. 



CEPHALIC NERVES. FIFTH PAIR, OR TRIGEMINUS. 683 



SPINAL CORD. MEDULLA OBLONGATA. BRAIN. 



Posterior or Sensory Division. 



Posterior portion f Decussating portion (?) of Posterior Pyramidal "j 

 of Lateral Columns \ columns L Thalami Optici 



{Non-decussating portion (?) of Posterior Pyra- j 

 midal columns ..... J 

 Restiform columns Cerebellum. 



713. Nerves of the Spinal Axis. With the Spinal Cord (in its limited 

 sense) there are connected thirty-one pairs of nerves ; each of these corre- 

 sponding to a vertebral segment of the body, and having a double set of roots, 

 as already described ( 696). The anterior roots are usually the smaller ; and 

 this is particularly the case with those of the cervical nerves, in which the pos- 

 terior roots are of remarkable comparative size. In the first Cervical " or sub- 

 occipital" pair, the anterior roots are sometimes wanting; but there is then a 

 derivation of fibres from the Spinal Accessory, or from the Hypoglossal, or from 

 both. The two roots of the ordinary Spinal nerves unite immediately beyond 

 the ganglion, which is situated on the posterior one ; and the trunk thus formed 

 separates immediately into two divisions the anterior and posterior each 

 of which contains both afferent and motor fibres. These divisions, of which 

 the anterior is by far the larger, proceed to the anterior and posterior parts of 

 the body respectively ; and are chiefly distributed to the skin and the muscles. 

 The anterior branch is that which communicates with the Sympathetic nerve. 

 In addition to these, however, the cranial prolongation of the Spinal Axis is the 

 centre of all the cephalic nerves, save those of special sensation, which termi- 

 nate in their respective ganglia ; and as these cephalic nerves are for the most 

 part distinguished by the peculiarity of their endowments, they require to be 

 separately noticed. 



714. The pair of nerves commonly designated as the Fifth of the Cephalic 

 series, or as the Trigeminus, is the one which more nearly resembles the 

 ordinary Spinal nerves (as was long since pointed out by Sir C. Bell), than 

 does any other of those originating within the cranium. It possesses two dis- 

 tinct sets of roots, of which one is much larger than the other ; on the larger 

 root, as on the posterior and larger root of the Spinal nerves, is a distinct gan- 

 glion ; and the fibres arising from the smaller root do not blend with the others, 

 until after the latter have passed through this ganglion. The trunk of the 

 nerve separates, as is well known, into three divisions the Ophthalmic, the 

 Superior Maxillary, and the Inferior Maxillary ; and it can be easily shown, by 

 careful dissection, that the fibres of the smaller root pass into the last of these 

 divisions alone. When the 'distribution of this nerve is carefully examined, it 

 is found that the first and second divisions of it proceed almost entirely to the 

 skin and mucous surfaces, only a very small proportion of their fibres being 

 lost in the muscles ; whilst of the branches of the third division, a large num- 

 ber are distinctly muscular. Hence analogy, and the facts supplied by anatomi- 

 cal research, would lead to the conclusion that the first two divisions are nerves 

 of sensation only, and that the third division combines sensory and motor 

 endowments. Such an inference is fully borne out by experiment. When the 

 whole trunk is divided within the cranium by the penetration of a sharp instru- 

 ment (which Magendie, by frequent practice, has been able to accomplish), 

 evident signs of acute pain are given. After the incision has been made 

 through the skin, the animal remains quiet until the nerve is touched ; and 

 when it is pressed or divided, doleful cries are uttered, which continue for some 

 time, showing the painful effect of the irritated state of the cut extremity. 

 The common sensibility of all the parts supplied by this nerve is entirely 

 destroyed on the affected side. The jaw does not hang loosely, because it is 



