60 THE ANATOMY OF THE HORSE. 



means of the absorbents, which remove it, and the arteries 

 which replace it. Its quantity also depends on the wants 

 of the animal and the stimulus applied : in the full -bred 

 horse, therefore, the bones will be found more solid than in 

 the bulky lower-bred varieties. Most cylindrical bones are 

 hollow, and are lined by a membrane, called the medullary, 

 or membrane of the marrow : the bones have also an exter- 

 nal periosteum, or outer covering. The medullary cavities 

 have the effect of diminishing the weight of bones without 

 weakening them, and at the same time are of service in sup- 

 plying cavities where nature can store away her superabun- 

 dant fat for a time of need. Bones, though furnished with 

 blood-vessels and nerves, have in health little sensibility, but 

 during disease they inflame, and then become acutely sensi- 

 tive. The varieties in the form of bones have occasioned 

 their division into cylindrical, fiat, short or long, and irre- 

 gular. They are furnished with cavities and eminences : 

 the cavities, in all excepting long bones, are small, and 

 lined with a medullary membrane, which secretes only in 

 these places a reddish fluid ; they have, also, pits, furrows, 

 notches, holes, sinuses, &c. ; the eminences are long and 

 short, to which tendons are attached through which the 

 muscles move the bones as by means of levers. Bones arti- 

 culate with each other by joints ; either like to a cup and 

 ball ; held loosely together by ligaments as a hinge ; bound 

 together more tightly by cartilage and ligament ; inter- 

 mingled, or fixed closely together by notches. 



DESCRIPTION OF PLATE I.— THE SKELETON. 



THE HEAD. 



A, The skull, face, and upper jaw, in one piece ; B, the 

 lower jaw ; a, the incisor teeth ; b, the tushes ; c, the 

 molares, or grinders ; d, the peak formed by the extremities 

 of the nasal bones ; e, the zygomatic spine, to the bottom 

 of which the masseter esternus takes its origin; /, the orbit; 

 g, the cavity above the orbital arch ; h, the pole ; i, the 

 zygomatic arch ; j, j, the styloid processes for the attach- 

 ment of the muscles ; k, the joint formed by the upper and 

 lower jaws ; /, the meatus auditorius, or opening to the 

 internal ear. 



