266 THE HORSE. 



CHAPTER XV. 



OF THE JOINTS AND MUSCLE. — THE TISSUES ENTERING INTO 

 THEIR COMPOSITION. 



Joints — Cartilage — Fibrous Tissue — Physiology of Muscle. 



THE JOINTS. 



The joints are all formed between two or more separate bones, 

 having a soft and elastic substance interposed, whose structure 

 varies with the amount of motion. Where this is extensive, as in 

 the joints of the limbs, the adjacent surfaces arc covered with a 

 peculiar kind of cartilage arranged in a thin and very smooth layer 

 upon them. In addition to this protection against friction and 

 vibration, the bones are firmly bound together by strong bands of 

 white fibrous inelastic tissue under the general name of ligaments, 

 each bundle receiving a distinct appellation. In those situations 

 where the motion is limited, a mixture of cartilage and fibrous 

 tissue is inserted between the ends of the bones and attached to 

 both, as in the vertebra?, ischio-pubic symphysis, &c. ; while in 

 order to reduce the vibration and friction in certain important 

 joints fibro-cartilages ai'e introduced, with both surfaces free, and 

 in contact only with the usual layer of cartilage, as in the stifle and 

 jaw. A lubricating fluid (called synovia) is required to reduce 

 the amount of friction ; and to produce it, as well as to keep it 

 within proper limits, a membrane (synovial) is developed. This 

 is attached to each bone in a peculiar manner, to be presently de- 

 scribed. Lastly, an elastic fibrous tissue (yellow) is met with in 

 certain situations, the most remarkable being the great ligament 

 of the neck. 



CARTILAGE. 



True CARTILAGE (which is familiarly known to all when it shows 

 the large white masses in a breast of veal, as dressed for the table) 

 is a homogeneous, white, semi-transparent substance, possessing a 

 certain amount of elasticity, and easily cut with a knife. In the 

 early embryo it exists as the sole foundation of the skeleton, bone 

 being afterwards deposited in its meshes and finally substituted for 

 it. This is called temporary cartilage. In alter life it invests 

 those parts of the bones which enter into the composition of the 

 joints (articular cartilage, which is what we arc considering just 

 now), and also forms the costal cartilages, the ensiform and carini- 

 form cartilages, and those of the larynx, trachea, and nose. Re- 

 ticular or membraniform cartilage, differing slightly from true 

 cartilage, is met with in the Eustachian tube, the external ear, 

 and the epiglottis 



