ANATOMY OF THE FROG 61 



The crural nerve arises from the seventh spinal at or just 

 before its union with the eighth, and runs to the ventral and 

 outer border of the thigh, supplying the muscles and skin of 

 that region. 



The sciatic nerve is the largest in the frog's body, and is of 

 great importance in certain physiological experiments. It 

 passes into the leg close to the posterior end of the urostyle, 

 and runs down between the muscles of the thigh on the inner 

 or hinder side, following, in its course, the biceps muscle of 

 the thigh. In the upper part of its course it gives off several 

 branches to the thigh muscles, and a little way above the knee 

 it divides into two branches, the peroneal and the tibial nerves. 

 The former runs down the crus on the outer side of the tibio- 

 fibula, and is eventually distributed to the foot. The tibial 

 nerve runs down the crus on the inner border of the gastroc- 

 nemius muscle, giving off branches to it, and eventually supplies 

 the sole of the foot. 



The tenth spinal nerve, which passes through the small 

 coccygeal foramen in the urostyle, besides contributing a 

 branch to the sciatic, sends twigs to the bladder, the cloaca, 

 and the adjacent parts. 



Each of the spinal nerves arises from the spinal cord by two 

 roots, a dorsal or posterior, and a ventral or anterior root. The 

 dorsal root is also called sensory or afferent, because it is com- 

 posed of fibres, which only transmit impulses from without 

 inwards from the tissues to the spinal cord and brain. The 

 ventral roots are also known as motor or efferent, because their 

 fibres only transmit impulses in the reverse direction, from the 

 cerebro-spinal axis to the tissues. The two roots unite to 

 form one just as they pass out through the inter-vertebral 

 foramina, and the dorsal afferent root has an enlargement or 

 ganglion on it just before it joins the ventral root. The spinal 

 nerves are connected with the sympathetic nervous system, 

 but before describing this it will be well to consider the brain 

 and cranial nerves. 



The brain is directly continuous with the spinal cord, and 

 a small canal, which traverses the centre of the latter, expands 

 into the brain to form a series of chambers known as the 

 ventricles of the brain. 



The brain may be considered as consisting of three parts 

 the hind-brain, the mid-brain, and the fore-brain. 



