72 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
nate the ventral branch of the first spinal nerve as nerve 1, the ventral 
branch of the second as nerve 11, ete. 
It has seemed necessary in comparing plexi to indicate in some way 
the relative value of the nerves entering them. I have therefore 
recorded their approximate diameters, which I shall designate as their 
strength, — a rough method, but accurate enough for the present 
purpose. 
The material which I studied consisted of thirty specimens of the 
large perennibranch salamander of the Middle United States, — Noc- 
turus maculosus Rafinesque. These specimens were obtained from the 
Great Lake Region, in the main from a single locality (near Sandusky, 
Ohio) on the south shore of Lake Erie. 
Unfortunately, I neglected to note the sex of each individual. The 
importance of this did not occur to me until the urogenital system had 
been removed, and dissection had gone so far as to prevent the deter- 
mination of the sex. The greater number (more than two thirds) were 
males, but which ones I am unable to say. 
The work represented by this paper was done at the suggestion, and 
under the direction, of Dr. G. H. Parker, to whom I am indebted for 
assistance and advice during its progress. 
Tue BRACHIAL PLEXUS. 
The brachial plexus (Plate 1, Fig. 1) is formed from nerves 1 to v in- 
clusive. In no case have I found indications that nerve v1 contributed 
to it. 
The greater strength of the plexus lies posterior to the pectoral 
girdle. Its major part is compacted into a large trunk, the brachial 
nerve (br.), which, after sending branches to the shoulder muscles, 
divides to supply the various muscles of the fore leg. But nerve distri- 
bution does not concern us here. It is my purpose to determine if, in 
the number and strength of the nerves which enter the brachial plexus, 
there be any variation which may be correlated with the variations in 
the sacral region. 
If, when the sacral rib is on the 19th vertebra, — the more usual 
position, — we find the brachial plexus constant in position, and if, when 
the sacral rib is on the 20th vertebra, —a common variation, — we find 
a backward displacement of the brachial plexus to the extent of one 
segment, it would be fair to infer that the seat of variation is in the 
pre-brachial region. 
