The Myelon
The spinal canal, and the cord which it contains, are lined by continuations of the three membranes which protect the encephalon. The cord is sub-cylindrical, and contains a median longitudinal canal, the canalis centralis, the remains of the primitive groove. It is divided by anterior and posterior median fissures into two lateral halves, which are, usually, connected only by the comparatively narrow isthmus, which immediately surrounds the canalis centralis. The cord may, in the adult, extend through the whole spinal canal, or it may come to an end at any point between the caudal extremity and the anterior thoracic region.The distribution of the two essential constituents of nervous tissue, ganglionic corpuscles and nerve - fibres, is very definite in the spinal cord, ganglionic corpuscles being confined to the so-called "gray matter" - which constitutes the isthmus, and spreads out into two masses, each of which ends in an anterior (or ventral) and a posterior (or dorsal) horn. Nerve fibres also abound in the gray matter; but the so-called "white matter," which constitutes the external substance of the cord, contains only the fibrous nervous matter, and has no ganglionic corpuscles.
The spinal nerves arise in opposite pairs from the two halves of the cord, and usually correspond in number with the vertebrae through, or between, which they pass out (Fig. 23). Each nervo has two roots, one from the dorsal, and one from the ventral, region of its half of the cord. The former root has a ganglionic enlargement, and only contains sensory fibres; the latter has no ganglion, and exclusively contains motor fibres. (Amphioxus appears to ho an exception to this, as to most other, rules of Vertebrate anatomy.) After leaving the vertebral canal, each spinal nerve usually divides into a dorsal and a ventral branch; but, in the Ganoid fishes, each of these branches is a distinct nerve, arising by its own proper roots.
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| Fig. 23. - A diagrammatic view of the Chief Trunks of the Cerebro-spinal and Sympathetic Nervous Systems of Rana esculenta eeen from below (twice the size of nature). - 1. The olfactory nerves. N. The olfactory sac, II. The optic nerve. 0. Tho eye. L. op. The optic lobes. Ta. Optic tracts passing from the optic lobes to the chiasma, behind which lies the pituitary body. III. Oculomotorius. IV Patheticus. V The trigeminal, with which the abducens (V. J.), facialis (VII.). and the upper end of the sympathetic (VS), are closely connected. Branches of this nervous plexus are V.a, the nasal and ophthalmic branches of the fifth and the abducens. V,b,c,d, the palatine, maxillary, and mandibular branches of the fifth. V, e, the tympanic branch into which the proper facial nerve (VII.) enters, and, with a brancli of the vagus, forms the socalled facial nerve of the Frog, F. VIII. The auditory nerve. X, with its branches X1, X2, X3, X4, represents the glossopharyngeal and the vagus. The medulla oblongata (Myelencephalon) ends, and the medulla spinalis (Myelon) begins, about the region marked by the letter M. M1-10, the spinal nerves. M2, the brachial nerves, M 7, 8. 9, the iachiatic plexus, from which proceed the crural (N. c.) and Ischiatic (N. i.) serves. S. The trunk of the sympathetic. SM. The communicating branches with tho spinal gaoglia, S 1-10. The sympathetic ganglia. |
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