60 TOPOGRAPHICAL ANATOMY OF 



a short diverticulum from the sheath of the common extensor follows 

 the thin tendon of Phillips' muscle for a distance towards its union 

 with the tendon of the extensor digiti quinti. There is, however, no 

 union between the synovial sheaths of the two tendons. 



The slanting abductor pollicis has a sheath that ends close to the 

 insertion of the tendon into the base of the second metacarpal bone. 



A small synovial bursa lies under the tendon of each of the 

 extensors of the digits as it crosses the joint between the third meta- 

 carpal bone and the first phalanx. 



The tendon of the flexor carpi ulnaris has no sheath. The radial 

 flexor has a sheath very similar in extent to that provided for the 

 radial extensor. The most proximal limit of this sheath is, roughly, 

 7 to 8 cm. above the radiocarpal joint, and the sheath is continued 

 almost to the insertion of the tendon into the second metacarpal bone. 



Two common sheaths enclose the tendons of the superficial and 

 deep flexors of the digit. One of these begins about 8 to 10 cm. above 

 the radiocarpal joint and ends near the middle of the metacarpus. 

 The second sheath extends from the distal fourth of the metacarpus 

 to the middle of the second phalanx. 



The tendon of the deep flexor muscle has, in addition, a third 

 sheath (to be examined later), where it plays over the sesamoid bone 

 of the third phalanx. 



Mm. lumbricales. — In the distal part of the metacarpus a pale, 

 weak, and variable lumbrical * muscle will be found attached to each 

 border of the deep flexor tendon. The slender tendon of each is lost 

 in the fibrous basis of the "ergot." These muscles cannot be regarded 

 as of functional value, but are of interest as being vestiges of structures 

 that formerly served a definite purpose. 



Mm. intekossei. — Like the lumbricals, the interosseous muscles 

 have been profoundly affected by the marked modifications that have 

 taken place in the manus during the evolution of an animal with only 

 one functional digit. Three interosseous muscles are represented in 

 the modern horse, but, of these, two, which may be named medial and 

 lateral, are rudimentary. They have a slender, pale fleshy belly arising 

 from the base of the second or fourth metacarpal bone as the case may 

 be. Very attenuated tendons end variously in the neighbourhood of 

 the metacarpo-phalangeal joint. 



The middle interosseous muscle (m. interosseous medius) has, 



during its evolution, increased considerably in size — in conformity with 



the great increase in the size of the digit with which it is associated — 



and has so changed in structure that it is now almost entirely tendinous. 



1 Lumbricus [L.], an earthworm. From the shape of the muscles in man. 



