44 ON THE NERVOUS SYSTEM IV. 



hypoglossus are In part connected by the arachnoid. In Bufo Americanus I have 

 found the hypoglossus arising very nearly as Yogt has described it. 



Voo-t's classification of this nerve among the true cranial nerves does not seem to 

 be satisfactorily sustained, and certainly will not hold as regards Frogs. In them it 

 certainly has all the characters of a common spinal nerve ; viz. two kinds of roots, a 

 ganglion on one of them, a mass of crystals attached as in the spinal nerves, and 

 makinjr its exit at an intervertebral foramen. There is still another anatomical 

 character, though perhaps not an important one, which allies it to the spinal more 

 than the cranial nerves ; this is the primitive direction of the nerve roots back- 

 wards instead of forwards, as is the case with the cranial nerves. 



II. Brachial Nerves. (Plate II. Figs. 1, 2.) — These are comparatively simple. 

 In most of the higher Reptiles which have limbs, they consist of three or more pairs, 

 which subsequently unite and form the brachial plexus, which in turn gives its 

 branches to the upper extremity. In Frogs, however, only a small filament is given 

 by the third spinal to the brachial nerve, and thus an approach merely to a bra- 

 chial plexus exists. All the nerves to the arms, with the exception of the above- 

 mentioned branch, are supplied by the single trunk of the second pair, which is at- 

 tached to the spinal chord by several distinct motor and sensitive filaments. The 

 trunk passes to the upper part of the arm before it divides, giving off, however, 

 collateral branches to the muscles of the scapular and clavicular regions ; also a 

 branch to the pectoral muscles and a circumflexus humeri. The remaining por- 

 tion of the nerve forms the ulnar and radial branches. 



III. IV. V. VI. are the smallest pairs of the spinal nerves. They all arise by 

 single anterior and posterior roots, and are mainly distributed to the muscles and 

 skin of the back and the abdomen. The distribution of the cutaneous nerves will, 

 however, be described hereafter. 



Lumbar Nerves, VII. VIII. IX. These likewise all arise by single anterior and 

 posterior roots from the spinal chord, but they are very much longer than those of 

 the preceding pairs, in consequence of the greater distance of their origins from 

 the intervertebral foramina, where the anterior and posterior filaments unite. The 

 three lumbar nerves thus formed descend to the neighborhood of the hip joint, 

 where the eighth and ninth pairs become intimately blended (c), and at the same time 

 receive an anastomosing branch from the seventh (6). After giving off the branch just 

 referred to, the seventh pair makes a turn outwards and upwards, so as to give its 

 terminal filament to the posterior walls of the abdomen ; from the convex side of 

 its curved portion are given off numerous branches, which descend in a parallel 

 series and are distributed to the skin of the thigh [g). This nerve, from the fact that 

 it escapes at the inguinal region, and in consequence of its distribution to the leg, 

 may be regarded as identical with the criiralis. 



By the union of the eighth and ninth, with a portion of the seventh, a large 

 sciatic trunk is formed, and this descends nearly to the knee without division, 

 giving off, however, as collateral branches, a glutajal nerve (e), and two larger 

 muscular nerves to the thigh (/). At the knee the sciatic is divided into two 

 branches, the tibial and peroneal nerves (h and i). 



