THE SENSORIAL FUNCTION. 117 



when that force is removed. It sustains perfectly the weight of the head. 

 That portion of tenacity or strength is given to it which will not give way to the 

 simple weight of the head, but which will yield to a very little additional 

 weight. Its resisting power is so admirably adjusted to that which it has to 

 sustain, that when certain muscles, whose action is to depress or lower the head, 

 begin to act, and add their power to the previous weight it had to bear, the 

 ligament stretches, and when the horse is browsing it is full two inches longer 

 than when the head is erect. 



When the animal has satisfied himself, these depressing muscles cease to act, 

 and other muscles which are designed to assist in raising the head, begin to 

 exert themselves ; and by their aid but more by the inherent elasticity of the 

 ligament the head is once more elevated, and remains so without the slightest 

 exertion of muscular power. This is one of the many applications of the prin- 

 ciple of elasticity which will be discovered and admired in the construction of 

 the animal frame. 



The ligament of the neck is inserted into the centre of the back part of the 

 occipital bone, and immediately below the vertex or crest of that bone ; and 

 therefore the bone is so thick ar this part (see fig. e, p. 111). 



Many large and powerful muscles are necessary to turn the head in various 

 directions, as well as to assist in raising it when depressed. The occipital bone, 

 as will be seen in the cut, presents a spine running down the centre, B, and a 

 large roughened surface for the attachment of these muscles C, C. 



Lower down, and still at the back of the 

 occipital bone, are two rounded 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 condyloid, 

 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, the foramen 

 magnum, or great aperture, E, through which 

 the continuation of the brain, termed the 

 spinal cord or marrow, passes out of the skull. 



As an additional contrivance to support the 

 enormous weight of the head, are two other projections of the occipital bone, pe- 

 culiar 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 prolongations, F, F, of the occipital bone. 



Running forward, and forming outwardly a part of the base, and inwardly 

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

 the cuneiform process of the occipital bone (fig. i, p. 111). It is thick, strong, 

 and solid, and placed at the bottom of the skull, not only to be a proper founda- 

 tion for, and to give additional strength to, the arch on either side, but speedily 

 to 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. 111). 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 this work is concerned. Internally (fig. k), the sphenoid forms a portion of 

 the cavitv of the skull. 



