COMPARATIVE ANATOMY OF THE MUSCULAR SYSTEM 157 



to the lateral head, but quite separable from the medial head, being covered on 

 the medial side by a shining aponeurosis. Origin, patella and femur; insertion, 

 by a thick tendon which passes in the middle of a sort of tube formed by the 

 tendon of the gastrocnemius and soleus onto the ventral surface of the calcaneus. 

 Here it broadens and finally divides into four slips, each attached to a digit. 

 Action, flexor of the digits. 



g) Flexor digitorum longus. On turning to the medial side of the shank 

 and clearing away the surface fascia, the following may be identified. Most 

 ventrally will be seen the tibialis anterior; next comes the exposed surface of the 

 tibia. Immediately dorsal to the bone is the flexor digitorum longus which 

 consists of two parts, somewhat separated. The other part is more lateral in 

 contact with the peroneus muscles. Separate the part of the flexor which appears 

 on the medial surface from the tibia by a cut, and lift it up. Internal to it is 

 seen a long tendon, and on the other side of this tendon is the other part of the 

 flexor, this part corresponding to the flexor hallucis longus of man. Both parts 

 of the flexor terminate in slender tendons which unite distally into a broad 

 tendon, which eventually divides into four tendons inserted on the digits. 

 Origin, tibia, fibula, and adjacent fascia; action, flexor of the digits. 



h) Tibalis posterior. The long tendon between the two parts of the preceding 

 muscle was noted above. It is the tendon of the tibialis posterior and on follow- 

 ing this tendon proximally the belly of the muscle will be located. Origin, 

 fibula, tibia, and fascia; insertion, scaphoid and medial cuneiform of the ankle; 

 action, extensor of the foot. 



E. SUMMARY 



1. The voluntary muscles are of two kinds — 'the somatic or parietal muscles which are 

 produced by the epimeres and the visceral muscles which arise from the hypomeres. Voluntary 

 muscles of hypomeral origin occur only in connection with the gill arches, from which, however, 

 they may spread to cover considerable areas. 



2. In primitive vertebrates the somatic muscles exist in the form of muscle segments or 

 myotomes which are repeated at regular intervals along the longitudinal axis. Each myotome 

 extends from the mid-dorsal to the mid-ventral line. The myotomes are bounded by connec- 

 tive tissue partitions, the myosepta. 



3. The myotomes are divided into dorsal or epaxial halves and ventral or hypaxial halves 

 by the horizontal skeletogenous septum. 



4. The girdles and paired appendages interrupt the series of myotomes. The myotomes 

 adjacent to the appendages send out muscle buds which produce the muscles of the girdles and 

 paired appendages. 



5. In higher animals these muscles of the girdles and appendages increase in size and impor- 

 tance and spread over the segmented musculature until the latter is scarcely recognizable. 

 The myotomes are further altered by processes of splitting and fusion so that the primitive 

 arrangement no longer exists in the adults of higher land vertebrates. 



6. The derivatives of the hypaxial muscles form "the larger part of the mammalian muscu- 

 lature. The epaxial muscles remain as a thick mass on either side of the vertebral column, 

 concealed from surface view by dorsal extensions of the hypaxial muscles. 



7. With the adoption of the air-breathing habit the visceral or gill musculature is greatly 

 changed and is found in mammals associated with the larynx, pharynx, hyoid apparatus, 

 upper and lower jaws, and the skin of the neck and face. The muscles of the head are prac- 

 tically all visceral muscles (except the muscles of the eyeball). 



