It is usually supposed by naturalists that in the descent of man from some organism low in the scale of existence, he has passed, at a remote period, through a stage resembling the Ascidians or Tunicata. In these animals respiration is maintained by water being driven through a perforated sac in the meshes of which the nutritive fluids of the animal circulate. The contractile motions of the sac by which the circulation of fluid is maintained probably depend on a nervous ganglion situated between the oral and anal apertures as represented in the diagram (Fig. 77). We do not know whether or not this ganglion may influence the circulation which is maintained by the rhythmical contractions of the simple tube which serves as a heart. These drive the fluid first in one direction, and then after a while the action of the tube is reversed, and its contractions drive the fluid in the opposite direction. This ganglion in its functions would correspond with the medulla oblongata in the vertebrata, and thus the medulla oblongata may be looked upon as a lower and more fundamental centre than the brain or spinal cord.

We see this more distinctly perhaps by looking at the two diagrams (Figs. 78 and 79) representing an amphioxus and a fish. In the amphioxus respiration is kept up in much the same way as in the ascidian, the water passing from the pharyngeal to the atrial sac and through the atrial aperture or abdominal pore. There is no head and no organs of special sense, and so we have no brain whatever. But the body is elongated so as to remind us of an ascidian, having its ganglion and the part of the body-wall containing it so much extended as to remove the anal considerably from the oral aperture. The muscles of this elongated body require innervation, and thus the ganglionic mass is elongated into a cord called the myelon, which represents the spinal cord as well as the medulla oblongata. In ascidians then we have a mass corresponding to the medulla; in the amphioxus we have a mass corresponding to medulla and spinal cord.

In a fish the pharyngeal or branchial sac, instead of opening into the atrial sac, opens directly into the surrounding water.

Fig. 77.   Diagram of an Ascidian.

Fig. 77. - Diagram of an Ascidian.

Fig. 78.   Diagram of Amphioxus. The water enters the oral aperture, passes through the openings in the pharyngeal sac into another cavity, whence it escapes by the abdominal pore.

Fig. 78. - Diagram of Amphioxus. The water enters the oral aperture, passes through the openings in the pharyngeal sac into another cavity, whence it escapes by the abdominal pore.

Fig. 79.   Diagram of fish.

Fig. 79. - Diagram of fish.

We have a head and organs of special sense, and therefore we have a large nervous mass or brain.

In these three members of the animal kingdom, therefore, we have the medulla as the lowest or fundamental centre, next the spinal cord, and lastly the brain. We might therefore expect that notwithstanding the apparently higher position and greater nearness of the medulla to the brain than to the spinal cord, the medulla would be less readily affected by many drugs than the cord or the brain, and this is what we find in the case of such drugs as alcohol, ether, or morphine, which appear to paralyse the nervous centres in the inverse order of their development - the brain first, spinal cord next, and medulla last.

There are some drugs, however, e.g. aconite, gelsemium, and hydrocyanic acid, which seem to have a special paralysing action on the respiratory centre.

If we look at the ganglionic mass in an asciclian, represented in the diagram, we shall see that it sends some fibres to the pharyngeal sac and some to the anal sac. If these two sacs were to contract together they would oppose each other's action, and thus the passage of water through the branchial apertures would be stopped, and respiration consequently arrested. They must therefore act alternately, and this alternate action is regulated by the ganglion. This ganglion consists of numerous nerve-cells and fibres. As some of these have a more special connection with the pharynx, the group which they form may be called the pharyngeal centre or inspiratory centre.

Similar arrangements occur in higher animals, and the terms used in regard to their nervous system may lead to some confusion of thought; thus we speak of the respiratory, of the inspiratory, of the expiratory, and of the vomiting centres.

By nerve-centres we simply mean the groups of cells and fibres which are concerned in the performance of certain acts. They are not necessarily entirely distinct from one another, and the same group of ganglionic cells may form a part of several centres. Thus in the accompanying diagram (Fig. 80), the respiratory centre includes both inspiratory and expiratory centres, and the vomiting centre includes some ganglionic groups which form part of the inspiratory, and others forming part of the expiratory centres, besides other ganglion groups which are concerned with the simultaneous dilatation of the cardiac orifice of the stomach. On analysing this subject still further we find also that the inspiratory centre affects many muscles, and that it does not always affect them to the same extent. Thus in men the diaphragm takes a more active share in inspiration during the day than the thoracic muscles. During sleep the diaphragm takes a much less active part, and may be entirely quiet, while the thoracic muscles are more active, and the chest rises and falls more than during walking.

The inspiratory centre might be thus still further divided into thoracic inspiratory centre, and diaphragmatic inspiratory centre.