rhombencephalon (see BRAIN), '3l1Cl the sensory are the axons and dendntes of cells situated in ganglia which have budded off from the brain. The evidence of comparative anatomy, however, shows that there are two ventral roots to one dorsal. In the fishes and higher Vertebrates. the dorsal and ventral roots unite, though in selachian (shark) embryos F. M. Balfour says that the dorsal and ventral W roots alternate (The Development of Elasmobranch W:  :¥ = 7 ' ' .4.l T h A Fishes, London, 1878). When limbs are developed, " - EL V x, v . ' beginning with fishes, limb plexuses are formed. b? 1f>'~ =fi . gr l Where the limbs are suppressed rudimentary A 'ii plexllll/es hma}d'perS1St' as in the Snake' th°“gh “sua Y f el' 1SaPP“'af~

-.'- - . »; The cranial nerves are only represented by two pags in Almghioxns. In the Fyclostomata, fishes

• °-  ;§ = an. ¥“P 1 la' ¥@“.Pa1FS ° "ewes are f°““d~"~

Ao,  ». -.<.' 2 which In the1r distribution do not always agree . With those Of man- In the AmHi0ta Of-f@DU1@S. A iff# LJ' birds alnd mtammalgh the eleventh and twelfth xx ', , nerves- ave een a e The researches of W. 6 n *"  »' - Gaskell (“ in the structulnre, distribution and " A "" " ~»l§ ;.' " ., f unctions o the nerves w ich innervate the visceral and vascular systems, " J. of Phys. vii. """' 1"' ' ¢ . 4- I, 1886), O. S. Strong (“ The cranial nerves of Amph1bia, ” J. Morph. x. IOI), J. B.

— Johnston (J. Comp. Neurol. xii. 2 and 87), “, . gg / ~ and others, show that the cranial nerves are n  ;§ f!, '§ j§ i Q formed of at least five components: (I) Ven~ AVS; f éfi A tral motor, (a) Lateral motor, (3) Somatic f" 'i' f =§ ' " " SQHSOYY, (4) V1SC€f<11 Sensory. (5) Lateral hue —~~ / “ewes

~ ""'1 The 'ventral motor components are those which -; V V, , rise from cells situated close to the mid line, and probably correspond to the ventral roots of the spina nerves. he nerves to the eye muscles (motor oculi, trochlearis and abducens) have this is 5 . ~, origm (see N ERVE: Cranial), as also has the hypo-13 glossalywhich doubtless is a cephalized spinal " ' nerve-23

- The lateral motor components rise from cells situ: ated more laterally, and comprise the motor roots == ' f== ""' *fl " F » oi, the fifth (trigeminal), seventh (facial), and 3 0

From A. M. Paterson, in Cunningham's Texhbouk of Anatomy. FIG. 7.—Development of the Spinal Nerves.

A Formation of nerve roots

ninth, tenth and eleventh (glossopharyngeal, vagus and spinal accessory). These nerves

supply muscles belonging to the bronchial

skeleton, instead of the muscles of the primi-C. F0rmati<>r1 Of 1'1€1'V'fS- ~ » five cranium, of which the eye muscles are the eg' gorsallmot' ' So, Somatic division. 1'€mH21r1fS-Nentréi mg; Vi, Visceral branch. .The somatic sensory components supply the - 1 'fum tu ' P Postel-lor primary division skin, and end In cells which, among the cyclo!-7;/'EBC iigtochord. I ' i stomes and fishes, form a considerable elevation memary Cana ' D, E, Formation of subordinate branches. 1I1 the rh0mb€I1C€Dh2110f1. kflOWr1 35 the lobus 412170. .éorEa» I Lat, Late, -al, and tr1gem1n1'(fig. 8, Nuo. V.). These components, M P Nfl; Ta rem' Ant, Anterior, branches, in the lower orms, are found In the fifth, seventh < » U-Q9 Pate~ and tenth nerves, but in mammals practically B, Formation of nerve trunk (N) F, Formation of nerve trunks in rglation only the fifth C0l'llI&111S them. They corres ond P

D.G, Dorsal ganglion. to the limb; dorsal and ventral to the d01'5al 1'00fS Of the Splflal Y1@fV@S-Sy, Sympathetic cord. trunks corresponding to lateral and The 5Pll"»Qh"1'6 -Y€"~°0f}' 91' W-95970 56"-YOU Wm W.D Wolffian duct. anterior trunks in D-and E ' ponents end in the brain In the medulla ' cells 1'Y

Co, Coelom. i g

the cranial nerves cannot be directly homologized with the spinal, nor can the fact of there being twelve of them justify us in assuming that the head contains the rudiments of twelve fuse or unsegmented somites.. To this we will return later. The case of the optic nerve is different to that of any of the others. A. Robinson (Journ. Anat. and Phys., vol. 30, p. 319) has shown that most of its fibres are the axons of ganglion cells in the retina, and, as the retina is part of the optic vesicle and an outgrowth from the brain, the so-called optic nerve is only comparable to a tract of fibres within the brain. The twelfth or hypoglossal nerve is regarded as a fusion of the motor roots of three spinal nerves, and embryology bears' this out, for Froriep has described a small and transitory ganglion corresponding to the posterior root ganglion of this nerve. ' Another link in the chain of reasoning is that the first spinal orsub-occipital nerve often has its posterior root suppressed.

The sympathetic system is developed from the posterior root ganglia of the s inal nerves, by cells which in man migrate a few at a time. A. Paterson, however, believes that the sympathetic is developed, independently of the cerebro-spinal system, in the mesoderm (Phil. Trans. clxxxi. pt. B. p. 159). In embryos of 14-5 m.m. there are found masses of 'cells on each side of the abdominal aorta, permeated with blood vessels, and having the same structure as the carotid and coccygeal bodies. They are known as the, organs of Zuckerkandl, and disappear soon after birth. I COMPARAIIVE ANATOMY,

The comparative anatomy of the brain and spinal cord is dealt with in the separate articles devoted to them.-Spinal Nerves.-In Amphioxus the dorsal and ventral roots do not unite with one another but alternate, a dorsal root on one side being opposite a ventral on the other. The dorsal roots are both sensory and motor, the ventral only motor. In the Cyclostomata (Petromyson) the arrangement is nearly the same, but in some regions known as the fasciculus communis in fishes, and fasciculus solitaries in mammals (see BRAIN, ii . 4), as well as in the lobus trigemini and lobus vagi (fig. 8, Nnc. XY). They are found 1n the fifth, seventh, ninth, tenth and eleventh nerves, and supply visceral surfaces. In mammals the lingual and palatine 1 its Wg I

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FIG. 8.-Brain (A) and Choroid Plexuses (B) of Lamprey. branches of the fifth, the chorda tympani and great superficial pctrosal (?) of the seventh, and all the sensory fibres of the ninth and tenth except Arnold's nerve, represent these. In fishes. and

Amphibians the palate' is supplied by the seventh nerve instead of