64 THE ANATOMY OF THE HORSE. 



womb. As these separate portions are originally soft, and 

 upon even slight pressure can be made to overlap. Within 

 the skull is contained the brain, and to hold this important 

 organ safely, seems to be its principal use. The hair and 

 skin covers the outside of the head ; next a layer, deep and 

 broad, of muscle laps over the sides of the skull, the bones 

 of which there are double, each consisting of two layers ; an 

 outer layer, which is soft and spongy, like common bone, 

 and an inner layer, which is hard and smooth, like ivory or 

 glass. There is an imperfect cavity between these two 

 layers, termed diploe, and then, internally, come three 

 membranes and some fluid before the brain is reached ; so 

 the reader will perceive how thoroughly this organ (the 

 brain) is protected within the skull. 



Behind the skull will be observed a large opening {Plate 

 VI. n), which is for the passage of the medulla oblongata, or 

 substance which unites the spinal marrow and the brain. On 

 every side of the skull will be perceived openings, some very 

 small and others of considerable size ; the larger are to admit 

 of the passage of veins, which carry blood from the brain ; 

 the smaller ones are for the entrance of arteries, which carry 

 blood to the brain, and to give egress to the nerves, which 

 bestow sense or motion on the whole head, and also upon 

 some other parts of the body. 



Yet, as the division of the skull gives attachment to vari- 

 ous muscles, it may be as well to mention the more promi- 

 nent of their peculiarities, for the sake of a better under- 

 standing hereafter. 



The cranium is said to be composed of twelve bones, four 

 pairs, which make eight bones, and four single bones, which 

 complete the twelve. The pairs are the squamous and the 

 petrous temporal, the parietal, and the frontal. The single 

 bones are the os triquatrum, the ethmoid, the sphenoid, and 

 the occipital. 



The OCCIPITAL BONE (single) is situated partly at the 

 bottom, and directly at the back of the skull. It has two 

 surfaces, an external and an internal. The external we will 

 consider first. Right at the back of the head is the large 

 hole of the skull, called the foramen magnum {Plate VI. n). 

 Projecting from the bottom of the foramen magnum, so as 

 to form part of the base of the cranium, is the basilar 



