Mention some other Plexuses of the Sympathetic system. They are very numerous; besides those already mentioned, the most important are the following: :

Tympanic Plexus,-described under Nerves of the Tympanum.

Meningeal Plexus, on the middle meningeal artery.

Facial Plexus, surrounding the facial artery.

Cardiac Plexuses, the deep in front of the bifurcation of the trachea, the superficial in front of the right pulmonary artery; the first named lies behind the arch of the aorta, the latter beneath it.

Coronary Plexuses, anterior and posterior, accompanying respectively the left and right coronary arteries.

Aortic Plexus, on the sides and front of the aorta between the superior and inferior mesenteric arteries.

Hypogastric Plexus, on and between the common iliac arteries; supplying the viscera of the pelvic cavity.

Inferior Hypogastric or Pelvic Plexuses, two in number, one on each side of the rectum and bladder. Their branches are the

Inferior Hemorrhoidal Plexus.
Vesical and Prostatic Plexuses.
Vaginal Plexus.

Small Cavernous Nerve.

Large Cavernous Nerve.
Uterine Nerve.

What is the Vidian Nerve? It has generally been described as a branch of the spheno-palatine (Meckel's) ganglion (see p. 137), running backwards through the Vidian canal, and dividing into a Petrosal Branch to the Facial nerve, and a Carotid Branch to the carotid plexus of the Sympathetic. It is now, however, more correctly described as a short nerve of communication, between the Facial and the Sympathetic on the one hand, and the Sphenopalatine Ganglion on the other, supplying that ganglion with its motor and vaso-motor roots. It is formed by the junction of the

Large Superficial Petrosal, Br. of the Facial (p. 138), with the—

Large Deep Petrosal, Br. of the Carotid Plexus (p. 140), which occurs in the cartilage filling in the foramen lacerum medium (p. 35). It then enters the Vidian Canal (p. 20), in the pterygoid process of the sphenoid bone, passing through which to finally join the posterior part of the spheno-palatine ganglion (p. 137). Some filaments, the

Upper Posterior Nasal Branches,—are apparently given off from the Vidian in the canal, to the septum and roof of the nose; but they are really branches from the ganglion, bound up in the same sheath with the Vidian

nerve.

VISCERAL ANATOMY.

What is a Viscus? Viscus, gen. visceris, pl. viscera, is a term which is applied to any internal organ of the body. The Viscera are the organs contained in the three great cavities-cranium, thorax and abdomen-with their appendages. Of these the Heart and Brain have been described, with the circulatory and nervous systems respectively. [See pp. 99, 120; also the COMPEND OF PHYSIOLOGY in this series.]

THE DIGESTIVE ORGANS.

What is the Alimentary Canal? A musculo-membranous tube, from 25 to 30 feet in length, extending from the mouth to the anus, lined throughout with mucous membrane, furnished with several accessory organs, and performing the functions of ingestion, digestion, and egestion.

Name its Subdivisions. They are the Mouth, Pharynx, Esophagus, Stomach, Small Intestine (duodenum, jejunum and ileum) and Large Intestine (cæcum, colon and rectum). The first three lie above the diaphragm, the rest below it.

Name the Accessory Organs of Digestion. They are the Teeth, Salivary glands (parotid, sub-maxillary, sub-lingual), Liver, Pancreas and Spleen.

THE TEETH.

What are the Teeth? They are 32 organs of digestion (20 being temporary, or milk-teeth), situated one-half in each jaw, imbedded in the alveolar processes, and partly surrounded by the Gums, which are composed of fibrous tissue and covered with mucous membrane. In each half of each jaw there

are

Temporary Teeth (5) -2 Incisors, I Canine, 2 Milk-molars.

Permanent Teeth (8)—2 Incisors, I Canine, 2 Bicuspids, 3 Molars.

What are their General Characteristics? Each tooth presents a—

Crown, or Body,—the part seen projecting above the gum.

Neck, the constricted portion between the crown and the fang.

Fang, or Root,—imbedded in the alveolus; and surrounded by the periodental membrane or periosteum lining the alveolus.

Pulp-cavity, in the interior, opening at the apex of the fang for the entrance of vessels and nerves.

State the Characteristics of each class of Teeth.

Incisors, or Cutters.-Crown chisel-shaped, beveled posteriorly. Fang single, long, thickest antero-posteriorly.

Canines, or Tearers,-Crown thick and conical. Fang longest and thickest of all the teeth, forming a projection on the alveolar arch. The 2 upper canines are the "eye-teeth."

Bicuspids,-Crown has 2 cusps. Fang single but grooved deeply, showing a marked tendency to bifurcate.

FIG. 65.

Molars, or Grinders,-Crown large, low and cuboid in shape, has 4 cusps on upper molars, 5 on the lower ones. Fangs multiple, usually 3 on the first two upper molars, 2 on the first two lower ones; the third molar of either jaw, having but one fang, is called the "wisdom tooth," and is the smallest of the three.

The 2d or 3d Temporary Molar is larger than the first.

Describe the Structure of a Tooth. Each consists of

Dentine or Ivory,2-composed of tubules sur-
rounded by the inter-tubular tissue or Matrix,
and opening into the pulp cavity. It resembles
compact bone in appearance and in composi-
tion, consisting of 28 parts Animal matter and
72 Earthy matter. The Tubules are delicate
wavy canals, diameter about 300 of an inch,
which branch outwardly and anastomose with
each other, forming concentric shadings or
Schreyer's Lines.

Enamel,3-
3-covers the crown; consists of very
dense tissue, which contains but 31⁄2 per cent. of
animal matter. It is covered by a very delicate
epithelial cuticle, Nasmyth's Membrane, which
when intact withstands the action of acids.

Crusta Petrosa or Cement,1-the enamel of the fang; is a layer of true
bony tissue, containing lacunæ, canaliculi, and Haversian canals.
Pulp,-fills the pulp-cavity and is prolonged into the dental tubules; is soft,
vascular, and sensitive; and consists of connective and fibrous tissue,

nucleated cells, blood-vessels and nerves. anastomose with each other, those situated Odontoblasts.

The cells are caudate and superficially being termed

What Arteries and Nerves supply the Teeth? The Arteries are derived from the inferior dental, and from the alveolar and infraorbital branches of the internal maxillary. The Nerves are derived from the inferior dental branch of the inferior maxillary division of the 5th, and also from the anterior and posterior dental branches of the superior maxillary division of the same nerve.

When do the Temporary Teeth appear? Their eruption begins about the 7th month after birth, with the central incisors, and ends with the appearance of the second molars, about the age of two years. The lower teeth slightly antedate the upper. Their formula is as follows:

Mo. Mo. Ca. In.

(Upper... I

I

I

I

-=20

about the end of the 6th year, followed by the incisors about the 7th or 8th year, the bicuspids from the 9th to the 10th year, the canines about the IIth or 12th year, the second molars from the 12th to the 13th year, and the third molars from the 17th to the 25th year. Those of the lower jaw are slightly in advance of the corresponding upper ones. Their formula is as

follows:

Wis. Mo. Mo. Bi. Bi. Ca. In. In. In In. Ca. Bi. Bi. Mo. Mo.Wis.

What is the greatest Number of Teeth at one time in the jaws, and when? Forty-eight, namely, all the temporary and permanent teeth except the third molars,-occurring between the 5th and 7th years of age.

Describe the Development of the Teeth. They arise from the mucous membrane covering the maxillary arches, in which, about the 6th foetal week, appears a depression, the Primitive Dental Groove, from the floor of which arise papillæ of mucous membrane to form the pulp of the milk-teeth.

In the Follicular Stage, membranous septa form across the groove, and its margins become thick and prominent.

The Saccular Stage extends from the 13th week to the 16th, and is marked first by the projection of the papillæ from the follicles, next by the growth of the follicular margins, the formation of processes or Opercula thereon, which meet and close in the papillæ; and finally by the closing in of the dental

groove by the union of its margins. A Cavity of Reserve for each tooth is then formed by the closure of the secondary dental groove, from the floor of which another papilla arises to form the germ of the permanent tooth.

The dental pulps now take the forms of teeth, a thin lamina of dentine appears and increases from without inward, the enamel organ and membrane are formed, and when calcification has advanced sufficiently, the pressure of the teeth causes the absorption of the gum above them, the septa ossify and the eruption of the teeth occurs. The Cement is formed from the periodental membrane, at a later period of life.

THE MOUTH.

Describe the Mouth. It is an oval cavity formed by the lips, cheeks, jaws, palate and tongue, forming the superior portion of the alimentary canal, and opening posteriorly into the pharynx by the fauces. It presents the teeth (already described), the tongue (to be described), and also the— Hard Palate, formed by the palate processes of the superior maxillary and palate bones, and covered with mucous membrane; forms the roof of the mouth.

Soft Palate, formed by 5 muscles on each side, viz., the levator palati, tensor palati, palato-glossus, palato-pharyngeus, and the azygos uvulæ ;-the latter forming with its fellow the Uvula, a descending muscular projection. Anterior Pillars of the Fauces,-arch downwards and forwards to the base of the tongue, and contain the palato-glossi muscles.

Posterior Pillars of the Fauces,-arch downwards and backwards to the sides of the pharynx, and contain the palato-pharyngei muscles.

Isthmus Faucium,-the space bounded by the pillars, the free border of the palate, and the base of the tongue.

Tonsils, are small, elongated, glandular bodies, situate done on each side of the fauces, between the anterior and posterior pillars. Each has 12 or 15