ON THE MAMMALIAN NERVOUS SYSTEM. 425 



tages bas the greater one of furnishing us with the results of cord changes expressed 

 quantitatively, and, therefore, capable of strict comparison with one another, have 

 combined to cause us to devote a very large proportion of our experimental investi- 

 gations to the subject of the present and the succeeding chapters. The subject 

 matter of the present chapter becomes, therefore, an answer to the question, to what 

 extent are the electrical changes in the cord, and hence the nerve impulses, of which 

 these are the index, interrupted when between the stimulated nerve and the observed 

 region of the cord different intervening localised sections are made. The principle is, 

 therefore, that of the experiments carried out in LUDWIG'S laboratory, it is the index 

 alone which is novel. 



SECTION 4. ANATOMICAL RELATIONS OF THE LUMBAR NERVE. 



Since in all the experiments which bear upon the relations of the spinal cord to its 

 nerves the sciatic was chosen, it is desirable at this stage to refer to the anatomical 

 relations which exist between this and the roots of the lumbar and sacral nerves. 



These differ in the two sets of animals used, Cat and Rhesus Monkey, and as we 

 are unable to find that the comparison of their anatomical and physiological relations 

 has been anywhere* set forth, we must proceed to examine the case of each species 

 of animal in a little detail. 



In the Cat the photograph of the plexus (Plate 35) shows that of the two branches 

 of the sciatic nerve (I. P. and E.P.}, internal and external popliteal, the former derives 

 most of its fibres from the 6th, the latter from the 7th lumbar roots. Since the 

 portion of sciatic nerve which we used was that in the thigh, the fibres involved were 

 almost entirely those derived from the 6th and 7th lumbar roots, a few fibres coming 

 from the 1st sacral, and still fewer from the 5th lumbar. The obturator nerve derives 

 its fibres from branches of the 5th and 6th lumbar nerves, the anterior crural mainly 

 from the 5th, but to some slight extent from the 4th. 



In the Monkey the relations are somewhat different, as will be seen by referring to 

 the photograph of the lumbar plexus of the Macacus rhesus (Plate 34). It will be 

 then found that the two branches of the sciatic nerve (I. P. and E.P.} derive their 

 fibres from a more widespread origin. The majority of fibres come from the 5th, 6th, 

 and 7th lumbar ; a few fibres from the 1st sacral join to form the trunk of the nerve, 

 but these appear to pass entirely into that branch which leaves the trunk high up and 

 supplies the hamstring muscles (Hs.). The obturator nerve derives the majority of 

 its fibres from the 4th lumbar, receiving a few fibres also from the 5th, whilst the 

 anterior crural derives its fibres from the 4th, 3rd, and (?) 2nd in decreasing amount. 



Roughly speaking, therefore, the relations of the lumbar plexus in the two species 

 of animals differ in this respect, that with the same number of lumbar nerves (7) the 



* While this is passing through the press a very valuable paper, giving the anatomy of the plexus in 

 the Cat, has appeared, viz., LANGLEY, 'Journal of Physiology,' vol. 12, No. 4, 1891. 

 MDCCCXCI. B. 3 I 



