The reflex convulsions that occur in poisoning with strychnine, or as the result of some constant but slight stimulation, may be explained as follows:

Diagram of the paths taken by the impulses in the brain and cord. MM., motor channels; ss., sensory channels; CR., cranial nerves.

Besides the resistant protoplasmic fibrils in the gray part of the cord, there exist medullated fibres in the root zones-short cuts, as it were-by which impulses travel from one part of the cord to another. If we suppose the ordinary reflex traffic of the

cord cells to be carried on without the assistance of these direct lines of communication, we must assume that there is some special means of shutting these fibres out of the working of the reflex machine. Such special mechanisms in all probability exist, and are in relationship with or under the command of the inhibitory cells of the higher centres. We may then suppose that strychnine removes the power of these inhibitory agents, and the impulses finding the direct ways open, take these routes, and are simultaneously and irregularly diffused throughout all the cell territories (independent of the ordinary paths they have been educated to follow), and thus convulsive movements are excited in many parts of the body.

In like manner the unremitting activity necessary to keep in check the impulses arriving from a constant source of stimulation (such as intestinal worms), eventually fatigues the active elements in this inhibitory mechanism, and then-often suddenlythe force of the accumulated irritation rushes along the direct channels to all parts of the cord, and simultaneously exciting them, brings many discordant muscles into spasmodic action.

The reflexion of an impulse from a sensory nerve, through the cells of the spinal cord to a motor nerve, occupies a measurable length of time, which has been estimated at about of a second. The time required for the performance of a reflex act varies considerably in the same individual under different conditions; of these, high temperature and intense stimulation shorten the time, and fatigue or cold lengthen it.

SPECIAL REFLEX CENTRES.

Many of the groups of nerve cells in the cord are employed in executing familiar acts essential to the animal economy independent of the will. Many of these acts are very complex, and require the coördinated action of certain sets of muscles. Such groups of nerve cells have been called special centres, and many of them have already been described in the preceding chapters. The more important are:

1. A centre for securing the retention of the urine by the tonic contraction of the sphincter muscle of the bladder. This group

of nerve cells is probably kept in action by impulses arriving from the bladder by the afferent nerves passing from its walls to the spinal cord. The more distended the bladder becomes, the more powerful the stimulus sent to the cord, and therefore the more firmly the sphincter is made to contract.

2. Nearly related to the former is the centre which presides over the evacuation of the bladder. This is excited by impulses arriving from the urethra, near the neck of the bladder. It then sets the detrusor muscle in action, while the sphincter is relaxed by voluntary inhibition.

3. The ejaculation of the semen may also be said to be accom-, plished by a special spinal centre, capable of controlling movements, in which involuntary muscles play an important part.

4. In parturition a number of motions are called into play (as well as the uterine contraction) which are so regularly coördinated, though involuntary, as to entitle us to suppose that they are arranged by a special centre in the spinal cord.

5. The act of defecation is accomplished by means of a spinal centre also. The action of this centre might (like that presiding over the urinary bladder) be divided into two parts-retention and evacuation-in which volition and intestinal peristalsis play a very important part.

COÖRDINATION.

From what has been said concerning the more complex reflex actions, it is clear that the cells of the spinal cord are capable of arranging the discharge of nerve impulses, so as to bring about definite purposeful movements. This power of coördinating impulses, which is so striking in some reflex actions after the brain has been destroyed, is equally important in arranging efferent impulses and accomplishing ordinary voluntary movements. In fact, most of the details of the mode of working of the muscles are under the control of the cells of the spinal cord.

It will help us in formulating the mechanism if we suppose the resistance in the gray part of the cord to be much greater than that in the medullated nerve channels, and that throughout it the paths are so numerous that each individual nerve cell might be in communication with every other nerve cell. These paths

are made passable by use; the oftener an impulse traverses a given route the more adapted such a route becomes for future traffic. Thus, by practice, we constantly freshen certain channels of intercommunication between the various cells of the cord and thus make beaten tracks, along which impulses can pass without hindrance. In a similar way certain groups of nerve cells acquire the habit of working together and exciting complex movements which at first were impossible. The nerve paths, along which the impulses, producing common movements, have to pass, are no doubt prepared by the long practice of our ancestors, and the power of performing these actions is transmitted to us ready for immediate application. Other paths connecting groups of cells required for the production of unusual combinations of movements have to be practiced by the individual, and much of the difficulty of learning any trade of special manual dexterity depends on the necessity of making impulses readily traverse definite directions, so as to excite certain groups of cells to act synchronously and set the required combination of muscles in accurately coördinated motion. Indeed, the delicacy of manipulation required by some trades cannot be attained in the lifetime of one individual; thus, it is said to require three generations to make a perfect glassblower; the grandson having the benefit of the hereditary tendency to accomplish certain coördinations acquired by the lifelong habit of the parents.

The importance of this technical education of the cells of the spinal cord in the execution of delicate manipulations will be felt if one attempt to imitate the movements of precision which a skilled craftsman executes without attention or voluntary effort even in the most careless exercise of his craft. The practice required for such education is experienced by any one who attains skill in the simplest special manipulation, from writing to playing the violin.

AUTOMATISM.

Besides being excited to action by impulses coming from the brain-volition-and from the surface-reflexion-the groups of cells in the spinal cord may act without any obvious incoming impulse; that is to say, some of the cells appear to be capable of

independent activity. Such groups of nerve cells are commonly called automatic centres; the more important of those found in mammalia may be classified as follows:

1. Vasomotor centres: Though the central point controlling the contraction of the blood vessels is situated in the medulla, there is no doubt that even in man, centres are distributed throughout the gray matter of the spinal marrow, which are capable of keeping up the arterial tone in the regions over which they preside. As evidence of this may be mentioned the fact that the dilatation of the arteries, which follows the severance of the lumbar part of the cord from the medulla, only lasts a few days, after which the vessels again contract in a distinctly tonic manner. The arterial tonus only disappears completely and permanently when the spinal cord is destroyed. Thus, it would appear-although habitually all the vessels of the body are regulated by a centre in the medulla, nearly related to the cardiac centre-that every vascular region has a nervous mechanism of its own in the cord, which suffices to keep up the tonic contraction of the muscular coat of its vessels.

2. Sweating centres: Though closely related to the preceding, the centres which preside over the secretion of sweat in the lower part of the body and hinder extremities must, for many reasons which cannot now be mentioned, be regarded as separate centres.

3. Some smooth muscle fibres appear to be influenced by centres in the cord. In the lower part of the cervical cord is a group of nerve cells which keep the sphincter muscle of the iris in check; narrowing of the pupil has been described as following injury of this region.

4. The gray matter of the cord is also said to keep the skeletal muscles in a state of slight tonic contraction; elongation of the muscles is said to follow section of the anterior roots. When this muscular tone is absent the phenomenon known as "tendon reflex" is wanting, as the tap on the tendon ceases to excite the toneless muscle.

5. So-called trophic centres are also said to exist in the spinal cord. The best evidence in this matter is derived from the skeletal muscles. If the motor nerves or roots be cut, or the