Chapter XII

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TECHNICAL NOTE.--The teacher should be provided with several well-cleaned skeletons of the toad in order that the bones may be carefully studied. Boil in a soap solution a toad from which most of the muscles and skin have been removed (see p. 452). Leave in this solution until the muscles are quite soft and then pick off all bits of muscles and tissue from the bones. If this is carefully done, the ligaments which bind the bones will be left intact and the skeleton will hold together.

Note that the _skeleton_ (fig. 2) consists of a head portion which is composed of many bones joined together to form a bony box, the _skull_; of a series of small segments, the _vertebræ_, forming the _vertebral column_, which with the skull forms the _axial skeleton_; and of the _appendicular skeleton_, consisting of the bones of the fore and hind limbs. Note that the skull is composed of many bones joined together, some by _sutures_, while others are fused. Do the limbs attach directly to the axial skeleton? The anterior limbs (arms) articulate with the _pectoral_ or _shoulder-girdle_. The arms will be seen to be made up of a number of bones placed end to end. Note that the uppermost, the _humerus_, is attached to the pectoral girdle, while at its lower end it articulates with the _radio-ulna_. At the lower end of the radio-ulna is a small series of _carpal_ bones which afford attachments for the slender finger-bones, the _phalanges_ or _digital_ bones. The bones of the leg are articulated with a closely fused set of bones, the _pelvic girdle_. The leg-bones, proceeding from the pelvic girdle, are named _femur_, _tibio-fibula_, _tarsal_ bones, and _phalanges_ or _digits_. To what bones of the arm do these correspond? Determine the other principal bones of the skeleton by reference to figure 2.

[Illustration: FIG. 2.--Skeleton of the garden toad.]

TECHNICAL NOTE.--In a specimen which has been macerated for some time in 20% nitric acid dissect out the nervous system. Place the specimen in a pan ventral side uppermost and pin out. Carefully pick away the vertebræ and the roof of the mouth-cavity, thereby exposing the central nervous system, which will appear light yellow.

Examine the _brain_. In front of the true brain are the _olfactory lobes_, the nervous centre for the sense of smell. The brain itself is composed of several parts. The anterior portion consists of two elongated parts, the _cerebral hemispheres_; just back of these are the optic lobes or _midbrain_, consisting of two short lobes, which are followed by the small _cerebellum_, which in turn is followed by a long part, the _medulla oblongata_, which runs imperceptibly into the long dorsal nerve, the _spinal cord_. Note the large _optic nerves_ running out to each eye. How far backward does the spinal cord extend? Note the many pairs of nerves given off from the brain and spinal cord. These nerves branch and subdivide until they end in very fine fibres. Some end in the muscle-fibres, and through them the central nervous system innervates the muscles. These are _motor endings_. Still others pass to the surface and receive impressions from the outside. These last are _sensory endings_. Note that the _spinal nerves_ arise from the spinal cord by two roots, an _anterior_ or _ventral_, and a _posterior_ or _dorsal root_. Trace the principal spinal nerves to the body-parts innervated by them. These nerves are numbered as first, second, etc., according to the number of the vertebræ (counting from the head backward) from behind which they arise.

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