SYMPATHETIC NERVES. 81 



subcallosus of Zuckerkandl) are developed. These parts are separated even in the 

 adult by a sulcus, along which the mesial or internal olfactory root runs. 



The olfactory nerve fibres arise, according to His, from neuroblasts which 

 become formed within the thickened epiblast of the olfactory area (see p. 95). 

 This epiblast at a certain period of development resembles the neural epiblast, and 

 whilst some of the cells become spongioblasts, others become pear-shaped, or spindle- 

 shaped, and their processes grow as nerve fibres towards the olfactory lobe. Not 

 only, however, do these fibres emerge from the olfactory epiblast, but some of the 

 neuroblasts themselves also pass out, and these form a ganglion which lies between 

 the olfactory lobe and the olfactory area. Subsequently this ganglion, the cells 

 of which are prolonged at either end into nerve fibre processes, becomes attached 

 to and partially invests the olfactory bulb, with which it ultimately blends, forming 

 the part whence the olfactory nerve fibres pass to the Schneiderian membrane 

 (layer of olfactory-glomeruli and nerve fibres), whilst bands of fibres on the other 

 hand grow centripetally and become the olfactory roots. These are in fact 

 comparable to the centripetal (so-called " ascending ") roots of the trigeminus, 

 glossopharyngeal, and vagus. 



It is not until the third month that the part of the olfactory lobe which forms 

 the bulb, begins to grow forwards away from the trigonum, and thus to form the 

 olfactory tract. 



The cranial nerves, except the optic and olfactory, and the relations they bear to one 

 another and to the visceral arches of the head, are shown in fig 1 . 91 as they occur in the human 

 embryo of about four weeks. Fig. 92 distinguishes diagrammatically the nerves which grow 

 into the nerve centres (centripetal or afferent nerves) from those which grow out from the 

 centres (centrifugal or efferent nerves), and the extent of growth inwards of the former in the 

 same embryo. 



The sympathetic nerves and ganglia. That these are merely outgrowths 

 of the cerebrospinal nervous system, nearly all recent observations, both morpho- 

 logical and physiological, clearly show. But even before this fact had come to be 

 generally recognized, it was known that they are developed in connection with the 

 spinal nerves (Balfour), and indeed as offshoots from the posterior spinal ganglia 

 (Schenk and Birdsell, Onodi). They appear for a time as enlargements upon the 

 main stem of each spinal nerve, but afterwards become connected with this by a 

 short branch (r. communicans) (fig. 88), and with one another by a longitudinal 

 commissure. The branch in question contains the splanchnic fibres of the spinal 

 nerve, and the sympathetic ganglia are its splanchnic or vagrant ganglia (Gaskell). 

 The splanchnic ganglia of the cranial nerves are probably formed in a similar way, 

 but their mode of development has not as yet been worked out. 



The ciliary ganglion appears to be formed as an outgrowth of the Gasserian ganglion (fig. 

 91, e.g.} much in the same way as the sympathetic trunk ganglia are formed as offshoots of the 

 posterior spinal ganglia. In elasmobranchs it is derived from the ophthalmicus profundus 

 ganglion, itself an offshoot of the Gasserian (Ewart). 



Paterson has recently described the sympathetic chain of ganglia as developing in mammals 

 (rodents) from a continuous rod of mesoblast lying on either side of the aorta, and as becoming 

 only secondarily segmented and connected with the cerebro-spinal nerves. But observations 

 upon earlier embryos than were used by Paterson are necessary before the mesoblastic origin 

 of the rod can be admitted. 



RECENT LITERATURE. 



Ahlborn, Ueber die Bedeutung der Zirbeldruse. Zeitschr. f. wissensch. Zoologie. Bd. xl., 1884. 



Barnes, Will., On the development of the posterior fissure of the spinal cord and the reduction of 

 the central canal in the pig. Proc. Americ. Acad. Arts and Sciences, 1884. 



Beard, J., On the cranial ganglia and segmental sense organs of fishes. Zoolog. Anzeiger, 1885 ; 

 The system of branchial sense organs and their associated ganglia in Ichthyopsida. Quarterly Journal 

 of Micr. Science, 1885 ; The development of the peripheral nervous system of vertebrates. Quarterly 

 Journal of Micr. Science, Oct., 1888. 



Bedot, M., Redierches sur le developpement des nerfs spinaux chez les Tritons. Recueil zoolog. 

 Suisse, i., 2, 1884. 



VOL. I. G 



