The reflex stimuli which were supposed to be elicited through the medium of the submaxillary ganglion, probably depended on the escape of the stimulating electric current used, and the reflexion from a sporadic ganglion, such as the submaxillary, has never been satisfactorily demonstrated.
It has further been shown that direct stimulation of the chorda tympani nerve, although it be cut off from its central connections, causes a copious secretion of thin watery saliva, and this increased secretion is accompanied by a great dilatation of the small arteries going to the gland, so that a pulsation may be seen in the small veins, and the blood retains its bright arterial color when leaving the organ.
These two chief results of stimulation, activity of the secreting cells and vascular dilatation are distributed by different nervous agencies, as appears from the action of atropia, which stops the secretion of saliva, but does not prevent the dilatation of the vessels on stimulation of the chorda tympani; from which we conclude that its effect is restricted to a mechanism engaged exclusively in controlling the activity of the gland cells.
Stimulation of the chorda tympani causes the secretion to be carried on with great energy. The fluid was found to enter the duct with a pressure equal to 200 mm. (about 8 inches) of mercury, while the blood pressure in the carotid artery of the animal was only 112 mm. (about 4 1/2 inches) mercury; that is to say, the force by means of which secretion is driven outward is nearly twice as great as the pressure in the blood vessels in the gland. The secretion of saliva cannot then be a question of mere filtration, for if the physical agency - pressure - alone were acting, the saliva, if produced, would be forced into the lymph or blood vessels when the pressure in the duct exceeded that in the vessels.
The force and rate with which the secretion is produced vary with the strength of the stimulation. The flow of saliva steadily increases within certain limits as the stimulus gets stronger. It is not only the quantity of the secretion that depends on the amount of nerve impulse, but also its quality; that is to say, with a fresh gland, not wearied by previous experiment, the amount of solids in the saliva increases as the stimulus is increased, so that not only is the activity of the gland cells under the control of nerve influence, but the kind of work they perform is also regulated by the intensity of nerve impulse they receive.
It has been found that the increase in the blood flow is secondary to the secretion called forth by stimulation of the chorda tympani. This is shown by the fact that even when the blood supply is cut off by any means (strong sympathetic stimulation, ligature of the vessels, or even decapitation), an amount of saliva can be made to flow from the gland which could not have been stored up in its cells prior to the stimulation of this nerve.
II. With regard to the influence exerted by the sympathetic branches, the most obvious result of stimulation of these is a contraction in the arterioles, and a consequent diminution of the amount of blood flowing through the gland. The glands look pale, and the blood leaving them is intensely venous in character; the exact opposite, in fact, to the result obtained by stimulation of the cerebro-spinal nerves. But the sympathetic has also an effect on the gland cells, as it produces an increased flow of saliva. In the dog the secretion of "sympathetic saliva" is only temporary and scanty, having high specific gravity, and being overloaded with the solids. In the cat and rabbit "sympathetic saliva" is scanty, and not thicker than the "chorda saliva" of the same animal. So far as regards the blood vessels, the chorda is directly opposed to the sympathetic. To explain this antagonism we may either assume the existence of local nerve centres governing the muscular coats of the arterioles, and suppose that the sympathetic stimulates and the chorda inhibits the activity of these centres, or, what seems more simple, in the absence of anatomical evidence that such a centre exists, we may attribute to the arterial muscle cells themselves an automatic tonic power of contraction which can be increased by the sympathetic and diminished by the chorda tympani. It is singular that, if all the nerves leading to the gland be cut, a copious secretion of watery saliva begins after some hours, and lasts for some weeks, after which the cells undergo atrophic changes, and the gland becomes reduced in size. The explanation of the appearance of this so-called "paralytic saliva" is not clearly made out. Possibly the removal of some trophic nerve influences induces abnormal nutritive changes which cause stimulation of the cells, and ultimately lead to their degeneration.
Fig. 66. Sections of Orbital Gland of the Dog. (Heidenkain).
(A) After prolonged period of rest.
In (A) the secreting cells are clear, being swollen up with mucigen, and the half moon cells are very distinct and darkly stained.
(B) After a period of activity.
In (B) the accumulated material has been discharged from the cells, and the alveoli are shrunken.
The histological investigation of the elements of these glands in the various stages of secretion throws considerable light on the behavior of the cells during their periods of activity and rest.
It is now certain that the different stages are accompanied by constant structural changes in the cells, which doubtless are intimately connected with secretory activity. During the period of rest, that is, the time when the gland is not discharging its secretion, the cells slowly undergo a change in their appearance, which is the more obvious in proportion to the ease with which the material they secrete is recognized in the protoplasm. Thus, in mucous glands, or in mucus-yielding salivary glands, the changes are conspicuous; while in those which give a watery secretion they are less easily seen.
As an example we may take a mucous gland, such as the orbital gland of the dog, and follow the changes which occur in one of its cells, during the period which may be called its cycle of activity. (Fig. 66).
Immediately after the prolonged and active discharge of the secretion of the gland, the cells have all the characters of ordinary protoplasmic units, and the distinction between the polygonal cells and those next the wall of the acinus (demi-lune cells) is made out with great difficulty, because all the cells stain evenly with carmine, and have no special characters except those belonging to active protoplasm.
During rest certain changes gradually appear in those gland cells which are next the lumen of the saccule. They appear to swell toward the lumen, and at the same time become clear and resist staining with carmine, their protoplasm becoming impregnated with mucus-like material (mucigen), while the demi-lune cells remain protoplasmic and stain easily, and are thereby readily distinguished from the cell in the cavity of the saccule.
Fig. 67. Cells of the Alveoli of a Serous or Watery Salivary Gland. (Langley).
(A) After rest. (B) After a short period (C) After a prolonged period of activity. of activity.
If the discharge of secretion be induced either by normal reflex excitation, or by direct stimulus of the chorda tympani nerve, the cells discharge the contained specific material, some of them probably being destroyed by the act. If the active secretion be continued for some time, the cells return to their former protoplasmic state, and those which have been worn out are replaced by others from the demi-lune or marginal cells.
In the glands which do not produce any mucus the brilliant look of the cells after rest is wanting, but a corresponding change occurs. The secreting protoplasm becomes extremely granular during the resting period, and again clear after the discharge of the secretion. (Fig- 67).
Thus it would appear that during the so-called period of rest, when little or no fluid is poured into the duct, the gland cells are busy at their manufacturing process, diligently adding to their stock in hand, in order to be ready for a sudden demand which they could not meet by merely concurrent work. To sum up, then, we may conclude: -
1. That the manufacture of the specific materials of the secretion is accomplished as the result of the intrinsic power of the protoplasm of the gland cells.
2. That a vital process is called forth in the gland cells by the action of nerve impulses, because - (a) The force with which the secretion is expelled cannot be accounted for by the blood pressure. (b) The quantity and quality of the secretion is modified by the intensity of the nerve stimulation. (V) The temperature of the blood is raised. (d) Structural changes in the cells can be observed.
3. The normal stimulus to secretion passes from the centre in the medulla oblongata to the salivary glands along cerebro-spinal nerves.
4. This centre for salivary secretion, which at ordinary times is moderately active, may be excited to energetic action by impulses coming from taste, smell and ordinary sensory nerve terminals (particularly in the mouth), as well as by those which emanate from mental emotions.