300 ABRAM T. KERR 



the difference in the effect of the nitric acid on the epineurium 

 and the perineurium is due merely to greater density of the latter, 

 or if there is a difference in the kind of connective tissue forming 

 each of them. It is clearly evident that when two funiculi join 

 to form a third, we cannot, by this method, determine whether 

 the branches from this latter contain nerve fibers from one or 

 from both of the original bundles. 



Figure 9 shows the principal funiculi of a plexus from which 

 the epineurium has been removed by maceration. From this 

 it will be seen how complicated is the network of bundles and how 

 absolutely hopeless it is to attempt to trace to a definite spinal 

 nerve the elements which enter into some of the branches even 

 when the epineurium has been so completely removed. It will 

 be seen, therefore, how much more difficult, in fact, how impossi- 

 sible it is to trace these funiculi ; let alone nerve fibers accurately 

 by means of ordinary dissection. 



COMBINATION OF NERVES TO FORM A PLEXUS 



We have so far been speaking of the brachial plexus, while 

 as a matter of fact we have been dealing only with the nerves 

 which go to make up that plexus. These nerves are usually 

 described as combining to form the plexus in the following 

 manner. The fifth and sixth cervical nerves unite to form a 

 common stem, the cephalic trunk (upper or outer trunk or pri- 

 mary cord) and in the same way, the eighth cervical and first 

 thoracic nerves unite to form a caudal trunk (lower or inner, 

 trunk or primary cord) while the seventh cervical remains single 

 and represents an intermediate trunk (middle, trunk or primary 

 cord). Each of these trunks divides into ventral and dorsal 

 branches. The ventral branches of the cephalic and intermedi- 

 ate trunks join to form a lateral fasciculus (outer cord); the 

 ventral branch of the caudal trunk remains single as the medial 

 fasciculus (inner cord) ; while the dorsal branches of all three 

 trunks join to form the dorsal fasciculus (posterior cord) (figs. 

 1, 2, 3). 



