Quain's elements of anatomy . 838 THE jS^EEVOUS SYSTEM. marrow. It extends to the roof of the mid-brain, and there is the sameshifting downwards of the attachment of the roots to the neural crestas in the spinal neives. In most of the nerves it has been observedthat a subdivision occurs into portions representing a root, a ganglion,and a peripheral nerve-trunk. The change of the place of attachmentis most remarkable in the case of the third nerve, which is carried downquite to the lower surface of the mid-brain. The sixth and the twelfth (or hypoglossal) nerves may arise, accordingto Marshall,
Quain's elements of anatomy . 838 THE jS^EEVOUS SYSTEM. marrow. It extends to the roof of the mid-brain, and there is the sameshifting downwards of the attachment of the roots to the neural crestas in the spinal neives. In most of the nerves it has been observedthat a subdivision occurs into portions representing a root, a ganglion,and a peripheral nerve-trunk. The change of the place of attachmentis most remarkable in the case of the third nerve, which is carried downquite to the lower surface of the mid-brain. The sixth and the twelfth (or hypoglossal) nerves may arise, accordingto Marshall, as motor or lower roots of certain of the other nerves ; but Fig. Fig. 744.—Transverse section through the posterior part op the head of anEMBRYO CHICK OF 30 HOURS. (From Balfour.)hi, hind-brain ; vg, vagus nerve ; ep, epiblast; ch, notochord ; x, sub-notochordal rod;al, throat ; ht, heart; pp, body-cavity ; so, parietal mesoblast ; sf, visceral mesoblast;hy, hypoblast. the mode of origin of these two nerves, as weU as of the fourth, has notyet been fully ascertained, and Balfour doubts whether the cranial nervesbefore described as arising dorsally from a neural ridge or crest, are, likethe dorsal roots of the spinal nerves, exclusively sensory. Some of them,he holds, are undoubtedly mixed, and all of them may be so. (No. 205.) The olfactory nerve is certainly an otitgrowtli from the fore-brain ; but whetherfrom an extension forwards of the neural crest or not is still doubtful. In theolfactory nerve of mammalian embryoes, A. Eraser has observed the root ganglionand two main divisions of the nerve forking over the nasal pit (see fig. 745). The ori
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