132 MERISTIC VARIATION. [part l 



Birds investigated by himself. He also gives particulars of the 

 individual variations which were found in certain cases. From 

 this table the following statement is compiled, shewing the most 

 important diversities met with and the instances of individual Va- 

 riation. In the majority of cases the most posterior spinal nerve 

 of the cervical region was the most posterior nerve of the brachial 

 plexus, but in a certain number of cases it does not join the plexus 

 at all ; in some other cases the anterior spinal nerve of the dorsal 

 region also takes part in forming the plexus. As the table shews, 

 each of these plans has been likewise met with as an individual 

 variation. 



Fiirbringer's table shews 3 as the minimum number of spinal 

 nerves found taking part in the formation of the plexus of any bird 

 (Bucoi'vus abyssinicus) : the same number has been found as a 

 minimum by other observers in other birds (v. Furbrixger, p. 242, 

 note). The maximum number w T as 6, found in Gharadrius and 

 some specimens of Columba. The plexus is generally formed by 4 

 or 5 spinal nerves. 



In cases where several individuals were examined, individual 

 variation w r as generally found, as in Anser, Podargus, Picas, Geci- 

 nus and Garrulus ; in these cases the number of spinal nerves 

 which took part in forming the brachial plexus varied between 4 

 and 5, while in Columba, the number even varied between 4 and 6. 



Variations also occurred in this respect between the two sides 

 of the body. For example, in a specimen of Anser cinereus the 

 plexus was formed on the right side by the nerves XVI, XVII, 

 XVIII and XIX. while on the left side it received a strand from 

 the XXth nerve in addition to these. 



As has been stated, the last cervical nerve is generally the last 

 nerve supplying the brachial plexus but deviations from this plan 

 occur in both directions. These deviations may occur as individual 

 variations and they may even be unilateral, owing to the transition 

 between the cervical and dorsal vertebras being effected at different 

 points on the two sides of the body. 



Particulars are given respecting the average proportions of the 

 several roots in the different arrangements, but the arrangement 

 or size of the roots relatively to each other was not found to bear 

 any constant relation either to the systematic position of the bird, 

 or to its size, or to its capacity for flight. It was however generally 

 found that there was a certain relation betw r een the relative size of 

 the roots and the length of the neck in birds with a plexus com- 

 posed of four roots. In this case the greatest thickness was gener- 

 ally either in or anterior to the middle roots of the plexus in short- 

 necked birds, but posterior to the middle of the plexus in long- 

 necked birds, but even this rule was not at all closely observed and 

 many exceptions occurred. Furbrixger, I. c. p. 243. 



In Variation in the ordinal positions of the spinal nerves com- 

 posing the plexus, the pattern of the plexus as newly constituted 



