CRANIAL NERVES. 



77 



least in part at the expense of the sensory epiblastic thickenings. In connection with this 

 question it is worthy of note that from the sensory thickening which forms the olfactory area 

 a ganglionic rudiment becomes formed which joins the olfactory lobe of the brain, and gives 

 origin to the olfactory nerve-fibres (His). 



Even in the adult, as was shown by Thomsen, traces of pre-existent ganglionic structure 

 can be found in the root of the third nerve, and similar traces of ganglionic structure have 



Fig. 89. TRANSVERSE SECTION 



THROUGH THE POSTERIOR 

 PART OF THE HEAD OF AN 

 EMBRYO CHICK OF 30 HOURS. 



(From Balfour. ) 



hb, hind-brain ; vy, vagus 

 nerve ; ep, epiplast ; ch, noto- 

 chord ; x, sub-notochordal rod ; 

 al, throat ; ht, heart ; pp, body- 

 cavity ; so, parietal mesoblast ; 

 sf, visceral mesoblast ; hy, hypo- 

 blast. 



also been described by Gaskell 

 in the roots of the fourth 

 nerve, in the motor root of the 

 fifth, and in the root of the 

 seventh nerve. If these, as 

 Gaskell supposes, indicate the 

 pre-existence of sensory ele- 

 ments in the roots, it is pro- 

 bable that these nerves and 

 ganglia have all been origi- 

 nally developed like the posterior gangliated roots of the spinal nerves, as outgrowths from 

 the neural crest. Whether they are joined by outgrowths corresponding to the efferent fibres 

 of the spinal nerves, or whether they originally contain the elements of the efferent fibres, 

 and thus resemble the spinal nerves of Amphioxus, in which there are no anterior roots, 

 but both sensory and motor nerves are contained in the posterior roots, is not at present 

 known. What is however clear is that the ganglion cells and afferent fibres in the roots of 

 the third, fourth, motor of fifth and seventh nerves, eventually entirely disappear, the efferent 

 fibres alone remaining, while in the roots of the sixth, eleventh, and twelfth nerves efferent 

 fibres only are found, and ganglionic rudiments are not developed at all. 



As is shown in another part of this work (Neurology) the nuclei of origin of the efferent 

 cranial nerves are disposed in two longitudinal series. One of these series comprises the nuclei 

 of origin of the somatic efferent nerves of Gaskell, which correspond with the largest fibres 

 of a typical anterior spinal root, and the series of nuclei is a continuation of the cell-column 

 of the anterior horn ; the nuclei of this series are those of the hypoglossal, or twelfth, the 

 sixth, fourth, and third. The other series comprises the nuclei of origin of the splanchnic 

 efferent nerves of Gaskell, which correspond with the medium-sized and smallest fibres of a 

 typical anterior spinal root, and the series is a continuation mainly of the cells of the lateral 

 cornu or intermediolateral bract, and partly, perhaps, of the cells of the base of the posterior 

 horn ; the nuclei of this series are those of the spinal accessory, those of the efferent fibres 

 of the vagus and glosso-pharyngeal, the facial, and the motor nucleus of the fifth. 



According to the observations of His, the distinction into an anterior or ventral 

 (somatic) and a lateral (splanchnic) group of efferent fibres is well marked in the 

 embryo by the fact that the two kinds of efferent fibres take origin from entirely 

 different parts of the basal lamina of the neural tube, those which correspond with 

 the somatic efferent fibres originating from groups of neuroblasts near the middle 

 line, while the others take their rise near the junction of the basal with the alar 

 lamina (see fig. 90). "With the alar lamina itself the afferent fibres are con- 

 nected, but they do not arise from groups of cells within it, as do the efferent fibres 

 within the basal lamina : on the contrary, they effect their connection with the lamina 

 by growing into it from a ganglion in the manner already described for the posterior 

 roots of the spinal nerves, and they then appear in most cases to grow downwards 

 in the direction of the spinal cord, This is stated by His to be the case with the 



