Peripheral ISTervons System in Human Embryo 299 



where it is joined by the descendens cervicis, which can be seen as a 

 slender nerve made up of branches from the second and third cervical 

 nerves. 



The phrenic nerve is formed by anterior terminal branches from the 

 fourth and fifth cervical nerves. A contribution on the part of the 

 sixth could not be made out. A view of the nerve can be seen through 

 the arm in Plate I. Owing to the position of the diaphragm at this 

 time the course of the nerve is almost directly ventral, passing over the 

 lung anlage which is not represented in the drawing. Later, as pointed 

 out by His and Mall (Mall, '01), the points of origin and insertion of the 

 nerve draw gradually apart, due, on the one hand to the descent of the 

 diaphragm and the lengthening of the thoracic cavity, and on the other 

 hand to the subsequent elevation of the cervical nerves which accom- 

 panies the development of the structures of the neck. It is thus that 

 there results the long caudal course of this nerve that is characteristic 

 of the adult. 



The brachial plexus is shown in Plate I by representing the arm as 

 transparent. In the region of the fifth cervical to the first thoracic 

 nerves there is an exuberant growth of both the anterior and lateral 

 terminal branches, resulting in a solid flattened mass of fibres, which in 

 turn is split by the skeletal anlage into two laminae, from which the 

 various nerves arise. Arising from the anterior or ventral lamina one 

 can recognize the nn. musculocutaneous, medianus and ulnaris, and 

 from the posterior or dorsal lamina the nn. axillaris and radialis. These 

 nerves pass down into the arm and break up in the muscle masses, 

 which they are to supply. 



The lumbosacral plexus as compared with the brachial plexus is some- 

 what retarded in its development. It is formed by the fusion of the 

 trunks of the five lumbar and upper three or four sacral nerves. These 

 nerves and their ganglia, as with the cervical nerves, are enlarged as 

 though stimulated to extra growth by the presence of the limb bud. 

 The nerves unite into a flattened mass of fibres which enters lateral- 

 ward into the base of the leg bud, the division into anterior and lateral 

 terminal branches being lost in the formation of the plexus. The 

 further course of the fibres is determined by the framework of the leg. 

 Owing to the cell masses of the bony pelvis and the femur the fibres 

 become grouped into four bundles arranged in two pairs, each consist- 

 ing of a median and lateral trunk. Of the upper pair the median trunk 

 corresponds to the obturator nerve, and the lateral the femoral nerve. 



