The Multipolar cells of the anterior cornuæ are connected with the generation and transmission of motor impulses outward; are centres for reflex movements; are the trophic centres for the motor nerves and muscular fibres to which they are distributed. The anterior roots give passage to the vaso-constrictor and vaso-dilator fibres which exert an influence upon the calibre of the blood vessels. Complete destruction of the anterior horns is followed by a paralysis of motion, degeneration of the anterior roots, atrophy of muscles and bones and an abolition of reflex movements. 2. As an Independent Nerve Centre.

The spinal cord, by virtue of its containing ganglionic nerve matter, is capable of transforming impressions made upon the centripetal nerves into motor impulses, which are reflected outward through centrifugal nerves to muscles, producing movements. These reflex movements taking place through the gray matter, are independent of sensation and volition.

The mechanism involved in every reflex act is a sentient surface, a sensory nerve, a nerve centre, a motor nerve and muscle.

The reflex excitability of the cord may be

(1) Increased by disease of the lateral columns, the administration of strychnia, and in frogs, by a separation of the cord from the brain, the latter apparently exerting an inhibitory influence over the former and depressing its reflex activity.

2. Inhibited by destructive lesions of the cord, e. g., locomotor ataxia, atrophy of the anterior cornuæ, the administration of various drugs, and, in the frog, by irritation of certain regions of the brain. When the cerebrum alone is removed and the optic lobes stimulated, the time elapsing between the application of an irritant to a sensory surface and the resulting movement will be considerably prolonged. The optic lobes (Setchenow's centre) apparently generating impulses which, descending the cord, retard its reflex movements.

All movements taking place through the nervous system are of this reflex character, and may be divided into excito-motor, sensori-motor and ideo

motor.

Classification of Reflex Movements. (Küss.) They may be divided into four groups, according to the route through which the centripetal and centrifugal impulses pass.

I. Those normal reflex acts, e. g., deglutition, coughing, sneezing, walking, etc., pathological reflex acts, e. g., tetanus, vomiting, epilepsy, which take place both centripetally and centrifugally, through spinal nerves.

2. Reflex acts which take place in a centripetal direction through a

cerebro-spinal sensory nerve, and in a centrifugal direction through a sympathetic motor nerve, usually a vaso-motor nerve, e. g., the normal reflex acts, which give rise to most of the secretions, pallor and blushing of the skin, certain movements of the iris, certain modifications in the beat of the heart; the pathological, which, on account of the difficulty in explaining their production, are termed metastatic, e. g., ophthalmia, coryza, orchitis, which depend on a reflex hyperæmia; amaurosis, paralysis, paraplegia, etc., due to a reflex anæmia.

3. Reflex movements, in which the centripetal impulse passes through a sympathetic nerve, and the centrifugal through a cerebro-spinal nerve; most of these phenomena are pathological, e. g., convulsions from intestinal irritation produced by the presence of worms, eclampsia, hysteria, etc.

4. Reflex actions, in which both the centripetal and centrifugal impulses pass through filaments of the sympathetic nervous system, e. g., those obscure reflex actions which preside over the secretions of the intestinal fluids, which unite the phenomena of the generative organs, the dilatation of the pupils from intestinal irritation (worms), and many pathological phenomena.

Laws of Reflex Action. (Pflüger.)

1. Law of Unilaterality. If a feeble irritation be applied to one or more sensory nerves, movement takes place usually on one side only, and that upon the same side as the irritation.

2. Law of Symmetry. If the irritation becomes sufficiently intense, motor reaction is manifested, in addition, in corresponding muscles of the opposite side of the body.

3. Law of Intensity. Reflex movements are usually more intense on the side of the irritation; at times the movements of the opposite side equal them in intensity, but they are usually less pronounced.

4. Law of Radiation. If the excitation still continues to increase, it is propagated upward, and motor reaction takes place through centrifugal nerves coming from segments of the cord higher up.

5. Law of Generalization. When the irritation becomes very intense, it is propagated to the medulla oblongata; motor reaction then becomes general, and is propagated up and down the cord, so that all the muscles of the body are thrown into action, the medulla oblongata acting as a focus whence radiate all reflex movements.

Special Centres in the Spinal Cord.

Genito-spinal centre. In the lower portion of the spinal cord are located the centres which control the sphincter muscles of the rectum and

bladder, the erection of the penis, the emission of the semen, the action of the uterus during parturition, etc.

Cilio-spinal centre. Situated in the spinal cord between the 6th cervical and 2d dorsal nerves; stimulation of the cord in this situation produces a dilatation of both pupils through filaments of the sympathetic, which take their origin from this region of the cord.

Throughout the spinal cord are situated numerous centres which preside over the following reflexes, viz:—

The patellar tendon reflex takes place through the segments from which arise the 2d, 3d and 4th lumbar nerves; the cremasteric reflex through the segment from which arise the 1st and 2d lumbar nerves; the abdominal reflex through the segments between the 8th and 12th dorsal nerves; the epigastric reflex through the segments from which arise the 4th, 5th and 6th dorsal nerves.

Paralysis from Disease of the Spinal Cord.

Seat of Lesion. If it be in the lower part of the sacral canal, there is paralysis of the compressor urethræ, accelerator urinæ, and sphincter ani muscles; no paralysis of the muscles of the leg.

At the upper limit of the sacral region. Paralysis of the muscles of the bladder, rectum and anus; loss of sensation and motion in the muscles of the legs, except those supplied by the anterior crural and obturator, viz: psoas iliacus, Sartorius, pectineus, adductor longus, magnus and brevis, obturator, vastus externus and internus, etc.

At the upper limit of the lumbar region. Sensation and motion paralyzed in both legs; loss of power over the rectum and bladder; paralysis of the muscular walls of the abdomen interfering with expiratory move

ments.

At the lower portion of the cervical region. Paralysis of the legs, etc., as above; in addition, paralysis of all the intercostal muscles and consesequent interference with respiratory movements; paralysis of muscles of the upper extremities, except those of the shoulders.

Above the middle of the cervical region. In addition to the preceding, difficulty of deglutition and vocalization, contraction of the pupils, paralysis of the diaphragm, scalene muscles, intercostals, and many of the accessory respiratory muscles; death resulting immediately, from arrest of respiratory

movements.

Anterior half of spinal cord. Posterior half of spinal cord. ataxia or tabes dorsalis.

Paraplegia developing symmetrically.
Characteristic symptoms of locomotor

In the gray substance in the vicinity of the central canal and anterior horns. If the lesion be acute, symptoms characteristic of acute spinal paralysis manifest themselves; if chronic, symptoms characteristic of progressive muscular atrophy.

MEDULLA OBLONGATA.

The Medulla Oblongata is the expanded portion of the upper part of the spinal cord. It is pyramidal in form and measures one and a half inches in length, three-quarters of an inch in breadth, half an inch in thickness, and is divided into two lateral halves by the anterior and pos

View of Cerebellum in section, and of Fourth Ventricle, with the neighboring parts. (From Sappey.)

1. Median groove fourth ventricle, ending below in the calamus scriptorius, with the longitudinal eminences formed by the fasciculi teretes, one on each side. 2. The same groove, at the place where the white streaks of the auditory nerve emerge from it to cross the floor of the ventricle. 3. Inferior peduncle of the cerebellum, formed by the restiform body. 4. Posterior pyramid, above this is the calamus scriptorius. 5. Superior peduncle of cerebellum, or processus e cerebello ad testes. 6 6. Fillet to the side of the crura cerebri. 77. Lateral grooves of the crura cerebri. 8. Corpora quadrigemina.-After Hirschfeld and Leveille.

terior median fissures, which are continuous with those of the cord. Each half is again subdivided by minor grooves, into four columns, viz: anterior pyramid, lateral tract and olivary body, restiform body and posterior pyramid.

1. The anterior pyramid is composed partly of fibres continuous with those of the anterior column of the spinal cord; but mainly of fibres derived from the lateral tract of the opposite side, by decussation. The united fibres then pass upward through the pons Varolii and crura cerebri, and for the most part terminate in the corpus striatum and cerebrum.

2. The lateral tract is continuous with the lateral columns of the cord; its fibres in passing upward take three directions, viz: an external bundle joins the restiform body, and passes into the cerebellum; an internal bundle decussates at the median line and joins the opposite anterior pyramid; a middle bundle ascends beneath the olivary body, behind the pons, to the cerebrum, as the fasciculus teres.

The olivary body of each side is an oval mass, situated between the anterior pyramid and restiform body; it is composed of white matter externally and gray matter internally, forming the corpus dentatum.

3. The restiform body, continuous with the posterior column of the cord, also receives fibres from the lateral column. As the restiform bodies pass upward they diverge and form a space, the 4th ventricle, the floor of which is formed by gray matter, and then turn backward and enter the cerebellum.

4. The posterior pyramid is a narrow, white cord bordering the posterior median fissure; it is continued upward, in connection with the fasciculus teres, to the cerebrum.

The Gray Matter of the medulla is continuous with that of the cord. It is arranged with much less regularity, becoming blended with the white matter of the different columns, with the exception of the anterior. By the separation of the posterior columns, the transverse commissure is exposed, forming part of the floor of the 4th ventricle; special collections of gray matter are found in the posterior portions of the medulla, connected with the roots of origin of different cranial nerves.

Properties and Functions. The medulla is excitable anteriorly, and sensitive posteriorly to direct irritation. It serves (1) as a conductor of sensitive impressions upward from the cord, through the gray matter to the cerebrum; (2) as a conductor of voluntary impulses from the brain to the spinal cord and nerves, through its anterior pyramids; (3) as a conductor of coördinating impulses from the cerebellum, through the restiform bodies to the spinal cord.

As an Independent Reflex Centre. The medulla oblongata contains special collections of gray matter, which constitute independent nerve centres which preside over different functions, some of which are as follows, viz :

: