The involuntary nervous system / by Walter Holbrook Gaskell.

  • W. H. Gaskell
Date:
1920
Catalogue details

Licence: In copyright

Credit: The involuntary nervous system / by Walter Holbrook Gaskell. Source: Wellcome Collection.

    19/200 page 3
    which correspond to the infra-cesophageal nerves in the inverte¬ brate, are divisible into cranial and spinal segmental nerves, supplying motor fibres to the voluntary muscles of the cranial and spinal segments respectively. I pointed out in 1889 that the spinal and cranial segmental nerves differed in that the former arose from the central nervous system by two roots, ventral and dorsal, but the latter from three roots, ventral, lateral, and dorsal, and that the cranial region must be looked upon as more primitive than the spinal; so that the three-root system was more likely to give a clue to the segmental arrangement in the invertebrate ancestor than the two-root system. The work of V. Wijhe has given an explan¬ ation of the lateral root; for he has shown how in the embryo of the selachian the mesoblast in this cranial region has a double segmentation, and forms a dorsal and ventral series of segments out of which the muscles are developed. The muscles developed from the dorsal series are the eye muscles, and those supplied by the hypoglossal nerve. Those developed from the ventral series are the muscles supplied by the motor part of the trigeminal, the facial, the glossopharyngeal and the vagus, that is to say the muscles concerned with mastication and respiration. This double segmentation has been called respectively mesomeric and bran- chiomeric ; but because the latter includes the segments concerned with mastication, which have nothing to do with respiration, I called the ventral segmentation the splanchnic or visceral segmen¬ tation in contradistinction to the dorsal, which I called the somatic segmentation. In my book on the “Origin of Vertebrates” I have put forward my conceptions of the manner in which the voluntary muscles of vertebrates arose from the corresponding muscles of invertebrates. In the invertebrate (Limulus, Scorpion, etc.) the animal is divided into segments, which may be considered as forming a double segmentation, that of the body and that of the appendages, so that the voluntary musculature in these animals falls naturally into two groups, (i) the body musculature, (2) the appendage musculature. The body or somatic musculature forms in all cases the great longitudinal muscles and the dorso-ventral muscles. In the vertebrate the longitudinal muscles are found in the trunk region and are divided segmentally to form the
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