266 ZOOLOGY SECT. 



row of the tarsus the tibiale or astragalus (AST) and the fibulare 

 or calcaneum (CAL) are greatly elongated and provide the leg with 

 an additional segment. There are three tarsals in the distal row, 

 one of which appears to represent the centrale, another the first 

 distale, and the third the fused second and third distalia. There 

 are five well-developed digits, and on the tibial side of the first 

 is a spur-like structure or calcar (c), formed of three bones, a meta- 

 tarsal and two phalanges : such an additional digit is called a 

 pre-hallux. 



All the long bones of the limbs consist of the shaft formed of true 

 bone and of extremities of calcified cartilage. The distinction is a 

 very obvious one, both in the freshly-prepared and in the dried 

 skeleton. 



The muscular system has undergone great modifications in 

 correspondence with the complex movements performed by the 

 limbs. The dorsal muscles of the trunk are no longer divisible 

 into myomeres, but take the form of longitudinal or oblique bands 

 (extensores dorsi, &c.), lying partly above the vertebrae, partly 

 between the transverse processes, partly between the ilia and the 

 urostyle. The ventral muscles are differentiated into a paired 

 median band, the rectus abdominis (Fig. 939, ret. abd), with longi- 

 tudinal fibres, and a double layer of oblique fibres obliquus 

 externus (obi. ext) and internus (obi. int) extending from the 

 vertebral column to the recti. Both the extensor dorsi and the 

 rectus abdominis are traversed at intervals by transverse bands 

 of fibrous tissue, the inscriptiones tendinece (ins. ten), but the 

 segments thus formed do not correspond with the embryonic 

 myomeres. The right and left recti are united by a longitudinal 

 band of tendon, the linea alba (I. alb). 



The muscles of the limbs are numerous and complex, each 

 segment having its own set of muscles by which the various 

 movements of which it is capable are performed. There are 

 muscles passing from the trunk to the limb-girdles ; from the trunk 

 or the limb-girdles to the humerus and femur ; from the humerus 

 and femur to the radio-ulna and tibio-fibula ; from the fore-arm or 

 shank to the digits ; and from one segment of a digit to another. 

 For the most part the limb-muscles are elongated and more or less 

 spindle-shaped, presenting a muscular portion or belly which passes 

 at either end into a tendon of strong fibrous tissue serving to fix 

 the muscle to the bones upon which it acts. The relatively fixed 

 end of a muscle is called its origin, the relatively movable end its 

 insertion, e.g. in the gastrocnemius muscle of the calf of the leg 

 (gstr) the proximal end attached to the femur is the origin, the 

 distal end attached to the foot the insertion. According to their 

 action muscles are divided into flexors which bend, and extensors 

 which straighten, one part upon another ; adductors which draw 

 towards, and abductors which draw away from, the middle line ; 



