GELATINOUS SUBSTANCE OF MEDULLA.

263

THE UPWARD CONTINUATION OF THE POSTERIOR HORNS. -The arcuate fibers of the medulla separate the "substantia gelatinosa" from the rest of the gray substance, near to the line of junction of the spinal cord. This portion of the posterior horns maintains a superficial position in the lateral column of the medulla, as high as the point of escape of the trigeminus, or fifth nerve from the pons. In the opinion of some authors, this structure may be regarded as prolonged through the aqueduct of Sylvius as far as the level of its opening into the cavity of the third ventricle.

Spitzka is led to the conclusion that the gelatinous substance of the posterior horn is not composed of neuroglia tissue, but is to be regarded rather as imperfectly organized ganglionic tissue which is connected with sensation. He is brought to this view by the following facts: 1. It exists. where the sensory nerves enter the posterior horn; 2. It is found in the sensory nuclei of the medulla, chiefly in the trigeminal nuclei; 3. It is found in the olivary bodies which are connected with strands from the posterior columns of the cord, above the nuclei of those columns.

The posterior spinal cornua are continued upward into the substance of the medulla upon the mesial aspect of the ascending root of the fifth cranial nerve (Fig. 57) as two masses-the so-called "spongy" and "gelatinous" form of gray substance. The former merges with the gray matter of the reticular field; while the latter lies more closely related to the trigeminus tract. Spitzka is led to believe, from the apparent connections of the gelatinous substance, that it is the station for sensory impressions derived from the pharynx. He states his grounds for this belief as follows:

"Taking into account that the three nerves, into which the eighth pair of Willis has been divided by Soemmering, supply many peripheries conjointly, I think it unfortunate that investigation should seek for the special nucleus of this or that nerve, designated by a given numeral. Meynert made a step in the right direction when he considered the nuclei of the ninth, tenth, and eleventh pairs in the aggregate, and

classified the aggregate nuclear masses on the topographical principle. If there is any truth in the notion that, with a given animal species, cells have special connections, present a special type of structure, then it will be reasonable to seek for the nuclei of different peripheries. This procedure is far more rational than to seek for the nucleus of a nerve whose component filaments have different peripheral terminations, and are included in the same sheath, rather by anatomical accident, as it were, than because of their physiological propinquity. I think it sound to speak of a nuclear column representing the visceral periphery of the vagus, or the gustatory periphery of the glosso-pharyngeal and intermediate nerve of Wrisberg, or the tactile periphery of the ninth and tenth, the motor pharyngeal periphery of the same nerves, and the motor laryngeal of the tenth and eleventh pairs."

THE CONTINUATION OF THE CENTRAL GRAY COLUMN OF THE SPINAL CORD.-By a reference to Fig. 52 it will be seen that the gray substance of the spinal cord on either side of the central canal is represented as consisting in part of the "gray" or "vesicular column of Clarke," and again of an intervening portion between it and the central canal called the "central gray column."

1

The COLUMN OF CLARKE is well defined in the dorsal region of the cord, but it is not represented in the cervical or lumbar

1 The gray matter of the cord is divided by Spitzka into three parts, viz.: the anterior horns, or motor system; the posterior horns, or sensory system; and the intermediate portion, to which he applies the term "mixed system," because it partakes, to his mind, of sensory, motor, and trophic functions. He attributes a trophic function to the cells of Clarke's column.

The following formulæ, advanced by the same author respecting the functions of cell groups in the central tubular gray masses, are of interest in this connection:

"The nearer the muscle is to the ventral aspect of an animal, the nearer will be its nucleus to the median line of the cord. Per contra, the nearer the muscle is to the dorsal line of an animal, the nearer to the so-called lateral cornu will its nucleus have to be sought for. Flexor nuclei are therefore in internal, extensor nuclei in external and posterior, cell groups."

"Hypertrophied segments of the body, such as the extremities, are accompanied by lateral extensions of the cornua, in which flexor and extensor muscles probably occupy the same relative position as the one stated.

"Whether groups of muscles be flexor or extensor, it will be found that the nearer they are to the animal axis, the nearer will their nuclei be to the central canal. This is especially true of the flexor nuclei."

ACCESSORY NUCLEI OF MEDULLA.

265

regions. In the lower part of the medulla, however, a collection of cells situated at the posterior and external part of the central gray column may be discovered, which bear a close analogy to those of the column of Clarke encountered in the dorsal region of the cord. It contains bipolar cells which are markedly pigmented, and constitutes the posterior nucleus of the spinal accessory nerve, which is given off at this level. At higher levels, after the central canal has opened upon the floor of the fourth ventricle, this column seems to assist in the formation successively of the nuclei of the eleventh, tenth, and ninth cranial nerves.

The CENTRAL GRAY COLUMN of the cord appears to be continued into the medulla as a thin layer of gray substance which covers the floor of the fourth ventricle, and is prolonged as a lining to the aqueduct of Sylvius.

The term "nuclear formation" is applied to the central tubular gray, because it exhibits a tendency in higher animals to resolve itself into nuclei of origin for special

nerves.

The fourth ventricle may be regarded as an expanded portion of the central canal of the cord, in order that sufficient room shall exist for the nuclei of the cranial nerves situated upon its floor.

The aqueduct of Sylvius, with its surrounding gray matter, again represents from this standpoint the spinal canal and the central gray column, continued onward to the third ventricle.

ACCESSORY NUCLEI OF THE MEDULLA OBLONGATA.

Within the apparent continuation of the central gray column of the cord upon the floor of the fourth ventricle and the aqueduct of Sylvius, certain collections of cells, that have no representation in the architecture of the cord, are observed. These have been named the "accessory nuclei" of the medulla. They require a separate description. These include the inferior facial; the accessory nuclei of the spinal accessory and hypoglossal nerves; and four acoustic nuclei.

Iferior Facial Nucleus.-This was first described by Clarke. It consists of a collection of several small masses of cells, which lie to the inner side of the hypoglossal nucleus and close to the median line. Before the spinal canal opens into the ventricle, they lie between it and the hypoglossal nucleus of either side. The fibers which spring from them appear to enter the funiculus teretes, through which they ascend to the fasciculus teretes, and then join with fibers of the facial nerve. This nucleus has been divided by some authors into two, to which the names internal and external have been applied.

Accessory Nuclei of the Spinal Accessory Nerve.-These consist of two collections of cells of small size, which are situated posteriorly to the main nucleus of the spinal accessory nerve. According to Meynert, these nuclei are connected by commissural fibers which pass posterior to the central canal.

Accessory Nucleus of the Hypoglossal Nerve.-Ross describes this collection of cells as developed principally upon one side only of the medulla. It is composed, according to this author, of caudate cells of extremely small size, when compared with the cells of the hypoglossal nucleus. He believes that this accessory nucleus is concerned in the mechanism of articulation, and that it is reasonable to suppose that it is in some way connected with the third convolution of the left frontal lobe.

SPECIAL NUCLEI OF THE MEDULLA AND PONS VAROLLI.Ross includes under this head the acoustic nuclei, because they can hardly be said to be represented by any part of the gray substance of the cord. They are four in number, two median and two lateral.

The posterior median acoustic nucleus occupies the space between the ala cinerea and the inferior peduncle of the cerebellum, as high as the anterior border of the "striæ medullares." It gives origin to the posterior root of the auditory nerve. Some of these fibers pass out superficially, the "striæ acoustica"; while others traverse the substance of the medulla.

GANGLIA OF PONS AND MEDULLA.

267

The anterior median acoustic nucleus lies anteriorly to the striæ medullares, and gives origin to the anterior root fibers of the auditory nerve. It occupies the external angle of the ventricle.

The posterior lateral acoustic nucleus lies imbedded in the peduncle of the cerebellum, and is interposed between the superficial and deep fibers of the auditory nerve.

The anterior lateral acoustic nucleus is situated between the middle peduncle and the flocculus. It gives origin to the so-called "portio intermedia of Wrisberg." The view that this nucleus is associated with the special sense of taste in the anterior two thirds of the tongue (because its fibers appear to pass in the chorda-tympani branch of the facial nerve) is now held by some anatomists.

THE SUPERADDED GRAY MATTER OF THE MEDULLA AND PONS.-When successive cross-sections of the medulla and pons are compared with each other, nodal masses of gray matter are discovered, the analogues of which are not found within the brain or spinal cord; and some are apparently not directly connected with the fibers of origin of special cranial nerves. Most of these have been incidentally referred to in previous pages, but a few remain to be described. These nodal masses include (1) the so-called "triangular nucleus"; (2), the "clavate nucleus"; (3), the "olivary body"; (4), the "parolivary body"; (5), the "internal parolivary body" or the "nucleus of the pyramid"; (6), the "superior olivary body”; and (7) the "middle sensory nucleus of the trigeminus nerve."

The "red nucleus of the tegmentum" or the "superior olive" of Luys (which has been already described in connection with the crus cerebri), as well as the "external geniculate body" (which has been discussed in previous pages), are also classed by Ross among these superadded collections of gray

matter.

The Triangular Nucleus.-This is a gray nucleus (Fig. 53) which is inclosed within the substance of the cuneate bun

The parolivary bodies are often designated as the "accessory olivary nuclei."