172 The Development of the Arm in Man 



Ilagen '* has reconstnicted the cartilaginous skeletal system of a 

 hnman en'ibryo of abont this age. A comparison of the drawings from 

 the reconstructions shows that there is considerable variation in the 

 carpal region. In none of my stages does the metacarpal come irt 

 contact with the radius, either before or after the cartilages of the- 

 carpus and metacarpus appear, and there is a considerable area of dense 

 mesenchyma between metacarpus and radius. I am inclined to believe 

 what he calls metacarpal I, may be trapezium and his so-called first- 

 phalanx the metacarpal. 



The Muscular System. — Plates I and II, Figs. A and B. The 

 trapezius muscle has both clavicular and acromial attachments. The 

 muscle has extended posteriorly so that the muscle fibers run from the- 

 occiput to the level of the fifth rib. They are connected by a consider- 

 able interval of fascia with the dorsal ends of the cervical and all the 

 thoracic neural processes. 



The levator scapulce and serratus anterior muscles are greatly altered iit 

 shape. The latter forms a broad, thin sheet between the dorsal border 

 of the scapula and the first nine ribs, being attached by a digitation tO' 

 each rib. The scapular attachment is into the condensed tissue along 

 its dorsal border. 



The pectoral mass is now spread out into a large, thin sheet, which 

 has split into the major and minor muscles. The clavicular and sterno- 

 costal portions of the pedoralis major are separated by a considerable 

 interval. The clavicular fibers arise from the median one-third of the 

 clavicle and pass to the humerus. They overlap the humeral ends of 

 the sterno-costal fibers which arise from the first six ribs and the sternal 

 anlage. 



The pedoralis minor is a distinct muscle arising from the second, third 

 and fourth ribs and passing to the coracoid process. 



The subelavius muscle is quite Avell developed and runs from the first 

 ril) to the clavicle, having a course nearly at right angles to the latter. 



The latissimus dorsi has spread out into a broad, thin sheet of muscle 

 fibers, which are connected by fascia with the lower thoracic and lumbar 

 neirral processes. Its humeral end is closely miited with the teres major. 



The teres major muscle has about the relations found in the adult. 

 It and the latissimus dorsi are inserted together into the humerus. 



Tlie deltoid muscle is very much like the adult in its attachments and 

 shape. 



-'•* Hagen, Die Bildung des Knorpelskeletes beim mensclalicben Embryo, Arch, fiir 

 Anat. u. Pbys., 1900. 



