Part 83

=The Brain Structure.=—The brain (Fig. 62, C and Fig. 75, A and C) is made up of three principal parts: the cerebrum, the cerebellum and the medulla oblongata. In a fowl of medium size the brain weighs about 150 grains.

The pons varolii is absent in birds. The crura cerebelli (Fig. 75, No. 9) are immediately connected with the corpora restiformia. The lower face of the isthmus is convex posteriorly; in front, the tubercula bigemina (Fig. 75, No. 4) are united to each other by a comparatively large transverse cord, formed by the optic nerves intercrossing in the median line. The superior face of the medulla oblongata is depressed above to constitute a fourth ventricle; in front of this ventricle are the tubercula bigemina, or optic lobes. These two voluminous tubercles are separated from each other above, where they embrace the cerebellum, and are salient on the sides of the lower face. They are hollow internally, and communicate with the aqueduct of Sylvius. The thalami optici are not well developed.

A large transverse fissure divides the cerebrum from the cerebellum (Fig. 75, No. 5). The optic chiasm (Fig. 75, No. 15) behind which lies the hypophysis (Fig. 62, No. C, 20) covers the region of the middle brain. The large transverse fissure is the dividing line between the hemispheres and the optic lobes. If the hypophysis be removed there will be observed a slit surrounded by gray matter which is called the tuber cinereum et infundibulum (Fig. 75, No. 18). On the pars commissuralis and the after brain there are visible the roots of the fifth, the ninth, the tenth, and the twelfth pairs of cranial nerves. Close beside the median furrow which extends to the front part of the long furrow of the spinal cord there is observed the third pair of cranial nerves, which have their origin in the middle brain. The fourth pair of cranial nerves extend from the roof of the middle brain on both sides between the middle brain and the optic lobes. These nerves finally emerge and become visible on the ventral surface.

The sixth nerve is visible near the middle furrow and almost in the middle of the pyramids, and near the roots of the fifth, the seventh, the ninth, the tenth, and the twelfth pair of cranial nerves.

THE DIVISIONS OF THE BRAIN

(Thalamus (Pineal body (Infundibulum (Hypophysis Forebrain (Optic tract and chiasm (Cerebral hemispheres (Olfactory lobes (Third ventricle (Lateral ventricles

(Peduncles of the cerebrum Midbrain (Optic lobes (Aqueduct

(Medulla oblongata Hindbrain (Cerebellum (Fourth ventricle

The medulla oblongata (Fig. 62, No. C, 1; Fig. 75, A and C) terminates anteriorly in the pars commissuralis and the pars peduncularis. As the spinal cord approaches the head there is a gradual swelling, or lateral thickening, which merges into the medulla oblongata. The superior and the inferior surface of the medulla oblongata (Fig. 75, No. A, 1 and C, 17) are flattened. There is a shallow furrow and a slight swelling at the point where the hypoglossal nerve emerges. The central canal of the spinal cord gradually comes closer to the upper surface and communicates with the fourth ventricle, at which point the posterior raphe is shortened and the sulcus longitudinalis posterior becomes shallow. At the point where the central canal terminates in the fourth ventricle there is a V-shaped point called the calamus scriptorius (Fig. 75, No. 2). The fourth ventricle is located on the upper wall of the medulla oblongata below the cerebellum, and is bounded laterally by the peduncles of the cerebellum. It is marked posteriorly by the calamus scriptorius and anteriorly by the valve of Vieussens.

The valve of Vieussens is located at the posterior end of the aqueduct of Sylvius. The posterior part of the fourth ventricle is marked by grooves or furrows. Extending along the central part of the floor of the fourth ventricle there is a sulcus, or groove, called the sulcus centralis, which divides the superior pyramids. On the median lateral sides there are two points of gray substance which form the alæ cinereæ. The roots of the pneumogastric, the glosso-pharyngeal, and the spinal accessory may be traced to the alæ cineræ and the gray matter of the ridges of the medulla oblongata. The sixth pair of the cranial nerves emerge from the medulla near the sulcus centralis and to the side of the auditory nerve. Part of the trigeminus emerges from the rim of the furrow next to the ridge. At the outer border of the medulla are observed the thick ends of the roots of the pneumogastric, the spinal accessory, the glosso-pharyngeal, and the auditory facialis.

On both sides of the sulcus longitudinalis inferior are found the inferior pyramids, or pyramidal columns. These pyramids become expanded near the origin of the third pair of nerves and merge into the cerebral peduncles, or crura cerebri.

The pyramidal fibers may also be traced to the optic lobes. The roots of the abducens are found at a point between the crura cerebri and the optic lobes. The third pair lies to the side of this, and the roots of the trigeminus are adjacent to those of the third. A bundle of fibers from this region pass into the cerebellum and form the crura cerebelli; others pass into the cerebellum from the side, spreading out in fan-shaped radiation, and forming the white central substances peculiarly arranged, called the arbor vitæ, or tree of life. A third bundle fuse with the crura cerebelli anteriorly, and pass into the peduncles of the cerebrum, or crura cerebri.

The gray ganglionic substance forms columns which extend the whole length of the medulla oblongata and into the cerebri. The medulla oblongata contains numerous centers which preside over various visceral functions as deglutition, respiration, thermotactic, secretory, cardiac, and digestion.

In the medulla as in the spinal cord there are five main groups of cells. First, the posterior or upper horns, from which come the somatic sensory nerves. Second, Clark’s cells, located centrally, which are the origin of the ganglionated splanchnic nerves. Third, cell groups of the lateral horns which are the center for the non-ganglionated splanchnic nerves, and certain other nerves for the viscera, including the enteric muscles. Fourth, cell groups of the anterior horns, which are the center of all somatic voluntary muscles. Fifth, groups of single cells probably belonging to the posterior horns which are probably centers for other splanchnic nerve fibers.

FIG. 77, A.

G, A section through the tuberculum bigeminum. 1, Pia mater. 2, Nerve fibers. 3, Fine granular ground substance. 4, Thin layer of small cells. 5, Fine granular ground substance. 6, A second thin cell layer. 7, Fine granular ground substance. 8, Third thin cell layer. 9, Fine granular ground substance. 10, Fourth widely extended cell layer. 11, A fine granular layer. 12, A thick layer of spindle cells. 13, A layer of medullated nerve fibers. 14, Ependemal cells. 15, The sinus.

H, A transverse section through the medulla oblongata, 1, 3, 4 and 6 are ganglionic centers. 2, The dorsal groove. 8, The ventral fissure. 5, The central canal. 7, Fiber tracts.

I, A transverse section through the cervical portion of the spinal cord. 1, Ventral septum. 2, Dorsal septum. 3, The central canal. 4, The dorsal horn. 5, The ventral horn.

J, A section through the wall of the oil gland, or rump gland. 1, The stroma. 2, The tubular glands some of which are branched.

K, A section through a spike of a comb of a cock. 1, The epithelial layer. 2, A dense fibrous subepithelial layer. 3, A second fibrous vascular layer. 6, A denser central core supporting the large arteries and veins. 4, A vein. 5, An artery.

L, A section through a lobe of the wattle of a hen. 1, The epithelial layer. 2, A dense vascular layer. 3, A vein. 4, An artery. 5, A loose fibrous vascular structure. ]

The gray matter in the cord is in the form of two commas placed with their backs together with the central canal passing between them. The central canal is located in the center of the cord. As the cord nears the medulla oblongata the form of the gray matter changes. The second, the third, and the fifth ganglion centers are arranged laterally, and show a distinct side-horn group. The central canal separates the two Clark’s columns; later the side-horn group enlarges and extends ventrally around the anterior horn group and joins on the other side in half-moon shaped formation.

The central ganglionic mass of the spinal cord, the center for the enteric visceral system, is divided into three complex parts. First, there are cell groups on both sides of the posterior raphe. Second, the half-moon shaped ventral groups extending parallel peripherally described above. Third, lying between groups one and two a complex group, which possesses a large collection of cells. These furnish splanchnic nerve centers, the anterior supplying motor nerves and carrying motor impulses outward, and the superior sensory which carry impulses to the centers. The cell group of the superior horns takes a sidewise peripheral position (Gadow).

The cerebellum (Fig. 62, C, 2) is located above the medulla oblongata and posterior to the cerebrum. The cerebellum consists principally of a median lobe called the worm. The lateral lobes are conical and rudimentary. The under part of the worm forms the roof of the fourth ventricle.

On the upper surface of the cerebellum are numerous transverse markings (in the hen 13 or 14) which divide the lobe into leaves. When the cerebellum is cut lengthwise there is observed on the sectioned surface the peculiar arrangement of the white and the gray matter, the arbor vitæ, mentioned above. There is a small cavity in the cerebellum which communicates with the fourth ventricle.

The cerebellar cortex shows four parts, namely: First, the central part composed of white medullated fibers, between which are arranged neuroglear cells. Second, a rather reddish layer of cells of different sizes. These cells are embedded in a fine ground substance and are about 0.003 mm. in diameter. Third, a plain layer of large pear-shaped ganglion cells, the cells of Purkinje. Fine branches extend outward from the apex of these cells. Fourth, an outer gray layer which consists of small multipolar nerve cells with some neuroglear cells. The color of these three layers, according to Gadow, depends upon the color of the plumage; that is, if the plumage is dark, this layer is dark; if light, the layers are light. The marked coloring is said to be most distinct in the second layer.

The anterior portion of the cerebellum consists of medullated nerves whose fibers run crosswise. These are the extensions of the true cerebellar fibers, which become fewer and fewer as they proceed anteriorly. Finally, a few remaining fibers running crosswise fuse as the commissura Sylvii, and extend into the lobi optici. A layer of nerve cells extends under these medullated nerve fibers.

A bundle of fibers extending from the medulla oblongata. These fibers extend into the optic lobes, form the crura cerebri, or peduncles of the cerebri, and also form the inferior wall, or floor, of the aqueduct of Sylvius. The crura are sometimes spoken of as the partes pedunculares. These contain gray matter. The following division is made of the ganglion of this region. First, a group of cells near the base of the peduncles, which are divided from the ventral rim by the arciform fibers. This ganglionic formation may be considered an extension from the medulla oblongata. Second, a group of the ganglion cells of the lobus optici. Third, a group of cells lying near the lobus opticus, which give rise to the ascending roots of the trigeminus. Fourth, a group of ganglionic cells from which spring the roots of the motores occuli, and which lies near the middle line and under the sulcus centralis. Many ganglion cells are found to the right and to the left of the sulcus centralis, which show different arrangement of the cells at different levels. Fifth, a group of irregular cells which extends centrally on the superior walls of the third ventricle and to the superior part of the lobus opticus.