183 The Development of tlie Arm in Man 



thick perichondrium. The clavicle, some of the carpals and the second 

 row of phalanges are of condensed tissue, while the distal row of 

 phalanges are not differentiated as yet. The nerves, both sensory and 

 motor, are distributed much as in the adult. 



By the end of the seventh week all the skeletal elements are of car- 

 tilage except the distal row of phalanges from the second to fifth digits, 

 which are of condensed tissue. All the muscles are to be recognized 

 and are composed of muscle fibers. The tendons and ligaments, except 

 in the distal part of the digits, are well formed. The digits present a 

 very interesting picture of the differentiation of the cartilage, peri- 

 chondrium, ligaments, and tendons from the condensed tissue tip of 

 each. 



During the process of differentiation other important changes are 

 taking place, namely, the migration caudally of the whole arm, the 

 migTation or extension of certain muscles from the arm caudally along 

 the body wall and the migration of other muscles from more anterior 

 regions to the arm, shoulder girdle and thorax. 



We may consider the position of the scapula and the inclination of 

 the brachial plexus as indicators of the migration of the arm. We find, 

 in an embryo of four and one-half weeks, that the scapula lies in the 

 region of the fourth and fifth cervical vertebrae. The brachial plexus 

 and the nerves forming it run to the arm without any caudal inclination. 

 The nerves which leave the plexus do bend posteriorly in the arm. At 

 five weeks the scapula has greatly enlarged and extends from the fourth 

 cervical to the first dorsal vertebrae. Its center has evidently shifted 

 posteriorly. The brachial plexus and the anterior nerves which go to 

 it have a slight caudal inclination. By the end of the sixth week the 

 greater portion of the scapula lies below the level of the first rib, its 

 posterior angle, including the condensed tissue, having extended to the 

 level of the fifth rib. The brachial plexus has been pulled along with 

 the shifting of the arm and has a decided posterior inclination. By the 

 end of the seventh week very little of the scapula lies above the level of 

 the first rib, and its lower angle reaches to the fifth intercostal space. 

 The brachial plexus has a very marked caudal inclination and appears 

 to be bent over the first rib. Before the adult conditions are attained 

 the sca]nila must migrate some distance posteriorly. Part of this move- 

 ment will take place with the sinking posteriorly of the ventral portion 

 of the thoracic wall, for in these stages the ventral ends of the ribs are 

 as far anterior as the vertebra? from Avhicli they arise. 



The migration of the pectoralis major and minor and the latissimus 

 dorsi muscles from the arm posteriorly to the thoracic wall is very 



