The main cord of the sympathetic appears to be entirely composed of fibres presenting there were no tubular nerve-fibres present ; it consisted entirely of structures similar to those already described. As regards the communi cating branches, all of them contained more or fewer tubular nerve-fibres ; some appeared to be entirely composed of these, while others consisted chiefly of the partially-developed nerve-fibres. The difference in point of struc ture between the fibres in the cerebro-spinal nerves and those occurring in the sympathetic is at this period of embryonic life very re markable : while the former present for the most part the tubular character of the per fectly-formed nerve-tube, the latter appear to consist of a mass of blastema with numerous granular nuclei imbedded in it, corresponding, in short, to the fibres of the cerebro-spinal system in the fcetus measuring 6 to 8 inches in length. This also applies, though perhaps in a less degree, to the ganglia of the sympa thetic as compared with those on the cerebro spinal nerves, the latter being more fully developed, both as regards their ganglionic corpuscles and nerve-fibres, than the former.
As regards the further development of the nerve-fibres of the sympathetic, it would ap pear, from the observations of Volkmann and Bidder, that they undergo little further change during the whole period of embryonic life.
At least, in embryos near the full time, they observed little change in the sympathetic nerve-fibres.
It has been already stated that in the sym pathetic of embryos of 18 or 19 inches in length there are some tubular fibres present ; these, probably, are to be regarded as fibres sent from the cerebro-spinal system.
Puystormov.—The actions which take place in the animal body may be divided into two classes. Those which are included in the one class are entirely under the guidance of volition ; those which belong to the other not only take place independently of any effort of the will, but are also rnore or less completely removed beyond its control. The movements which occur in the muscles of the limbs, and in most of the muscles of the trunk, form examples of the former ; while the movements of the internal muscular organs, such as those of the heart, intestinal canal, and genito-urinary organs, afford ex amples of the latter. To the latter also be long the acts of nutrition, secretion, &c., commonly termed the vegetative processes. Several of the latter, as the movements of the heart, go on without interruption during the entire life of the individual ; while others, as the movements of the intestinal canal, take place at irregular intervals, depending ap parently. on the application of external stimuli to the free surfaces of the organs in which they are manifested. The exercise of the former class of actions is moreover attended by sensation; that of the latter, in the normal condition, not. The impressions which are constantly being made by the blood upon the inner surface of the heart and vessels never reach the sensorium ; we are also insensible to the impressions made by the food upon the free surface of the intestinal canal, as well as to the contractions thereby induced. In like
manner, the acts of nutrition and secretion take place entirely without our knowledge. The feeling of weariness also which ensues after exertion of the voluntary muscles, is never felt so far as the heart is concerned, although its action is constant, and just as little in re gard to the other organic muscles.
The organs in which the former class of actions takes place are supplied with nerves which proceed directly from the brain and spinal cord ; those whose actions belong to the second class derive their nerves chiefly from the sympathetic.
Guided by this difference in character be tween the vital phenomena, Bichat divided life into animal and vegetative ; the former characterised by the circumstance of its phenomena coming within the range of sen sation and volition ; the latter including those acts which are more or less completely re moved beyond the sphere of the will and of the consciousness. In accordance with this division, he also separated the nervous system into two portions : the one corre sponding to the cerebro-spinal system, pre siding over the functions of animal life ; the other corresponding to the sympathetic, pre siding over the involuntary movements, and over the processes of nutrition and secretion, or functions of vegetative life. The sympa thetic and its ganglia are, according to the views of Bichat, entirely independent of the cerebro-spinal system of nerves. The various ganglia of the sympathetic he regarded as so many distinct nervous centres, each presiding over the actions of the parts to which it sends nerve-filaments, and each discharging its func tions without any relation to the brain or the spinal cord. The involuntary nature of the processes which take place in organs supplied by the sympathetic, as well as the circum stance that the normal impressions which are made upon the free surfaces of these do not reach the sensorium, rendered the views of Bichat highly probable. In diseases of the brain and spinal cord, as in tetanus and chorea, where the muscles supplied by cerebro spinal nerves are all thrown into a state of more or less violent contraction, the muscular organs which derive their nerves from the sympathetic, such as the heart, continue their movements as before. So also a stimulus ap plied to the brain or spinal cord, causes con tractions in the muscles which derive their nerves from these parts, but does not, ac cording to Bichat, produce any effect on the movements of parts which are supplied by the sympathetic. The fact that embryos in which the central masses of the nervous system are wanting may reach an advanced stage of development, showed that the pro cesses of vegetative life might go on perfectly, independently of the influence of the cere bro-spinal system, while the circumstance that in these the sympathetic system of nerves was always present, and in a high state of develop ment, seemed at the same time to indicate the connection subsisting between it and the processes in question.