process to meet one from the squamosal, forming the zygomatic arch. 7. The two nasals, forming the roof of the nose. 8. The two maxillæ, that part of the upper jaw in which the canines, premolars, and molars are lodged. 9. The two premaxillæ, in which the upper incisors are situated. 10. The two palatines, which, with the maxillary bones, form the roof of the mouth. There are two appendages to the skull: the mandible, or lower jaw, whose condyles, or rounded extremities, fit into a cavity (the glenoid) in the temporal bone; and the hyoid bone, situated at the root of the tongue.

The simplest form of the skull is a cartilaginous box, as in sharks, inclosing the brain and supporting the cartilaginous jaws and gill arches. In higher fishes this box is overlaid with bony plates and partly ossified. In frogs the skull is mainly bony, although a good deal of the cartilage remains inside the bones. In higher vertebrates the cartilage never makes an entire box, and early disappears.

The cervical vertebræ, or bones of the neck, are peculiar in having an orifice on each side of the centrum for the passage of an artery. The first, called atlas, because it supports the head, has no centrum, and turns on the second, called axis, around a blunt peglike projection, called the odontoid process. The centra are usually wider than deep, and the neural spines very short, except on the last one. The number of cervical vertebræ ranges from I in the frog to 25 in the swan.

The dorsal vertebræ are such as bear ribs, which, uniting with the breastbone, or sternum, form a bony arch over the heart and lungs, called the thorax. The sternum may be wanting, as in fishes and snakes, or greatly developed, as in birds. When present, the first vertebra whose ribs are connected with it is the first dorsal.

The neural spines of the dorsal series are generally long, pointing backward.

The lumbar vertebra are the massive vertebræ lying in the loins between the dorsals and the hip bones.

The sacral vertebra lie between the hip bones, and are generally consolidated into one complex bone, called

sacrum.

The caudal vertebræ are placed behind the sacrum, and form the tail. They diminish in size, losing processes and neural arch, till finally nothing is left but the centrum. They number from 3 or 4 in man to 270 in the shark.

Besides the lower jaw, hyoid, and ribs, vertebrates have other appendages to the spinal column two pairs of limbs.1 132 The fore limb is divided into the pectoral arch (or shoulder girdle), the arm, and the hand. The arch is fastened to the ribs and vertebræ by powerful muscles, and consists of three bones, the scapula, or shoulder blade, the coracoid, and the clavicle, or collar bone. The scapula and coracoid are generally united in mammals, the latter being a process of the former; and the clavicles are frequently wanting, as in the hoofed animals. The humerus, radius, and ulna are the bones of the arm, the first articulating by ball and socket joint with the scapula, and by a hinge joint with the radius and ulna. The humerus and radius are always present, but the ulna may be absent. The bones of the hand are divided into those of the carpus, or wrist; the metacarpus, or palm; and the phalanges, or fingers. The fingers, or "digits," range in number from 1 to 5.

The hind limb is composed of the pelvic arch (or hip bones), the leg, and the foot. These parts correspond closely with the skeleton of the fore limb. Like the shoulder, the pelvic arch, or os innominatum, consists of three bones ilium, ischium, and pubis. The three are

distinct in amphibians, reptiles, and in the young of higher animals; but in adult birds and mammals they become united together, and are also (except in whales) solidly attached to the sacrum. The two pelvic arches and the sacrum thus soldered into one make the pelvis. The leg bones consists of the femur, or thigh; the tibia, or shin bone; and the fibula or splint bone. The rounded head of the femur fits into a cavity (acetabulum) in the pelvic arch, while the lower end articulates with the tibia, and sometimes (as in birds) with the fibula also. An extra bone, the patella, or kneepan, is hung in a tendon in front of the joint between the femur and tibia of the higher animals. The foot is made up of the tarsus, or ankle; the metatarsus, or lower instep; and the phalanges, or toes. The toes number from I in the horse to 5 in man.

Certain parts of the skeleton, as of the skull, are firmly joined together by zigzag edges or by overlapping; in either case the joint is called a suture. But the great majority of the bones are intended to move one upon another. The vertebræ are locked together by their processes, and also by a tough fibrous substance between the centra, so that a slight motion only is allowed. The limbs furnish the best examples of movable articulations, as the ball and socket joint at the shoulder, and the hinge joint at the elbow. The bones are held together by ligaments, and to prevent friction, the extremities are covered with cartilage, which is constantly lubricated with an unctuous fluid called synovia.

A chemical analysis of bone shows it to consist mainly of phosphate and carbonate of lime and phosphate of magnesia mingled with glutin, chondrin, and oil, the amount of each varying in different animals.

FIG. 309.Skeleton of the Perch (Perca fluviatilis): 1, frontal; 4, postfrontal; 7, parietal; 8, supraoccipital; 9, exoccipital; 11, proötic; 12, pterotic; 15, nasal; 17, premaxillary; 18, maxillary; 19, prenasal; 20, suborbital plates; 21, supratemporal (peculiar to Fishes); 23, mastotemporal; 24, ectopterygoid; 27, metapterygoid; 28, operculum, or flap, closing the gill openings; 30, preoperculum; 31, quadrate; 32, suboperculum; 33, interoperculum; 34, dentary- that part of the lower jaw containing the teeth; 35, articular; 36, angular; 42, urohyal, lying between the branches of the os hyoides; 46, post temporal; 47, supraclavicle; 48, clavicle; 50, digital rays of pectoral fin; 51, coracoid; 52, scapula; 53, basal cartilages; 67, 68, abdominal vertebræ; 69, caudal; 70, attachment of the caudal fin, 71; 72, ribs; 73, styliform processes; 74, 79, interspinous bones; 75, dorsal fins; 80, pelvic bone; 81, ventral fin; 83, b, hæmal spines; 85, a, parapophyses; 86, anal fin.