anterior gray motor columns injured, the paralyzed muscles soon undergo fatty degeneration, which does not depend on mere inactivity, for it does not follow cerebral paralysis, in which the integrity of the muscle can be preserved by suitable electric stimulation. Similar trophic agencies probably influence the other tissues. Thus, many affections of the skin, herpes, etc., are attributed to nervous lesions.

On account of the elaborate and purposeful reflex movements performed by decapitated frogs or eels, it has been suggested that in the lower vertebrates the spinal cord is capable of sensation and volition-mental activity-but to follow this assumption we should have to modify our ideas of volition and sensation, for which consciousness is commonly taken to be a necessary factor. It is, however, important to note that the lower we go in the scale of vertebrate animals the less powerful are the mental faculties, and the more important are the functions presided over by the spinal marrow.

CHAPTER XXXV.

THE MEDULLA OBLONGATA.

The direct continuation of the spinal cord is called the medulla oblongata. It consists of representatives of the various parts of the cord, with some additional gray matter. The relationship of the different parts of the medulla to those of the spinal cord may be best understood by supposing the posterior median fissure and underlying nerve substance at its upper limit to be split vertically down to the central canal, and the lateral masses separated, so that the gray part becomes spread out on the posterior surface, and there forms the floor of the fourth ventricle. The gray matter of the medulla oblongata consists of two sets of nuclei ; one being the continuation of the gray columns of the spinal marrow, and the other made up of certain additional gray nodules embedded here and there among the white strands.

The anterior motor gray columns, which are cut off from the central gray substance by the passage of the pyramidal tract to the opposite side, are continued along the floor of the fourth ventricle near the median line. The posterior gray columns are continued upward to form the nucleus of Rolando, and are spread out on the lateral part of the floor of the ventricle. Important nuclei of gray matter lie in the olivary bodies, and numerous collections of cells forming the nuclei from which arise the chief cranial nerves. For an adequate description of these groups of nerve cells and their connections, works on anatomy must be consulted.

The various white columns of the spinal cord are so distributed in the medulla that their course gives some indication of the channels by which impulses are carried through it.

In ascending to the medulla the posterior white columns become differentiated into three. (1) Goll's column is more distinctively marked off, and enlarges to form the funiculus gracilis, containing the clavate nucleus; the funiculus gracilis tapers away to nothing above. (2) Burdach's column widens in a wedge-like fashion

and is called funiculus cuneatus, which contains the cuneate nu

FIG. 251.

cleus. It passes on to help to form the inferior peduncle of the cerebellum. (3) By the projection of the enlarged posterior gray column, Tubercle of Rolando, a prominence is produced called the funiculus of Rolando. This also helps to form the inferior peduncle of the cerebellum.

The greater part of the lateral white columns of the spinal cord pass, at the decussation of the pyramids, to the opposite side to form the pyramidal prominence on the front of the medulla, and are thence continued upward directly to the motor areas of the cortex.

Diagram of Brain and Medulla Oblongata. (Cleland.) tract

a, Spinal cord; b, b, Cerebellum divided, and above

The direct cerebellar which forms the

it the valve of Vieussens partially divided; c, superficial part of the Corpora quadrigemina; d, d, Optic thalami: e,

pineal body; ff, Corpora striata; g, g, Cerebral lateral column joins the hemispheres in section; h, Corpus callosum; i,

Fornix;,, Lateral ventricles; 3, Third ventricle; cuneate and Rolando's 4, Fourth ventricle; 5, Fifth ventricle, bounded on each side by septum lucidum.

bands to form the inferior cerebellar peduncle.

The majority of the fibres of the anterior gray columns pass into the medulla beneath the pyramids by which they are quite concealed. They can be traced some distance through the pons Varolii. The fibres of the direct pyramidal tracts join the pyramid of their own side.

It must be remembered that the medulla is the only route between the spinal cord and the upper nerve centres.

Through it all the afferent and efferent channels must pass, as they do through the spinal cord. From it, and the prolongation of its gray nuclei in the pons Varolii, several cranial nerves take origin. Thus, the medulla is to the cranial nerves (from the fifth to the ninth) as the spinal cord is to the spinal nerves, but their mode of distribution is different.

THE MEDULLA OBLONGATA AS A CENTRAL ORGAN.

A number of groups of ganglion cells with special duties are located in the medulla. Those acts which are most important

Diagram showing the position of the nuclei of the cranial nerves in the medulla oblongata, etc., as if seen in antero-posterior section, looking from the median line toward the right side. The nuclei near the median line are more darkly shaded. Py, Pyramidal tracts; Py Kr, Pyramidal decussation; O, Olivary body; Os, Superior olivary body; V, motor; V', Middle sensory; and V, Lower sensory nuclei of fifth nerve; R v. Roots of fifth nerve; VI, Nucleus of sixth nerve; R VI, Root of sixth nerve; VII, Nucleus; Gf, Knee; and R VII, Root of portio dura of seventh nerve; VIII, Auditory; IX, Glosso-pharyngeal; x, Vagus; XI, Accessorius; and XII, Hypoglossal nuclei; Kz, Clavate nuclei.

for the execution of the vegetative functions, are arranged and governed by the nerve cells of the medulla. Some of these centres are called automatic, though they are variously affected

by many impulses arriving from distant points, while others are purely reflex in their action.

The former are the more essential, and will therefore be first considered.

RESPIRATORY CENTRE.

The centre which regulates the respiratory movements is situated in the floor of the fourth ventricle, at the upper and back part of the medulla. Flourens long since showed that injury of this spot-the vital point--was followed by almost instant cessation of respiration.

This is a good example of a so-called automatic centre; that is to say, the blood flowing through the medulla and nourishing the cells suffices to supply them with the energy necessary for their activity. Even slight variations in the quality or temperature of the blood reaching this part modifies the activity of the cells. The less oxygen and waste products contained in the blood, the more powerfully does it act as a stimulant on the

centre.

Although we take the respiratory centre as an example of an automatic centre, its working is arranged by afferent impulses, so that the normal rhythm of breathing is regulated by reflex action. The mechanical state of the lungs-whether distended as in inspiration or contracted as in expiration-seems to excite the terminals of certain fibres of the vagus, which carry impulses to the centre, and thus excite or restrain movements.

This automatic centre can also be influenced by the higher centres of the brain, for by our will we can regulate our breathing movements or stop breathing altogether for a time. Independent of volition the higher centres control the respiratory rhythm, as seen in sleep, when their action is partially in abey ance while the vagi are active, and respiration becomes periodic, or when the brain functions are impaired and respiration becomes intermittent (Cheyne-Stokes respiration). Further, the action of the respiratory centre can be altered by impulses arriving from the surface, as may be seen by the gasping inspirations which involuntarily follow the sudden application of cold.

Again, the activity of the centre may be altered by stimula