776 EEPOBT— 1887. 



of a spinal nerve instead of from several ? In otker words, why does not one 

 primary division contain witliin itself all the strands of the nerve P How is it that 

 a nerve does not pass from the cord directly to the parts which it supplies, but, on 

 the contrary, gives fibres to and gets fibres from adjacent nerves in its course ? 



With the object of discovering if there is any fundamental plan upon which the 

 formation of the plexuses and the distribution of the nerves is based, complete dis- 

 sections were made often different animals — porcupine, rat, koala, rabbit, guinea-pig, 

 cat, camel, brindled gnu, capucinus, and entellus monkeys. 



These dissections have led to certain conclusions regarding the constitution of 

 the plexuses, which are supported by a reference to the condition of things which 

 obtains in man ; and, still further, to the formulation of a general hypothesis which 

 appears adequate to explain their existence. 



Without describing mimitice, it is enough to say that by an analysis of the 

 plexuses of the fore and hind limbs one finds on the one hand that there are certain 

 minor differences in the arrangement of the nerves in different cases ; on the other 

 hand that there are certain fundamental points in which all agree. 



(A) The points of difierence are, speakuig broadlj', two : — 



1. In the number of nerves entering such plexus. 



2. In the position of the limb-plexuses in the series of spinal nerves. 



1. Examining the number of nerves which enter into the composition of the 

 brachial and lumbo-sacral plexus, both in the animals dissected and in all other 

 recorded cases to which I have had access, I find \\vitjii-e nerves, or thereabouts, are 

 engaged in the formation of the nerves of distribution to the limb proper. The 

 number, however, varies in different animals, being sometimes more and sometimes 

 less than five. 



In the Brachial Plexus. — Five nerves entered into the formation of the plexus in 

 five cases, four in four cases, and in the entellus monkey six nerves assisted in 

 forming the plexus. 



In the Lumbo-sacral Plexus. — In all cases but one five nerves were engaged in 

 forming the branches which supplied the limb proper. In tliis category the ilio- 

 hypogastric ilioinguinal, small sciatic, and internal pudic are omitted, as only one of 

 them (the small sciatic) supplies any portion of the hind limb ; and in most of the 

 animals that nerve merely supplies the skin of the buttock and upper part of the 

 flank. In other recorded cases I find the number of plexus-forming nerves in relation 

 to the hind limb to vary between four (in thylacine) to seven (in man). 



While, however, there are slight individual variations, the number in all cases 

 is about Jive. This fact gives support to the late Professor Goodsir's hypothesis, 

 that the mammalian limb derives elements from five vertebral somites. 



2. With regard to the second point of difierence, tlie position of the liml)- 

 plexuses in the series of spinal nerves, the discrepancies are only slight. 



In the brachial plexus the nerves which form it are always the sixth, seventh, 

 eighth, and ninth spinal nerves, with, m addition in certain cases, tlie fifth or fourth. 

 Occasionally in man the tenth nerve also joins the plexus. 



In the lumbo-sacral plexus there is at first sight no regidarity in the arrange- 

 ment of the nerves entering into it, with regard to the vertebral segments. The 

 plexus for the hind limb may be formed wholly b}' lumbar nerves or by the addi- 

 tion of sacral nerves as well. 



This discrepancy is due to two causes : first, tlie varying number of thoracico- 

 lumbar vertebrae ; and secondly, the varying position of the lumbo-sacral articu- 

 lation. It disappears to a certain extent ■\\hen the numbers of plexus-forming 

 nerves are counted in relation to the series of the spinal nerves. AVben this is done 

 it is seen that there is still considerable A-ariation, from the twenty-first to the 

 twenty-ninth spinal nerves. The twenty- fifth nerve is the only one present iu every 

 case. 



(B) Points of agreement : — 



Notwithstanding these minor differences, there are certain fundamental points 

 of agreement in the composition of the limb-plexuses of the limbs and in all the 

 animals dissected : — 



