1376 HUMAN ANATOMY. 



visceral divisions. Such typical division, however, is displayed only by those spinal nerves dis- 

 tributed to that part of the trunk in which the primary segmentation is retained, namely, the 

 thoracic region, where the skeletal muscular, and vascular segments, as well as the nerves, 

 retain their identity. In the other parts of the spinal series, the cervical and the lumbo-sacral, 

 where provision is made for the supply of the highly differentiated musculature of the ex- 

 tremities from a number of cord-segments, the nerves early unite to form plexuses from which 

 the limb-trunks grow out, an arrangement well adapted for the distribution of fibres from 

 different sources without undue multiplication of nervous paths. Concerning the factors which 

 guide the young nerve to its destination with such remarkable constancy, nothing is known, 

 but it may be assumed that these are probably influences of a physical character, the developing 

 nerve taking the path offering least resistance. The visceral division of the spinal nerve, to 

 which reference has been made, corresponds to the white ramus communicans given off by 

 certain of the thoracic and lumbo-sacral nerves. These splanchnic fibres differ from the 

 somatic efferent ones in taking their origin from cells which occupy a more lateral position 

 within the gray matter of the spinal cord than do the root-cells giving rise to the motor fibres 

 destined for the skeletal muscles. Whilst the great majority of the splanchnic fibres reach the 

 ramus of communication by way of the anterior root, some few perhaps traverse the posterior 

 or sensory root and its ganglion before continuing their course to the sympathetic. The sensory 

 fibres described within the anterior roots of the spinal nerves are not actual constituents of these 

 roots, which are exclusively motor, but recurrent meningeal twigs destined for the membranes 

 of the cord. 



The Cranial Nerves. — From the preceding account of these nerves, it is evident that the 

 optic nerve differs morphologically widely from an ordinary nerve, since it may be regarded as 

 a modified outlying portion of the brain. Its development may be omitted, therefore, from 

 this series and appropriately considered in connection with the development of the eye (page 

 1482). There is sufficient reason, as will appear later, for regarding the hypoglossal ner\-e as a 

 cranially displaced member of the spinal series. Of the remaining nerves, only the olfactory 

 and auditory are purely sensory ; the third, fourth, sixth and eleventh are exclusively motor ; 

 and the fifth, seventh, ninth and tenth are mixed, the motor strands taking origin from the neu- 

 rones within the brain-stem, while the sensory ones are derivations from the neurones lying 

 within the ganglia connected with the afferent fibres. Although at first sight the trigeminus 

 closely corresponds to a spinal nerve in the possession of a gangliated sensory and a 

 motor root, critical examination of the origin of its motor fibres discloses an important differ- 

 ence, namely that they arise from the lateral nuclei and not from the mesial, which correspond 

 to collections of ventral root-cells. A similar difference also appears between the efferent 

 trigeminal fibres and those of the eye-muscle nerves, the latter arising from groups of root-cells 

 occupying a position close to the mid-line. In order to appreciate the significance of this differ- 

 ence, reference must be made to the primary division of the musculature of the head already 

 referred to in connection with the grouping of the muscles (page 472 ). It was there pointed 

 out that it may be assumed that the segmented condition of the trunk musculature, as expressed 

 by the metameres, is continued into the cephalic region but with subsequent suppression of the 

 middle members of the possible nine or ten segments which constituted the original quota of 

 head-metameres. Of those persisting two groups are recognized — one including the first three 

 metameres, giving rise to the ocular muscles and being supplied by the third, fourth and sixth 

 nerves ; the other including the last three or four, producing the tongue-muscles, and being sup- 

 plied by the twelfth nerv-e. To these groups of cephalic metameres is added a third, the 

 branchiomeres, which are regarded as representing a supplementary series connected with the 

 branchial arches and not present in the trunk. The branchiomeres receive the mixed cranial 

 nerves, whose motor filaments supply muscular masses surrounding the visceral tubes ( digestive 

 and respiratory), and arise from the lateral motor nuclei. It follows that none of the cranial 

 nerves contain fibres from all these sources, in the case of the fifth, seventh, ninth and tenth, the 

 fibres being derived from the lateral motor and the sensory- nuclei, and in the case of the third, 

 fourth and sixth, from the mesial (ventral) nuclei alone. From the primary conditions, as 

 revealed by studies on the lower vertebrates, it is probable that the dorsal fibres also are by no 

 means of similar morphological value, since some represent a somatic sensory system, as those 

 distributed to the integument, and others belong to a visceral sensory one, as those distributed 

 to the walls of the mouth, pharynx and larynx. Following the principle already emphasized, 

 the mentor fibres of the cranial nerves grow from the brain outward, while the sensory ones extend 

 centrally from the ganglia of the nerves associated with the brain. The cranial and spinal nerves 

 appear on the surface of the neural tube at a very early period, their presence being conspicuous 

 by the end of the fourth week (Fig. 1141). 



The olfactory nerve is developed in connection with the epithelial lining of the primary 

 olfactory pit (page 1429). As early as the end of the first foetal month, in the human embryo, 

 cells corresponding to neuroblasts appear in the anlage of the olfactory organ. From these 

 elements processes soon grow brainward, nucleated tracts indicating the formation of the later 

 olfactory fibres. The cell-bodies of the young neurone migrate so that for a time their position 



