. Outlines of zoology. Zoology. 442 STRUCTURE OF VERTEBRATA. into "dorsal," "ventral," and other branches. As these typically innervate a gill arch and slit, as may be well studied in 9, the branches may be called (as Beard proposes) supra-branchial (dorsal), post- branchial, prte-branchial, &c. In the course of growth the nerve often shifts from the position whence its root originated. (2) Some of the cranial nerves mark distinct segments of the head, while others are secondary derivatives. It is likely that i, 3, 5, 7, 8, 9, and several parts of 10 mark segments. It i
. Outlines of zoology. Zoology. 442 STRUCTURE OF VERTEBRATA. into "dorsal," "ventral," and other branches. As these typically innervate a gill arch and slit, as may be well studied in 9, the branches may be called (as Beard proposes) supra-branchial (dorsal), post- branchial, prte-branchial, &c. In the course of growth the nerve often shifts from the position whence its root originated. (2) Some of the cranial nerves mark distinct segments of the head, while others are secondary derivatives. It is likely that i, 3, 5, 7, 8, 9, and several parts of 10 mark segments. It is possible that the oculo- motor is a ventral root associated with the third or ciliary nerve, that the trochlear is a ventral root of the trigeminal, that the abducens is a ventral root of the facial. (3) It is possible that each truly segmental nerve supplied a primitive gill slit, as 7 supplies the spiracle, 9 the first branchial, 10 the second, third, fourth, and fifth branchials. (4) It is likely that each segmental nerve was associated with a branchial sense organ (Beard and Froriep). These organs arise above the gills, and grow thence into various parts of the head, and along the trunk as the " lateral ; It is possible tfet a branchial sense organ. Fig. 141.—Diagrammatic section of Spinal Cord. Posterior fissure; ^ posterior column of white matter; , dorsal, posterior, sensorj' or afferent, root; ^., ganglion; , ventral, anterior, motor or efferent, root; c,n., compound spinal nerve witli branches; ^., sympathetic ganglion; , anterior column; the anterior fissure is exaggerated; ^.c, a ganglion cell; , grey matter ; !c., white matter. lay over each primitive gill cleft, and had an associated ganglion. The ganglia known as ciliary, gasserian, &c., may be the ganglia of branchial sense organs, and it seems that parts of them arise in developinent independently of the brain. It may be that nose and ear were origina
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1895
