THE BRACHIAL PLEXUS. 379 



this, ventriduct the head and insert one blade of the nippers into 

 the foramen magnum (Fig. 56, 57, 59), and remove the O. supraoc- 

 cipitale piecemeal. Remove also the parietal and frontal bones in 

 the same way until the dorsal aspect of the brain is entirely uncov- 

 ered. The bony tentorium (Fig. 59, 88) will remain, but its dorsal 

 border will be free. To remove it, separate slightly the hemispheres 

 and cerebellum, insert the nippers and break the tentorium on both 

 sides. It may then be removed with the coarse forceps. 



1014. Removal of the Dura. After the hard parts are re- 

 moved from the central nervous system, the dura, a tough membra- 

 nous sac enclosing it, should be partly removed by grasping the 

 membrane at some point where it is relaxed and cutting away a 

 piece at a time. Grasp the cut edge of the dura with the forceps 

 and lift it away from the underlying nervous matter so that the 

 scissors may be inserted. Remove the dura, proceeding with ex- 

 treme caution, especially around the nerve roots. Sometimes it 

 will be necessary to employ fine scissors and fine forceps and the 

 tripod magnifier, especially in cutting the dura from the roots of 

 the cranial nerves. The fan-like appearance of some of the nerve 

 roots on their emergence may be well seen with the magnifier. 



THE BRACHIAL PLEXUS AND THE PRINCIPAL NERVES OF THE RIGHT 

 ARM AND SCAPULAR REGION. 



References. Quain, A, I, 582; Gray, A, 671 ; Hyrtl, A, 543 ; Gegenbaur (Lankester), 

 A, 514; Chauveau, A, 800 ; Chauveau (Fleming), A, 754; Gurlt, A, 749; Oweu, A, III, 

 170, 176 ; Milne-Edwards, A, XI, 239 ; Leyb, A, 537. 



Instruments and material, see 1009. 



1015. The Brachial Plexus is the network of nerves from which the scapular 

 region and the arm are supplied. It is formed by the intimate connection of the ventral 

 (anterior) primary divisions ( 1007) of the 6th, 7th and 8th cervical and the 1st tho- 

 racic nerves. 



Fig. 105 and the description are given to illustrate the relations and distribution of 

 spinal or myelonal nerves in a well denned region. 



For the study of this subject, the student should have before him a natural skeleton of 

 the arm, including the scapula ( 252). 



1016. Specimen, Posture and Preparation. The same specimen may be employed 

 as for Fig. 104, but it is better to use a different one. It should, however, be of the same 

 character, viz., young and lean. 



Preparation of Fig. 105. The cat was injected from the abdominal aorta with thin 

 plaster ( 345, 363). Then the skin was removed from the axillary and pectoral regions, 

 and from the caudal aspect of the arm to the wrist. The pectoral, claw-mastoid, daco- 

 trapezius, epitrochlearis and pronator teres muscles ( 680, 681) were then cut and turned 



