tions of certain parts of the air passages; so much so, that convulsive actions of the respiratory muscles are brought about, which induced some to speak of a sneezing centre and a coughing centre in the medulla. But sneezing and coughing may be equally well explained as a peculiar form of activity of the respiratory centre, or a reflex alteration in the respiratory rhythm, caused by irritation of the nasal or laryngeal mucous membranes. Though the action of the respiratory centre can be modified by (1) the will and (2) various peripheral stimulations, and is habitually regulated from the periphery through (3) the vagi by the state of the lungs, the condition of the blood supplied to the centre may be such that these remoter influences are quite powerless. This uncontrollable condition of the centre is established when the blood flowing through it is abnormally venous and the cells become over-stimulated. We know how short a time we can hold our breath by voluntary checking of the centre, and most people have had occasion to observe the inordinate and painful efforts of a person whose respiration is interfered with by disease. When the dyspnoea becomes intense, nearly all the muscles in the body are called into action. Thus, in quiet breathing comparatively few nerve cells in the medulla carry on the work of respiration, but under certain emergencies they can call to their aid the entire motor areas of the gray substance of the spinal cord, and thus give rise to a general effort. Hence, we often hear of a convulsive centre in the medulla being placed in close relation to the respiratory centre. In some cases, irritation of the air passages or imperfect oxidation of the blood, the convulsive centre comes under the command of the cells of the respiratory centre, which can then excite coughing, sneezing or convulsive inspiratory effort.

As already mentioned, the convulsions of asphyxia may be explained by the impure blood acting as a stimulus on the cells. of the cord itself.

THE VASOMOTOR CENTRE.

It has already been stated that groups of cells exist in the gray part of the spinal cord, which, according to the class of animal, have more or less direct influence upon the muscles in

the coats of the vessels. Thus, in a frog whose brain and medulla have been destroyed, in some hours the vessels of the web regain a considerable degree of constriction, which is again lost if the cord be destroyed. In the dog the vessels of the hinder limb recover their tone more or less perfectly in a few days after the spinal cord has been cut in the dorsal region, although just after the section they are widely dilated from the paralysis of their muscular coats. In a few days the cells of the cord can learn to accomplish, of their own accord, work which they have been in the habit of doing, only under the direction of the higher centre. From this we conclude that though the cord contains local vasomotor centres distributed throughout its gray matter, these are normally under the control of the vasomotor centre in the medulla, and this centre is really the chief station from which impulses destined to affect all the blood vessels must

emanate.

This arrangement is quite comparable with that by which the ordinary muscles are made to contract. When the will causes a muscular contraction, the impulse starting from the cerebral cortex does not travel directly to the muscle, but it passes from the brain to certain cells in the cord and thence to the muscles. In fact, to these spinal agents the ultimate arrangement and coördination of the act is confided. So, also, the chief vasomotor centre in the medulla executes its function through the medium of numerous under centres placed at various stations along the cord.

The vasomotor centres-like nearly all other controlling groups of ganglion cells-may be considered as composed of two parts antagonistic one to the other, viz., a constricting and dilating centre, the impulses from which travel by separate nerve channels. The constricting impulses are mainly distributed by the sympathetic nerve, while the dilating impulses accompany those which are employed in calling forth the ordinary function of the part in question.

From what has been said as to the wide distribution of centres influencing the blood vessels, an attempt to localize exactly the position of the medullary vasomotor cells is not satisfactory. In

lower animals-frogs-the cells are evenly diffused throughout the medulla and cord. In man the localization is difficult to demonstrate, though we have reasons for thinking it much more definitely circumscribed. In the rabbit it has been localized to the floor of the fourth ventricle in the immediate neighborhood of the respiratory and cardiac centres. From this the nerves pass by the cord to the spinal roots, by which they reach the sympathetic.

The vasomotor centre exerts a tonic or continuing action on the vessels, holding them in a state of partial constriction or tone. In this it may be said to have an automatic action. Although this tonic state of activity of the centre may be called automatic, it is really under the control of many reflex influences, which constantly vary the general tone, or effect local changes in the degree of constriction of this or that vascular area. Among the most striking afferent regulating impulses are those arriving from the heart, the digestive organs and the skin. In some animals, a special nerve-the depressor-has been discovered, which, passing from the heart to the medulla, keeps the vasomotor centre informed as to the degree of tension, etc., of the heart cavities. When the heart becomes over-full, impulses pass from it, and check the tonic power of the centre, so as to reduce the arterial pressure against which the ventricle has to act. Electric stimulation of this nerve causes a remarkable fall in the general blood pressure. The vasomotor centres regulate the distribution of blood to the viscera and skin, according to the condition of activity of these parts as described in another chapter (XXXI).

THE CARDIAC CENTRE.

Although the heart beats periodically when cut off from the nervous centres, its normal rhythm is under the control of a group of nerve cells in the medulla, from which some fibres of the vagus carry special regulating impulses to the heart. The action of this centre is habitually that of a restraining agent lessening the rate of the heart's contractions, and is, hence, called a tonic inhibitory centre. The activity of the centre is influenced by the condition of many distant parts, such as the

cortex of the brain, the abdominal viscera, etc., which exert a kind of reflex action on the heart through the centre. The degree of inhibitory power, as well as the share taken in the action of the centre by automatism and reflexion, differs in different animals. A centre (accelerator) antagonistic to the latter also exists in the medulla. It is weaker in action than the inhibitory centre, and is not tonic.

In the medulla there also exist many other centres connected with the organic functions. Among these, the centres for swallowing and vomiting may be mentioned. For further details on this subject, the reader may consult the chapter on Digestion.

CHAPTER XXXVI.

THE BRAIN.

As we pass upward in attempting to trace the conducting channels of the medulla, we come to the more elaborate system of nervous textures which, together, are called the brain. This is anatomically the most highly developed, and physiologically the most intricate, part of the central nervous organs. Besides the nerve cells and various kinds of conducting channels with which we have already become familiar in the cord, etc., there are in the brain a vast number of smaller elements which do not possess the distinctive characteristics of nerve cells. These granular bodies are tightly packed together in many parts of the centres, and must have some important function.

To form a general idea of the plan of construction of the brain, it is well to follow its development in the earlier stages of the embryo, from the time when it forms an irregular and thickened part of the tube of tissue, from which is developed the cerebrospinal axis. From this it will be seen that the brain is but a modified part of the primitive nervous tube, in which swellings may be observed at an early period of embryonic life. These swellings are called the fore-brain, mid-brain and hind-brain, and in the future development of the parts give rise to (1) the hemispheres and basal ganglia; (2) the corpora quadrigemina, pons and cerebellum; and (3) the medulla oblongata. The great mass of the brain-the hemispheres-is formed by an excessive development of bud-like processes which grow out from the sides of the fore-brain at an early period, and become elaborately folded, so that in the adult it is difficult to trace the relationship to the original form. For further details of the development of the brain, vide chapter on that subject.

The cells of the brain are, like those of the cord, grouped together in the complex gray substance, while the white part is made up of conducting fibres. The gray substance is distributed