THE FROG 85 



and second vertebrae. It runs forward in the floor of the buccal 

 cavity ventral to the glosso-pharyngeal nerve, and is distributed to 

 the muscles of the tongue and floor of the mouth. The hypoglossal 

 nerve in the mammals has become shifted forward and forms one of 

 the cranial nerves, of which there are twelve in this group, and not 

 ten as in the frog. 



The second is a large nerve running straight outwards. Branches 

 from the first and third, a small nerve, also join it, forming a complex 

 termed the Brachial plexus, which supplies by a large branch, the 

 coraco-clavicularis, the muscles of the shoulder girdle. The main 

 trunk, the Brachial, goes on into the arm, dividing just above the 

 elbow into radial and ulnar branches. 



The succeeding three pairs of nerves, four, five and six, are small 

 nerves supplying the skin and body wall in the trunk region. 



The seventh, eighth and ninth nerves unite in a somewhat com- 

 plex manner to form the large Sciatic plexus well outside the vertebral 

 column. Inside the neural canal these nerves, together with the 

 tenth and the filum terminale of the spinal cord, run downwards in 

 a brush-like group known as the cauda equina. From the sciatic 

 plexus, the exact constitution of which is subject to a certain amount 

 of variation, come off three nerves : the ihohypogastric supplying 

 the muscles of the abdomen, the crural supplying the muscles of the 

 thigh and the large sciatic, which runs down the thigh and divides 

 into tibial and peroneal branches, supplying the muscles and skin of 

 the leg and foot. The last and smallest of the spinal nerves is the 

 coccygeal, which leaves the vertebral column by a foramen in the 

 urostyle and after giving a branch to the sciatic, branches over the 

 bladder, cloaca and surrounding tissue. 



Minute Structure of the Nervous System. 



The structural unit of the nervous tissue is the nerve cell 

 or ganglion cell. A typical nerve cell is very large compared with 

 most other cells of the body, and consists of a cell body from which 

 various processes are given off. The cell body is composed of a 

 basis of ordinary protoplasm, which is very granular owing to the 

 presence in it of a substance, the tigroid substance, in the form of 

 small grains, Nissl's granules. These appear to form a store of 

 reserve material that is utilised in the periods of activity of the cell, 

 for far more of it is present in a cell that has been resting for some 

 time than in a similar cell that has been very active. Appropriate 

 staining also shows in it a number of very fine strands, the neuro- 

 fibrillae, which interlace freely and are continued out into the various 

 processes, linking them up with one another in all possible direc- 

 tions. Near the middle of the cell is a spherical nucleus which is 



