308 HUMAN EMBRYOLOGY AND MORPHOLOGY. 



student's while to briefly review the corresponding muscles in 

 the two limbs. He has already seen that the arm and leg buds 

 are similar in origin and made up of similar elements. Each is 

 subsequently modified for its special function. The key to the 

 homology of the muscles lies in their relationship to the digits, 

 and their position on the limb. 



Pollex and Hallux. — The extensor ossis metacarpi pollicis 

 corresponds to the tibialis anticus. The thumb muscle has 

 commonly a carpal insertion as well as metacarpal. The extensor 

 brevis or primi internodii pollicis is constant in man only; it is a 

 segment of the extensor ossis metacarpi pollicis. 



The extensor brevis hallucis is not represented in the thumb. 



Second Digit. — In the lower primates each finger has two 

 extensors — a deep and superficial. The deep in the second 

 digit becomes the extensor indicis ; in the little finger it forms 

 the extensor minimi digiti. The deep extensor muscles have 

 disappeared in man from the 3rd and 4th digits, but occasion- 

 ally reappear. In the leg the deep extensors have migrated to the 

 foot, and form the extensor brevis digitorum. That for the little 

 toe, however, has not descended ; it is always vestigial, if present, 

 and is commonly absent. It runs beneath or with the peroneus 

 brevis, and is known as the peroneus quartus or peroneus quinti 

 digiti. 



Flexors and Extensors of the Metacarpus. — These have 

 retained their primitive insertions in the hand; their modifications 

 in the foot have been already mentioned. Both at the knee and 

 elbow joint the origins of these muscles have undergone much 

 shifting and migration. 



The comparison already made between the scapula and ilium 

 (p. 291), will help the student to understand the correspondence 

 between the muscles of the thigh and arm. 



Vestigial and Abnormal Muscles in the Limbs and 

 Trunk. — (1) The muscles of the human ear and scalp may be 

 described as vestigial when compared to the development in 

 other mammals. Although their action on the ear and scalp is 

 feeble, yet they serve as most important bases into which certain 

 psychological states are reflected. 



