DEVELOPMENT OF THE NERVES. 761 



•which they permanently occupy, and after they have attained some size 

 they iniite with the posterior roots beyond the ganglion. 



But although the roots of the nerves may thus be traced in their 

 commencement to elements of epiblastic nature, it is probable that 

 their sheaths and blood-vessels arise from mesoblastic tissue introduced 

 later into them. The ganglia are at first of proportionally very large 

 size, causing even a considerable part of the projections on the surface 

 of the body, as in the human embryo of from four to seven weeks, 

 ■which are usually attributed to the vertebrate segmentation (Kolliker). 

 The union of the roots with the grey matter soon becomes apparent, 

 being most obvious in the anterior roots. 



The nerves, like the other elementary parts, are at first composed 

 entirely of cells, but fibres are soon formed by transformation of the 

 cells. On the sixth day in the chick, Foster and Balfour found the 

 fibres developed, but were unable to trace them into connection with 

 the ganglionic cells, but at a later period the connection was observed 

 by Lockhart Clarke to be formed. 



The very early development of the trunks of the nerves of the limbs, 

 and their progress outwards into the first part of the commencing limb, 

 were observed and figured by Remak. 



Cranial Nerves. — The optic nerve and the retina, arise from epi- 

 blast by an extension of the primary medullary wall of the brain, as 

 already described, and may therefore, in some sort, be regarded as an 

 extension of the brain itself. The olfactory tract and bulb are still 

 more to be looked upon as constituent parts of the cerebrum ; but the 

 manner in which the peripheral olfactory nerves which pass through the 

 cribriform plate into the nose are formed has not been yet observed. 



Among the other cranial nerves there are four important ones of which 

 the rudiments are seen to be formed at an early period ; taking their 

 origin as has been supposed in the mesoblastic wall of the cranial cavity, 

 and extending thence into the facial or visceral plates. These are the fifth 

 pair or trifacial, the facial or portio dura of the seventh pair, the glosso- 

 pharyngeal and the pneumogastric nerves. The two first of these are 

 situated in front, and the two last behind the otic vesicle and tympano- 

 eustachian passage ; and according to Parker each of these nerves forks 

 or divides into two above one of the visceral clefts. Thus the fifth pair 

 gives its naso-maxillary division in fi-ont, and its infero-maxillary 

 division behind the oral cleft ; the facial gives its vidian or superior 

 petrosal before and its descending part behind the tympano-eustachian 

 passage ; the glosso-pharyngeal has its lingual and pharyngeal branches 

 divided by the first branchial cleft, and in animals the pneumo-gastric 

 is similarly divided at the second and succeeding clefts. 



The third, fourth and sixth pairs of nerves are of subordinate import- 

 ance, and may be considered as related, the two first to the fifth pair, 

 and the last to the facial nerve. Their peripheral parts are developed 

 in connection with the muscles of the eyeball, but the mode of the 

 formation of their roots in connection with the nervous centres has not 

 been ascertained. 



The hypoglossal nerve, although it passes through the exoccipital 

 bone in man, may be compared to a spinal nerve, and probably takes its 

 origin much in the same manner. 



The Sympathetic Nerves. — Remak observed the development of 

 the great sympathetic nerves from the lateral plates in loops or arches 



