38 TOPOGKAPHICAL ANATOMY OF 



extensor which provides tendons for the fourth and fifth digits. There 

 can be little question that the main tendon in the horse is the homo- 

 logue of that tendon of the dog's common extensor which ends in the 

 third digit. The greatest difficulty arises with the muscle of Thiernesse 

 — assuming that it is really something apart from the main mass of the 

 muscle. Its suggested homology with the m. extensor indicis proprius 

 of man gives ground for debate. 



M. EXTENSOE DiGiTi QUiNTi. — Though here named the extensor of 

 the fifth digit, it is conceivable that this muscle may represent a 

 combination of the extensors of the third, fourth, and fifth digits of the 

 dog, and, to avoid difficulties of homology, it is frequently called the 

 lateral extensor of the digit. The muscle is narrow and semipenniform, 

 and is enclosed in its own compartment of antibrachial fascia. Its 

 position is along the lateral aspect of the forearm, insinuated between 

 the common extensor of the digits and the ulnar extensor of the carpus. 



The origin of the muscle is from the lateral collateral ligament, the 

 back of the projection on the head of the radius, and a narrow area of 

 both bones of the forearm bounding the lateral line of their union. In 

 the distal part of the forearm the muscle gives place to a tendon that 

 traverses a groove on the lateral part of the distal end of the radius, 

 then travels down the corresponding aspect of the carpus partly 

 embedded in ligament, and enters the metacarpus, where it will be 

 examined subsequently. Like the preceding extensors, its tendon is 

 surrounded by a synovial sheath. 



M. EXTENSOR CAEPi ULNARis. — Though morphologically an extensor 

 and in receipt of a nerve supply (radial) like that of the other extensors, 

 this muscle in the horse is doubtless physiologically a flexor. Unlike 

 its fellow-extensors, it is placed behind the lateral border of the radius. 



The origin of the muscle is from the distal part of the lateral 

 epicondyle of the humerus. Its insertion is double. A strong tendon 

 connects it with the proximal edge of the accessory carpal (pisiform) 

 bone ; while a narrow, rounded tendon runs somewhat obliquely along a 

 groove on the lateral face of the accessory bone and across the lateral 

 border of the carpus (embedded in ligament) to reach the base of the 

 fourth metacarpal bone. The second tendon is provided with a synovial 

 sheath. 



M. ABDUCTOR POLLicis LONGUS.i — This thin, triangular muscle arises 

 from the lateral border and adjacent area of the dorsal (anterior) 

 surface of the middle third of the radius. At first the muscle is 

 beneath the common extensor of the digits, but its narrow tendon 



1 This muscle, in all probability, represents the short extensor as well as the long 

 abductor of the poUex. PoHct FL.! the thumb. 



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