NERVES OF THE SPINAL AXIS. 615 



a complete network, the principal chains of which have in general a longi- 

 tudinal extension, and follow the course of the longitudinal fasciculi of the 

 anterior pyramids. The general relations of the Sensory tract were repre- 

 sented by Sir Charles Bell in the accompanying diagram (Fig. 224), in which 

 the Medulla is opened on its posterior aspect, the restiform columns sepa- 

 rated and turned aside so as to bring into view the posterior pyramids, some 

 of the fibres of which may be traced upwards into the Thalami Optici, 

 whilst they pass through the posterior pyramids into the posterior portion of 

 the lateral columns, and also into the posterior columns of the Spinal Cord. 

 It will be seen, however, from the foregoing description, that the posterior 

 and lateral columns of the Cord to a great extent terminate in the lower 

 part of the Medulla Oblongata, and that the posterior pyramids and resti- 

 form bodies, with their gray nuclei, are essentially new formations, the real 

 function of which is as yet only partially determined. The Motor tract 

 (Fig. 225) is brought into view by simply raising the superficial layer of 

 the Pous, and following upwards and downwards the longitudinal fibres 

 which there present themselves. These fibres may be traced upwards into 

 the Corpora Striata, and downwards into the anterior pyramids and a por- 

 tion of the olivary columns, so that they connect the Corpora Striata with 

 the anterior, and with the anterior portion of the lateral columns of the Spinal 

 Cord. 



489. Nerves of the Spinal Axis. With the Spinal Cord (in its limited sense) 

 there are concerned thirty-one pairs of nerves ; each of which corresponds to 

 a vertebral segment of the body, and has two sets of roots, an anterior and 

 a posterior, differing in their functional endowments, as already described 

 ( 461 ). The anterior roots are usually the smaller ; and this is particularly 

 the case with those of the cervical nerves, in which the posterior roots are of 

 remarkable comparative size. In the first Cervical or "suboccipital" 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 Sympa- 

 thetic nerve. In addition to these, however, as we have seen, the cranial 

 prolongation of the Spinal Axis is the centre of all the cephalic nerves, the 

 functions of which, since they are for the most part distinguished by the 

 peculiarity of their endowments, require to be separately noticed. 



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

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

 ordinary Spinal nerves, than does any other of those originating within the 

 cranium. It possesses two distinct 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 ganglion, known as the " Gasserian ;" and the fibres 

 arising from the smaller root, the number of which has been estimated at 

 9000 to 10,000, do not blend with those of the larger, until the latter have 

 passed through this ganglion. The trunk of the nerve separates into three 

 divisions, the Ophthalmic, the Superior Maxillary, and the Inferior Max- 

 illary ; 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 distribu- 

 tion 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 



