When an animal is provided with an internal bony structure, it indicates a high rank in the scale of organization. An elaborate texture of bone is found in no class below the vertebrates. Even in the lower order of this sub-kingdom, which is the highest of animals, bone does not exist, as is the case in some tribes of fishes, such as sharks, etc., and in all classes below that of the cartilaginous fishes, the inflexible substance which sustains the soft parts is either shell or some modification of bone, and is usually found on the outside of the body.
True bone, on the contrary, is found in the interior, and, therefore, in higher animals, the skeleton is always internal, while the soft parts are placed external to the bony frame. While many animals of the lowest species, being composed of soft gelatinous matter, are buoyant in water, the highest type of animals requires not only a bony skeleton, but also a flexible, muscular system, for locomotion in the water or upon the land. Each species of the animal kingdom is thus organically adapted to its condition and sphere of life.
CHAPTER III.
PHYSIOLOGICAL ANATOMY.
THE MUSCLES.
[Illustration: Fig. 19.
Muscular fillers highly magnified.]
The _Muscles_ are those organs of the body by which motion is produced, and are commonly known as _flesh_. A muscle is composed of _fascieuli_, or bundles of fibers, parallel to one another. They are soft, varying in size, of a reddish color, and inclosed in a cellular, membranous sheath.
Each _fasciculus_ contains a number of small fibers, which, when subjected to a microscopic examination, are found to consist of _fibrillae_, or little fibers; each of these fibrillae in turn being invested with a delicate sheath. The fibers terminate in a glistening, white _tendon_, or hard cord, which is attached to the bone. So firmly are they united, that the bone will break before the tendon can be released. When the tendon is spread out, so as to resemble a membrane, it is called _fascia_. Being of various extent and thickness, it is distributed over the body, as a covering and protection for the more delicate parts, and aids also in motion, by firmly uniting the muscular fibers. The spaces between the muscles are frequently filled with fat, which gives roundness and beauty to the limbs. The muscles are of various forms; some are longitudinal, each extremity terminating in a tendon, which gives them a _fusiform_ or spindle-shaped appearance; others are either fan-shaped, flat, or cylindrical.
[Illustration: Fig. 20.
1. A spindle-shaped muscle, with tendinous terminations. 2. Fan-shaped muscle.
3. Penniform muscle. 4. Bipenniform muscle.]
[Illustration: Fig. 21.
Striped muscular fibre showing cleavage in opposite directions. 1. Longitudinal cleavage.
2. Transverse cleavage. 3. Transverse section of disc. 4. Disc nearly detached. 5. Detached disc, showing the sarcous elements. 6. Fibrillae. 7,8.
Separated fibrillae highly magnified.]
Every muscle has an _origin_ and an _insertion_. The term _origin_ is applied to the more fixed or central attachment of a muscle, and the term _insertion_ to the movable point to which the force of the muscle is directed; but the origin is not absolutely fixed, except in a small number of muscles, as those of the face, which are attached at one extremity to the bone, and at the other to the movable integument, or skin. In most instances, the muscles may act from either extremity. The muscles are divided into the Voluntary, or muscles of animal life, and the Involuntary, or muscles of organic life. There are, however, some muscles which cannot properly be classified with either, termed Intermediate. The _Voluntary Muscles_ are chiefly controlled by the will, relaxing and contracting at its pleasure, as in the motion of the eyes, mouth, and limbs. The fibers are of a dark red color, and possess great strength. These fibers are parallel, seldom interlacing, but presenting a striped or striated appearance; and a microscopic examination of them shows that even the most minute consist of parallel filaments marked by longitudinal and transverse _striae_, or minute channels. The fibers are nearly the same length as the muscles to which they belong. Each muscular fiber is capable of contraction; it may act singly, though usually it acts in unison with others. By a close inspection, it has been found that fibers may be drawn apart longitudinally, in which case they are termed _fibrillae_, or they may be separated transversely, forming a series of discs. The _Sarcolemma_, or investing sheath of the muscles, appears to be formed even before there are any visible traces of the muscle itself. It is a transparent and delicate membrane, but very elastic. The _Involuntary Muscles_ are influenced by the sympathetic nervous system, and their action pertains to the nutritive functions of the body. They differ from the voluntary muscles in not being striated, having no tendons, and in the net-work arrangements of their fibers. The _Intermediate Muscles_ are composed of striated and unstriated fibers; they are, therefore, both voluntary and involuntary in their functions. The muscles employed in respiration are of this class, for we can breathe rapidly or slowly, and, for a short time, even suspend their action; but soon, however, the organic muscles assert their instinctive control, and respiration is resumed.
[Illustration: Fig. 22.
Unstriated muscular fiber; at _b_, in its natural state; at _a_, showing the nuclei after the action of acetic acid. ]
[Illustration: Fig. 23.
A view of the under side of the diaphragm.]
THE DIAPHRAGM, or midriff, is the muscular division between the thorax and the abdomen. It has been compared to an inverted basin, the concavity of which is directed toward the abdomen. The muscles receive their nourishment from the numerous blood-vessels which penetrate their tissues. The voluntary muscles are abundantly supplied with nerves, while the involuntary are not so numerously furnished. The color of the muscles is chiefly due to the blood which they contain. They vary in size according to their respective functions. For example, the functions of the heart require large and powerful muscles, and those of the eye, small and delicate ones. There are between four hundred and sixty and five hundred muscles in the human body.
[Illustration: Fig. 24.
A representation of the superficial layer of muscles on the anterior portion of the body.]
[Illustration: Fig. 25.
A representation of the superficial layer of muscles on the posterior portion of the body.]
Very rarely is motion produced by the action of a single muscle, but by the harmonious action of several. There is infinite variety in the arrangement of the muscles, each being adapted to its purpose, in strength, tenacity, or elasticity. While some involuntarily respond to the wants of organic life, others obey, with mechanical precision, the edicts of the will. The peculiar characteristic of the muscles is their contractility; for example, when the tip of the finger is placed in the ear, an incessant vibration, due to the contraction of the muscles of the ear, can be heard. When the muscles contract, they become shorter; but what is lost in length is gained in breadth and thickness, so that their actual volume remains the same. Muscles alternately contract and relax, and thus act upon the bones. The economy of muscular power thus displayed is truly remarkable. In easy and graceful walking, the forward motion of the limbs is not altogether due to the exercise of muscular power, but partly to the force of gravity, and only a slight assistance of the muscles is required to elevate the leg sufficiently to allow it to oscillate.
Motion is a characteristic of living bodies. This is true, not only in animals, but also in plants. The oyster, although not possessing the power of locomotion, opens and closes its shell at pleasure. The coral insect appears at the door of its cell, and retreats at will. All the varied motions of animals are due to a peculiar property of the muscles, termed _contractility_. Although plants are influenced by external agents, as light, heat, electricity, etc., yet it is supposed that they may move in response to inward impulses. The sensitive stamens of the barberry, when touched at their base on the inner side, resent the intrusion, by making a sudden jerk forward. Venus's fly-trap, a plant found in North Carolina, is remarkable for the sensitiveness of its leaves; which close suddenly and capture insects which chance to alight upon them. The muscles of the articulates are situated within the solid framework, unlike the vertebrates, whose muscles are external to the bony skeleton. All animals have the power of motion, from the lowest radiate to the highest vertebrate, from the most repulsive polyp to that type of organized life made in the very image of God.
The muscles, then, subserve an endless variety of purposes. By their aid the farmer employs his implements of husbandry, the mechanic deftly wields his tools, the artist plies his brush, while the fervid orator gives utterance to thoughts glowing with heavenly emotions. It is by their agency that the sublimest spiritual conceptions can be brought to the sphere of the senses, and the noblest, loftiest aims of to-day can be made glorious realizations of the future.
CHAPTER IV.
PHYSIOLOGICAL ANATOMY.
THE DIGESTIVE ORGANS.
_Digestion_ signifies the act of separating or distributing, hence its application to the process by which food is made available for nutritive purposes. The organs of digestion are the Mouth, Teeth, Tongue, Salivary Glands, Pharynx, Esophagus, the Stomach and the Intestines, with their glands, the Liver, Pancreas, Lacteals, and the Thoracic Duct.
[Illustration: Fig. 26.
A view of the lower jaw. _1_. The body.
_2, 2_. Rami, or branches. _3, 3_. Processes of the lower jaw. _m_. Molar teeth. _b_. Bicuspids, _c_. Cuspids. _i_. Incisors.]
The _Mouth_ is an irregular cavity, situated between the upper and the lower jaw, and contains the organs of mastication. It is bounded by the lips in front, by the cheeks at the sides, by the roof of the mouth and teeth of the upper jaw above, and behind and beneath by the teeth of the lower jaw, soft parts, and palate. The soft palate is a sort of pendulum attached only at one of its extremities, while the other involuntarily opens and closes the passage from the mouth to the pharynx. The interior of the mouth, as well as other portions of the alimentary canal, is lined with a delicate tissue, called _mucous membrane_.
The _Teeth_ are firmly inserted in the alveoli or sockets, of the upper and the lower jaw. The first set, twenty in number, are temporary, and appear during infancy. They are replaced by permanent teeth, of which there are sixteen in each jaw; four incisors, or front teeth, four cuspids, or eye teeth, four bicuspids, or grinders, and four molars, or large grinders. Each tooth is divided into the crown, body, and root.
The _crown_ is the grinding surface; the _body_, the part projecting from the jaw, is the seat of sensation and nutrition; the _root_ is that portion of the tooth which is inserted in the alveolus. The teeth are composed of dentine, or ivory, and enamel. The ivory forms the greater portion of the body and root, while the enamel covers the exposed surface. The small white cords communicating with the teeth are the nerves.
The _Tongue_ is a flat oval organ, the base of which is attached to the os hyoides, while the apex, the most sensitive part of the body, is free. Its surface is covered with a membrane, which, at the sides and lower part, is continuous with the lining of the mouth. On the lower surface of the tongue, this membrane is thin and smooth, but on the upper side it is covered with numerous papillae, which, in structure, are similar to the sensitive papillae of the skin.
[Illustration: Fig. 27.
The salivary glands. The largest one, near the ear, is the parotid gland. The next below it is the submaxillary gland.
The one under the tongue is the sublingual gland.]
The _Salivary Glands_ are six in number, three on each side of the mouth. Their function is to secrete a fluid called _saliva_, which aids in mastication. The largest of these glands, the _Parotid_, is situated in front and below the ear; its structure, like that of all the salivary glands, is cellular. The _Submaxillary_ gland is circular in form, and situated midway between the angle of the lower jaw and the middle of the chin. The _Sublingual_ is a long flattened gland, and, as its name indicates, is located below the tongue, which when elevated, discloses the saliva issuing from its porous openings.
The _Pharynx_ is nearly four inches in length, formed of muscular and membranous cells, and situated between the base of the cranium and the esophagus, in front of the spinal column. It is narrow at the upper part, distended in the middle, contracting again at its junction with the esophagus. The pharynx communicates with the nose, mouth, larynx, and esophagus.
The _Esophagus_, a cylindrical organ, is a continuation of the pharynx, and extends through the diaphragm to the stomach. It has three coats: first, the muscular, consisting of an exterior layer of fibers running longitudinally, and an interior layer of transverse fibers; second, the cellular, which is interposed between the muscular and the mucous coat; third, the mucous membrane, or internal coat, which is continuous with the mucous lining of the pharynx.
[Illustration: Fig. 28.
A representation of the interior of the stomach.
_1_. The esophagus. _2_. Cardiac orifice opening into the stomach. _6_. The middle or muscular coat.
_7_. The interior or mucous coat. _10_. The beginning of the duodenum. _11_. The pyloric orifice.]
The _Stomach_ is a musculo-membranous, conoidal sac, communicating with the esophagus by means of the cardiac orifice (see Fig. 28). It is situated obliquely with reference to the body, its base lying at the left side, while the apex is directed toward the right side. The stomach is between the liver and spleen, subjacent to the diaphragm, and communicates with the intestinal canal by the pyloric orifice. It has three coats. The peritoneal, or external coat is composed of compact, cellular tissue, woven into a thin, serous membrane, and assists in keeping the stomach in place. The middle coat is formed of three layers of muscular fibers: in the first, the fibres run longitudinally; in the second, in a circular direction; and in the third, they are placed obliquely to the others. The interior, or mucous coat, lines this organ.
The stomach has a soft, spongy appearance, and, when not distended, lies in folds. During life, it is ordinarily of a pinkish color. It is provided with numerous small glands, which secrete the gastric fluid necessary for the digestion of food. The lining membrane, when divested of mucus, has a wrinkled appearance. The arteries, veins, and lymphatics, of the stomach are numerous.
[Illustration: Fig. 29.
Small and large intestines. _1, 1, 2, 2_.
Small intestine. _3_. Its termination in the large intestine. _4_. Appendix vermiformis.
_5_. Caecum. _6_. Ascending colon.