CHAPTER XXVIII 

 BRIEF ANATOMICAL STUDY OF THE HORSE 



In all animal life the cell is the structural and functional unit. A 

 tissue is a collection of similarly differentiated cells. A number of 

 tissues grouped together form an organ. The body is an aggregation 

 of organs. The systems of organs are seven in number. They are 

 (1) skeletal, (2) muscular, (3) digestive, (4) respiratory, (5) genito- 

 urinary, (6) nervous, and (7) integumentary. 



Skeletal system. — The skeletal system is important as it largely 

 determines the conformation of the horse. The divisions of the 

 skeleton are (1) head, (2) neck, (3) trunk, and (4) legs. The head 

 consists of numerous bones, mostly fiat, united by sutures which gradu- 

 ally undergo obliteration with age. The lower jaws are strong and in 

 each jaw there are six molar teeth (24 in all). Twelve of these are 

 temporary — three in each jaw. There are also six incisors in the upper 

 and six in the lower jaws, all of which are temporary and are entirely 

 replaced by the time the animal is 43^ years old. 



The vertebral column is a chain of 54 to 56 irregular bones (verte- 

 brae) extending from skull to end of tail. There are seven cervical 

 (neck) vertebrae, 18 dorsal (back), 6 lumbar (loin), 5 sacral (croup), 

 and 18 or 20 caudal (tail) vertebrae. The 3rd, 4th, and 5th dorsal 

 vertebrae have the highest spines which form the withers. (See Fig. 

 146.) 



There are 18 pairs of ribs, 8 of which are true and 10 false. The 

 8 true pairs join the 8 segments of the sternum or breast bone. The 

 7th or 8th ribs are longest. There is no collar bone (clavicle) as in 

 man, and the fore legs are not attached to the trunk but are connected 

 by intervening muscles. The hind legs are united to the trunk by 

 the pelvic girdle which is composed of three segments on each side. 



Muscular system. — The horse comprises a large number of sys- 

 tems of levers represented in the bones and joints, each supplied with 

 a system of muscles which furnish the power. Muscles exert a force 

 in only one way, and that by shortening, giving a pull. For this 

 reason muscles are arranged in pairs, as illustrated by the biceps and 

 triceps which move the forearm in man. The flexor muscles are al- 

 ways inside the joint and the extensors outside. The theory has been 

 advanced that the shortening of muscles is due to a change in the form 

 of the muscular cell from an elongated form to one nearly round when 

 stimulated by nerve action. Muscles act through very short distances 



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