THE SENSORIAL FUNCTION 



77 



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 liead, begin to exert them- 

 selves ; 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 principle of elasticity which will 

 be discovered and admired in the construction of the animal frame. 



The lio-ament 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 at this part (see fig. e, p. 72). 



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

 tions, 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 occi- 

 pital 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 maiTow, passes out of the skull. 



As an additional contrivance to support the 

 enormous weight of the head, are two other pro- 

 jections 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 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. 72). 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 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. 72). Its body, likewise called the cuneiform 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 loings, 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 cavity of the skull. 



Of the ethmoid, sieve-like, hone, little can be seen outwardly. A small portion is 

 found in the back part of the orbit, and in the cavity of the cranium ; but the most 

 important part of it is that which is composed of a great number of thin plates, form- 

 ing numerous cavities or cells (fig. /, p. 72), lined with the membrane of the nose, 

 and entering into its cavity. 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 {d, p. 72). 



The cavity of the skull may be said to be arched all round. The builder knows 

 the strength which is connected with the form of an arch. If properly constructed, it 

 isi e<i^ual to a solid mass of masonry. The arch of the horse's skull has not much 

 7* 



