to surround the alveoli with a very complete network. It is in these capillaries that the oxidation of the blood takes place, the gases having to traverse only the capillary wall, an extremely small amount of fibro-elastic tissue, and the thin epithelial lining of the alveoli. The veins return by the side of their respective arteries.

Fig. 52.-Section of lung: a, Cross-section of small bronchus; b, alveolus; c, alveolar wall lined with flat epithelium; d, top wall of alveolus entirely cut across; e, section into infundibulum, showing several alveoli opening into it.

Other small branches from the arteries occur which serve entirely to supply nutrition to the tissue.

The lymphatics form three systems: the first is found in the connective tissue beneath the pleura and communicates with the pleural cavity.

The second accompany the blood-vessels between the lobules.

The third begin under the epithelium of the bronchi, follow the blood-vessels to the root of the lung, where they unite with the previous two systems and pass into the bronchial glands.

Numerous masses of diffuse adenoid tissue occur in the submucosa of the trachea and bronchi, and small nodes occur especially near their bifurcations.

Both medullated and nonmedullated nerve-fibers are found following the course of the blood-vessels and

Fig. 53. Thyroid gland: a, Acini completely filled with colloid material; b, same partly filled.

lymphatic vessels. unknown.

The termination of the fibers is

THE THYROID.

The thyroid is a compound tubular gland, the excretory ducts of which are closed. It falls within the classification of "ductless glands." It is surrounded by a capsule of fibrous tissue, which penetrates the body of the organ, dividing it into irregular lobules.

The acini are lined with short, columnar epithelium, resting upon a well-marked basement membrane. The lumen of the acini contain epithelial cells, leukocytes, granular matter, and occasional red blood-corpuscles, held together by a yellowish, gelatinous mass of colloid substance.

The blood-vessels enter the fibrous tissue between the lobules, and send rich capillary network to surround the acini.

The lymphatics are quite numerous, being found in the connective tissue of the capsule and between the lobules.

QUESTIONS FOR CHAPTER XII.

270. Describe the structure of the larynx.

271. Describe the structure of the trachea.

272. Describe the general structure of the air-passages.

273. What is the structure of the alveoli ?

274. What structures separate the air from the blood in the alveolar capillaries?

275. Describe the blood-supply of the lungs, the lymphatics, and the nerve-supply.

276. What type of gland is the thyroid? Describe its structure. 277. What substance is found in its acini?

278. Describe its blood-vessels and lymphatics.

CHAPTER XIII.

THE CUTANEOUS SYSTEM.

THE cutaneous system embraces the skin, nails, hair, and glands. The teeth are also sometimes included, but have been described with the digestive system.

THE SKIN.

The skin consists of two distinct portions—the cuticle, epidermis, or false skin, and the corium, cutis vera, or true skin.

The epidermis is the more external of the two, and consists of several layers of epithelial cells, varying much in thickness in different locations.

The innermost layer of these cells (the stratum Malpighii, or rete mucosum) rests upon the irregularly projecting papillæ of the corium and consists of two or three layers of columnar cells. These cells are in rapid process of growth and multiplication, which is indicated by their darkly and easily staining. In the colored races, and in some situations in the white, these cells are found to contain pigment, to which the color of the skin is due.

External to these cells a layer of irregularly polyhedral cells connected with each other by minute spines is found, called the prickle cells. Outside of these a narrow layer of flattened polyhedral cells containing many granules of eleidin occur, called the stratum granulosum.

Above this is a narrow layer of still flatter polyhedral cells which are largely transformed into keratin, the nuclei showing very indistinctly (the stratum lucidum).

Fig. 54.-Skin: A, Epidermis; B, cutis vera; a, stratum Malpighii; b, prickle cells; c, stratum granulosum; d, stratum lucidum; e, stratum corneum; f, duct of sweat-gland; g, coil of sweat-gland; h, adipose tissue.