Nervous System (continued).

B. The Peripheral Nervous System.

1. The Spinal Nerves.

Remove the abdominal viscera carefully so as to expose the spinal nerves which are seen as white cords on each side of the vertebral column as follows:—

(a) The first or hypoglossal nerve arising between the 1st and 2nd vertebrÆ. It runs forwards on the under surface of the head beneath the mylo-hyoid muscle and supplies the tongue. It was noted in the third day's work.

(b) The second and third nerves unite immediately after escaping from the spinal cord to form the brachial plexus, which supplies the muscles of the shoulder, and then extends along the arm.

(c) The fourth, fifth, and sixth nerves are small and supply the muscles and the skin of the body wall.

(d) The seventh, eighth, and ninth nerves run obliquely backwards, the eighth and ninth uniting to form the sciatic plexus, which gives rise to the large sciatic nerve which extends along the thigh, and down the leg. The sciatic plexus also receives a branch from the seventh nerve.

(e) The tenth or coccygeal nerve, which escapes through a small aperture in the side of the urostyle, and is distributed to the walls of the cloaca and urinary bladder. It is connected by a branch with the sciatic nerve. This nerve is frequently small or absent in the Platana.

[In the Frog, the seventh, eighth, and ninth nerves unite to form the sciatic plexus which gives rise to the large sciatic nerve extending along the thigh, and dividing above the knee into the tibial and peroneal nerves to the leg and the foot. The tenth nerve is present.]

2. The Sympathetic Nervous System.

This system consists of a cord on each side closely following the course of the systemic arches and the dorsal aorta. It has on it six ganglionic swellings, and there are connections between the ganglia and all the spinal nerves. Anteriorly each of the cords is joined to the vagus nerve; it ends posteriorly as a broad swelling on the ninth spinal nerve.

[In the Frog the system consists of a chain of ganglia on each side of the body closely following the course of the systemic arches and the dorsal aorta. The cords have connections with the spinal nerves. Anteriorly each cord ends in the Gasserian ganglion of the trigeminal nerve and posteriorly it is united with the coccygeal nerve by a varying number of branches.]

Make a complete sketch showing the spinal nerves and the sympathetic nervous system.

3. The Cranial Nerves.

Expose the brain and note the following cranial nerves:—

(a) The olfactory nerve, arising from the anterior end of the olfactory lobes, supplying the olfactory capsule.

(b) The optic nerve, arising from the ventral surface of the brain below the optic lobes, supplying the retina of the eye.

(c) The trigeminal nerve, arising just behind the optic nerve, giving rise to the following branches:—

(i) The ophthalmic nerve, which passes over the muscles of the eye and supplies the skin in the region of the olfactory capsule.

(ii) The maxillary nerve, which runs forward under the eyeball, and supplies the skin covering the upper jaw.

(iii) The mandibular nerve, which is at first continuous with the maxillary, and then bends round the inner surface of the upper jaw to extend along the outer surface of the mandible.

(d) The glossopharyngeal and the vagus (or pneumogastric) nerves, leaving the skull together, immediately behind the auditory capsule. The glossopharyngeal passes along in front of the hypoglossal nerve and proceeds along the base of the tongue. The vagus, after giving off branches to the dorsal muscles, reaches the walls of the pharynx and run obliquely backwards to supply the pharynx, heart, lungs and stomach.

Fig. 18.—Brain. Cranial Nerves, Spinal Nerves, and Sympathetic Nervous System of Xenopus and Rana.