CHAP. IX] 



SURGICAL DISEASES AND CONFORMATION 



101 



stag, whose head is naturally heavy. When a 

 horse wishes to lower his head, the muscles in 

 the under portion of the neck draw the head 

 down, the superior muscles relaxing. 



399. The joints of the limbs are securely 

 formed by very powerful ligaments, with the 

 exception of the scapulo-humeral joint between 

 the shoulder-blade and arm, which is kept 

 together chiefly by muscle and by vacuum. 

 These ligaments consist of interosseous liga- 

 ments, which are inside the joints, and binding 

 ligaments, which are on the outside. There are 

 also annular ligaments that form loops to keep 

 the tendons in place, and also capsular liga- 

 ments that do not hold a joint together, but 

 merely keep the synovia, or joint oil, from run- 

 ning out of the joint. In a broken knee one or 

 more of the capsular ligaments of the carpal 

 joints may have become ruptured or torn, but, as 

 a rule, a bursa of the tendons has become broken. 



Tendons are strong, non-elastic, white, 

 fibrous cords that usually have an active 

 function ; they generally connect the distal end 

 or insertion of a muscle to a bone. A tendon 

 is like the wire of a railway signal, and the 

 bone the signal arm, the signal cabin being the 

 muscle, the electric wire leading to the cabin 

 being the nerve that runs to the muscle. 



400. Structure of the Bone. — Before passing 

 on to the detection of lameness it will be well 

 to give the reader some idea of the structure of 

 the bony frame. Bone is a bluish-pink, hard, 

 insensitive substance that gives attachment to 

 muscles, acts as pillars of support, encloses 

 cavities and protects vital organs, forms joints 

 and acts as levers. 



Diseased bone is highly sensitive ; dead bone 

 is yellowish-white in colour. Bone is composed 

 of two substances : animal matter, which is very 

 elastic, and mineral or earthy matter, which is 

 very brittle, but gives hardness and rigidity to 

 the bone. If bone is placed in diluted hydro- 

 chloric acid for some minutes the mineral matter 

 becomes dissolved out, leaving an elastic mass 

 of the same shape as the original bone. On the 

 other hand, if bone is burnt in the atmosphere 

 the elastic animal substance will become burnt, 

 leaving a brittle earthy mass the same shape as 

 the bone. 



In the embryonic state bone is composed 

 entirely of animal matter. As the animal 

 advances in age the percentage of earthy matter 

 increases until, at very old age, the bones become 

 quite brittle. At birth the foal has a consider- 

 able amount of earthy matter in its bones ; it 

 is able to stand on its legs without their show- 

 ing any sign of bending within half an hour of 

 birth. But in the case of the human there is not 

 sufficient earthy matter to enable walking for 

 one to two years. Children's legs become bowed 

 through using them before the bones have 

 sufficient earthy matter in them. 



In the adult horse (five to six years of age) 

 the average composition of bone is one-third 

 animal and two-thirds mineral or earthy matter. 

 The hardest bone in the body is the triangular- 

 shaped ear bone (petrosal), which has 93 Jj per 

 cent, mineral matter. The ribs are softer than 

 the limbs. A foal one year old has about 55 per 

 cent, animal matter and 45 per cent, mineral ; 

 at ten years it would have about 25 per cent, 

 animal and 75 per cent, mineral. When bones 

 are too soft and liable to bend too much, phos- 

 phates of lime and iron should be given in the 

 feed. 



401. Histology of Bone. — Bone is composed of 

 a hard outside layer of compact tissue and a soft 

 inner portion of cancellated tissue. Normally, 

 this latter is never found at the outside of the 

 bone. The thoroughbred, that has small limbs, 

 has a great deal of compact tissue in its bones, 

 to render them sufficiently strong, whilst the 

 heavy draught horse has a greater amount of 

 cancellated tissue. Compact tissue is composed 

 of minute honeycombs. Inside all long bones, 

 such as those of the limbs, is a hollow space, 

 the medullary canal, which runs throughout 

 most of their length. In this canal we find bone 

 marrow, which is red in early life, but yellow 

 in the adult. Inside the honeycomb of the com- 

 pact tissue there are little spaces called lacunae 

 and little canals called canaliculae which join 

 the little spaces together. The average diameter 

 of these canalicul^ is l-7,000th of an inch. The 

 lacunae contain bone-producing cells. 



402. Cancellated tissue is composed of thin 

 plates, which contain lacunae and canaliculae. In 

 between these plates are spaces called cancelli, 

 which contain bone marrow. The cancellated 

 tissue is chiefly at the extremities of the bones 

 of the limbs. It gives bulk without increasing 

 weight ; the larger the end of a bone the greater 

 will be the area for attachment of muscles and 

 ligaments. 



Blood-vessels pass in between the various 

 layers of the bone tissue and also amongst the 

 marrow, thus supplying food to the bone cells 

 and tissue. Every bone is surrounded by a thin, 

 very vascular skin (the periosteum), except at 

 the articular ends, which are covered with arti- 

 cular cartilage. The periosteum also has an 

 outer fibrous, thick, non-sensitive layer. In a 

 joint the two articular cartilages of each of the 

 ends of the bones forming the joints are lubri- 

 cated by synovia, which, as stated above, is kept 

 in place by the capsular ligament that surrounds 

 the joint. This ligament is lined internally 

 by the synovial membrane that secretes the 

 synovia. 



The periosteum is very easily damaged ; 

 when this happens, the bone underneath is very 

 liable to die. Thus, when a bone is damaged, 

 great care must be taken to prevent necrosis, or 

 death, setting in. In the periosteum there is a 



