158 The Development of the Arm in Man 



base of the tongue. The cephalic portion of it is supplied by two nerves, 

 one a branch of the first and second cervical, and the other a branch of 

 the third cervical nerve. These two branches form a loop on the siu'- 

 face of the mass. They correspond to the ramus descendens n. hypo- 

 glossus and ramus communicans hypoglossus uniting to form the ansa 

 hypoglossus. Hence I have called this the infra-hjoid premuscle mass. 

 I have not been able to determine the fate of the condensed tissue on 

 the median side of the pectoral premuscle mass, and caudal to the infra- 

 hyoid mass. The phrenic nerve ends very close to it, and very likely 

 this is diaphragm premuscle mass. 



The rJwmhoid premuscle mass lies lateral to the second division of the 

 lateral premuscle mass and is an ill-defined plate of condensed tissue. 

 It lies at the level of the fifth cervical vertebra and receives a branch 

 from the fifth cervical nerve, arising in connection with a nerve to the 

 levator scapulae mass. 



The caudal end of the trapezius premuscle mass is seen in Fig. 7, 

 lateral to the levator scapulse mass. The main portion of the trapezius 

 premuscle mass lies opposite the cephalic four cervical vertebrae. It is 

 supplied by the spinal accessory and communicating branches from the 

 first four cervical nerves. 



Arm premuscle slteath. — The skeletal core of the arm is surrounded by 

 a mass of tissue which shows no signs as yet of splitting into separate 

 masses. Along the median side of the humerus this sheath is inter- 

 rupted by the entrance of the brachial plexus and nerves. In places 

 the sheath is separated from the skeletal core by blood-vessels, but in 

 most places no sharp line of separation can be seen. Toward the distal 

 end of the arm the sheath merges into the more primitive mesenchymal 

 tissue which fills most of the distal end of the arm. In Fig. 8 the 

 ,distal limit of the premuscle sheath is indicated, a portion of the primi- 

 tive arm mesenchyma having been removed to show the limit of the 

 sheath, the hand plate, the border vein and the venous plexus between 

 the hand plate and the mesenchyma. 



The Nerves. — The muscle plate column is supplied by branches of 

 the dorsal rami from all the nerves in this region. They enter the 

 median side of the muscle-plates branch Avithin them, one branch pass- 

 ing through to the subcutaneous tissue. 



Branches from the anterior rami of the III, IV, V, VI and VII cervi- 

 cal nerves supply the levator scapulfe and serratus anterior premuscle 

 mass. The rhomboid premuscle mass is supplied by a branch which 

 comes off with the one from the V cervical. 



