270 



STUDIES IN PHYSIOLOGY 



cord. Ten pairs of spinal nerves branch off from the cord 

 and are distributed to all parts of the trunk. Three of these 

 pairs (the first, second, and third) are distributed to the 

 arms ; the legs are supplied by the last four pairs. From 

 each of the spinal nerves, branches connect with the chains 

 of sympathetic ganglia which lie on the dorsal wall of the 

 body cavity. Hence, in general plan 

 there is a close resemblance between 

 the nervous system of a frog and that 

 of man (compare Figs. 121 and 126). 



Summary. For convenience we 

 have described the central nervous sys- 

 tem as though it were made up of two 

 distinct parts, brain and spinal cord, 

 each having its separate set of nerves. 

 In reality there is no such division. 

 We must rather consider the brain as 

 a prolongation and modification within 

 the head, of the spinal cord. In the 

 region of the hindbrain this cylindrical 

 rod or rather tube of nerve tissue is 

 enlarged to form the medulla and cere- 

 bellum ; farther forward are the two 

 FIG. 126. Ventral View enlargements of the midbrain, namely, 

 ^e optic lobes; while the greatest 

 expansion is noticed in the cerebral 

 hemispheres and olfactory lobes of 

 the forebrain. 



Each of the ten spinal nerves has a 

 dorsal and ventral root, and is therefore composed cf both 

 afferent and efferent fibers. The ten cranial nerves may be 

 arranged in three groups : the first (olfactory), the second 

 (optic), and the eighth (auditory) pairs always carry mes- 

 sages to the brain, and are therefore afferent; the third, 

 fourth, and sixth are efferent, since they convey impulses to 

 the eye muscles j the four other pairs (fifth, seventh, ninth, 



Spinal Cord, and Sym- 

 pathetic System. 



I-X= cranial nerves. 

 1-10 = spinal nerves. 



