72 bulletin: museum of comparative zoology. 



nate the ventral branch of the first spinal nerve as nerve i, the ventral 

 branch of the second as nerve ii, etc. 



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 perenuibranch salamander of the Middle United States, — Nec- 

 turus maculosus Ilafinesque. 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. 



The Brachial Plexus. 



The brachial plexus (Plate 1, Fig. 1) is formed from nerves i to v in- 

 clusive. In no case have I found indications that nerve vi contributed 

 to it. 



The greater strength of the plexus lies posterior to the pectoral 

 girdle. Its major part is compacted iuto a large trunk, the brachial 

 nerve (Jr.) 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 tlie 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. 



