Articles from "The reference handbook of the medical sciences" (William Wood & Co., New York, third edition, 1914) / by C. Judson Herrick.

  • Charles Judson Herrick
Date:
[1914?]
Catalogue details

Licence: In copyright

Credit: Articles from "The reference handbook of the medical sciences" (William Wood & Co., New York, third edition, 1914) / by C. Judson Herrick. Source: Wellcome Collection.

    11/44 page 327
    Cranial Nerves tube and the adjacent ectoderm to form the neural crest, from which a part of the cranial and spinal ganglia and of the sympathetic system are to be de- rived. And in the head region farther laterally patches of thickened sensory epithelium are differ- entiated to form ectodermal placodes. These lie in two rows, one more dorsal, the suprabranchial series, and one more ventral, the epibranchial series (Fig. 1589). The latter are formed in intimate relation with the external openings of the gill clefts. Fiq. 1588.—The Medulla Oblongata and Cerebellum of the Lake Sturgeon (Aci-penser rubicundus), to Show the Longitudinal Columns which are Developed as Centers for the Four funda- mental Functional Systems of the Peripheral Nerves. A dorsal view with the chorioid plexus of the fourth ventricle removed. B, C and D, sketches of sections at the levels indicated by the refer- ence lines. The dark area with the light circles is the somatic motor column, continuing the ventral horn of the spinal cord. The dark area with rectangular spaces is the visceral motor column, continuing the lateral horn of the spinal cord. The area with oblique cross hatching is the visceral sensory column, or lobus visceralis, containing both general visceral and gustatory centers. The area with vertical cross hatching is the somatic sensory column, containing below the general cutaneous centers and above the special somatic sensory nuclei (tuberculum acustieum and lobe of the lateral line). (After Johnston.) 1. Lobus lineae lateralis; 2, tuberculum acustieum; 3, fasciculus longitudinalis medialis; 4, lobus visceralis. In the trunk region the neural crests become seg- mented and their cells crowd together in each segment and migrate ventrally (Fig. 1592). Some cells are di- rectly transformed into spinal ganglion cells, others migrate outward to form sheath cells of the periph- eral nerves, while still others enter the sympathetic nervous system. In the head the development of the cranial nerve ganglia is much more complex. The neural crest, as in the trunk, becomes segmented and contributes important components to the cere- bral ganglia. Rudiments of the neural crest extend forward beyond the trigeminus ganglion, probably representing vestiges of segmental nerves which have disappeared in the course of vertebrate evolution. Figs. 1590 and 1591 illustrate forms taken by the neural crest in two species of fishes, and Figs. 1592 and 1593 in human embryos. Embryo of Petromyzon ia the Region of the Trigeminus Ganghon. (After von Kupffer.) During the process of differentiation of cells of the cranial ganglia from the neural crest the suprabran- chial and epibranchial placodes are undergoing an independent differentiation in the ectoderm, and finally cells are proliferated from the placodes which Fig. 1590.—A Diagram of the Head of the Embryo of Petro- myzon at a Stage when Two Gill Clefts are Open and the Neural Crest is Segmented into the Rudiments of the Cranial Ganglia. From the leftside. The suprabranchial (dorsolateral) placodes are indicated by vertical and horizontal cross lines, the epibranchial placodes by oblique lines. lAfter Koltzoff, from Johnston’s Nervous System of Vertebrates, P. Blakiston’s Son & Co.) enter into the ganglia of some of the cranial nerves. The exact role played by placodal cells in the forma- tion of the cerebral ganglia is not in all cases clear and it appears that there is considerable diversity among different species of vertebrates in this respect.
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