ; they are called the coracoid, beak-like, processes or prolon- 

 gations F, F, of the occipital bone. 



Running forward, and forming outwardly a part of the base, and in- 

 wardly a portion of the floor of the skull, is what from its wedge-like shape 

 is called the cuneifor7Ji process of the occipital bone (fig. i, p. 68). It is 

 thick, strong, and solid ; and placed at the bottom of the skull, not only to 

 be a proper foundation for, and to give additional strength to the arch on 

 each side, but speedily to break and stop all vibration and concussion. 



At the base of the skull, and anterior to, or below the occipital, lies the 

 sphenoid, wedge-like bone (fig. k, p. 68). Its body, likewise called the cunei- 

 form, or wedge-shaped process, is a continuation of the same process of the 

 occipital, and like it is thick and solid, and for the same important purpose. 

 This bone branches out into four irregular bodies or plates, two of which 

 are called the wings, and two running to the palate, the legs. They could 

 not be represented in the cut, and there is nothing important belonging to 

 them so far as our work is concerned. Internally (fig. k), the sphenoid 

 forms a portion of the cavity of the skull. 



Of the ethmoid, sieve-like, bone little can be seen outwardly. A small 

 portion is found in the back part of the orbit, and in the cavity of the cra- 

 nium ; but the most important part of it is that which is composed of a 

 great number of thin plates, forming numerous cavities or cells (fig. I, p. 68) 

 lined with the membrane of the nose, and entering into the cavity of the nose. 

 The upper portion is called the cribriform, or sieve-shaped plate, from its 

 being perforated by a multitude of little holes, through which the nerve 

 connected with smelling passes and spreads over the nose. 



Altogether these bones form a cavity of an irregular oval shape, but 

 the tentorium penetrating into it, gives it the appearance of being divided 

 into two (rf, p. 68). 



The cavity of the cranium may be said to be arched all round. The 

 builder knows the strength which is connected with the form of the arch. 



