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The olfactory cells are long, very narrow cells, with a nuclear expansion containing a spheric nucleus, situated at varying distances from the end of the cell. The cells are situated at different levels between the susten

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Fig. 66.-Olfactory mucous membrane: а, Sustentacular cells; b, olfactory cells; c, basal cells; d, submucous fibrous tissue; e, glands of Bowman; l, nervefibers.

tacular cells, and are in connection with delicate filaments of the olfactory nerve at their inner ends.

The basal cells are a row of modified cuboid, sustentacular cells which lie immediately upon the subepithelial tissue.

The subepithelial tissue consists of a rather loose

a

mixture of white fibrous and yellow elastic tissue, into which a number of branched tubular glands, called the glands of Bowman, extend.

The blood-vessels are usually found deeply situated in the subepithelial tissue, sending very numerous branches, however, to the tissue immediately under the epithelium as a capillary network.

The lymphatics are quite numerous, consisting of many nodes and follicles, with lymph-spaces in the upper portion communicating with the large lymphvessels deeper in the tissue.

The nerves are both sensory and special sense, from the trifacial and olfactory nerves respectively. The fibers of the olfactory nerve are nonmedullated, and eventually communicate with the olfactory cells. The sensory fibers are medullated, and are distributed to the under layer of the epithelial, but not to the olfactory cells.

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QUESTIONS FOR CHAPTER XVI.

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345. What are the two divisions of the nasal mucous membrane?

346. Describe the structure of the respiratory mucous brane.

347. What are the two layers of the olfactory mucous membrane ? 348. Describe the sustentacular cells; the olfactory cells; the

s basal cells.

349. Describe the subepithelial tissue. What are the glands of Bowman?

350. Describe the blood-vessels of the nasal mucous membrane ; the lymphatic; the nerves.

CHAPTER XVII.

THE CENTRAL NERVOUS SYSTEM.

THE SPINAL CORD. The spinal cord is continuous with the medulla oblongata, and extends downward in the spinal canal to the beginning of the lumbar region, at which locality it divides into a number of fibers, called the fibræ terminales.

The cord is covered externally by three membranes,the dura mater, the arachnoid, and the pia mater, — which serve as a support to itself and its blood vessels.

The dura mater is the outermost of the membranes of the cord, and is composed of bundles of fibro-elastic tissue, interspersed with thin connective-tissue cells, granular cells, and narrow lymph-spaces. Most of its external surface is attached directly to the bone, but areas occur in which spaces exist containing lymph and lined by endothelium. These spaces are called the epidural spaces.

The inner surface of the dura mater is also covered with endothelium, excepting where it is united by small branches to the arachnoid.

The arachnoid lies internal to the dura, and, while in contact with it throughout most of its extent, is attached only by but a few delicate fibers.

Between the two membranes occasional spaces exist, which are called the subdural spaces.

also occur between the internal surface

Other spaces

of the arachnoid and the pia mater, which are termed the subarachnoidean spaces. All the spaces of the arachnoid are covered by endothelium. Projecting upward into the subdural spaces occasional papillæ occur, which in certain localities are much enlarged, being then called the Pacchionian bodies.

The substance of the arachnoid is of much finer connective tissue than the dura, and its structure is very loose in texture.

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Fig. 67.-Spinal cord, cervical region (diagrammatic), showing secondary tracts of white matter, the dark area representing descending tibers, dotted area ascending fibers, cross-lined area inixed fibers, and clear area gray matter: l.p., direct pyramidal tract; a.g.b., anterior ground bundle; d.c., direct cerebellar tract; a.a.l., ascending anterolateral or Gower's tract; c.p., crossed pyramidal tract; d.a.l., descending anterolateral tract; m.l., mixed lateral tract: g, column of Goll; b, column of Burdach.

Neither blood-vessels nor nerves occur in the arachnoid, but lymph-spaces are numerous.

The pia mater lies internal to the arachnoid, and is applied closely to the substance of the spinal cord. It presents two layers :

The outer layer is very vascular, is composed of fibrous tissue, and is covered on its external surface by endothelium, excepting where small fibers connect it with the arachnoid.

The inner layer is less vascular, and sends prolongations down into the substance of the cord, and between the anterior and the posterior fissure.

A few scattered nonmedullated nerve-fibers occur.

The substance of the cord is composed of white matter and gray matter.

The gray matter is arranged in the form of an H- • shaped mass, situated in the central portion of the cord, and almost entirely surrounded by white matter. The two upright lines of the H are crescentic in outline, and run anteroposteriorly, so that the middle piece will lie transversely. The two anterior portions are slightly shorter and somewhat thicker than their posterior counterparts, and are called the anterior horns, in contradistinction to the latter, which are known as the posterior horns.

The anterior horns have numerous projecting processes pointing outward to the surface, but do not themselves extend as near to the anterior surface as do the posterior horns to the posterior surface.

The posterior horns are narrower, and in most localities extend to the posterolateral surface of the cord.

The middle piece of the H is termed the gray commissure, and contains in its center an opening which is lined with ciliated epithelium, and, being continuous with the ventricles of the brain, contains cerebrospinal fluid. This structure is called the central canal, and serves to divide the gray commissure into two parts, that in front of the canal being called the anterior gray commissure, and that behind the canal the posterior gray commissure.

The white matter surrounds the gray matter, giving

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