Text-book of the embryology of man and mammals / by Oscar Hertwig ; translated from the third German edition by Edward L. Mark.

Date:
1892
Catalogue details

Licence: Public Domain Mark

Credit: Text-book of the embryology of man and mammals / by Oscar Hertwig ; translated from the third German edition by Edward L. Mark. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

    490/704 (page 462)
    Since the .spinal cord comes to lie in deeper layerrf of the body far away from its place of origin, the dermal nerves must follow it, and therefore their origins are cori'espondingly farther separated ■from their terminations. Since also, on the other hand, the muscle- plates grow around the neural tube, certain motor and sensory nerve-cords are brought near to each other in their passage to theii- peripheral distribution. And this will occur especially in all cases where the motor and sensory peripheral terminations lie at a gi-eat distance from the origin of the nerves out of the spinal cord, as, for example, in the case of the limbs. The mutual appi'oximation of sensory and motor nerve-tracts thus brought about will finally lead to the formation of common tracts, according to the same principle of simplified organisation in accordance with which the blood-vessels also adapt themselves closely to the course of the nerves. (c) The Develojmie^it of the Sym2)athetic System. The development of the sympathetic nervous system has as yet been investigated by only a few observers. Balfour first announced that it arose in coimection with the cranial and spmal nerves, and therefore was, like the latter, really derived from the outer germ- layer. In the Selachians he found the sympathetic ganglia (fig. 262 sy.g) as small enlargements of the chief trunks of the spinal nerves [si^.n) a little below their ganglia (sp.g). In older embryos, according to Balfour's account, they recede from the spinal ganglia, and then at a later period unite with one another, by the development of a longitudinal commissure, into a continuous cord (Greflzstrang). The origin of the sympathetic system has been the most thoroughly studied by Onodi in researches covering several classes of Verte- brates. According to him the sympathetic ganglia arise directly, as Balfour suggested and as Beard has also lately reiterated, from the spinal ganglia. The ventral ends of the spinal ganglia undergo proliferation, as is best seen in Fishes. The proliferated part de- taches itself, and, as fundament of a sympathetic ganglion, moves ventrally. The fundaments of the individual segments are at first separate from one another. The cord (Grenzstrang) is a secondary product, produced by the growing out of the indi\adual ganglia toward each other and the union of the outgrowths. Afterwards the sympathetic ganglia and plexuses of the body-cavity are derived from this part.