748. The ABDUCENTES (sixth pair) passes through the opening by which the carotid artery enters the cranial cavity. It is the smallest of the cerebral nerves, and is appropriated to the external straight muscle of the eye.

749. The PORTIO MOLLIS (seventh pair) enters the hard portion of the tem'po-ral bone at the internal auditory opening, and is distributed upon the internal ear. (Fig. 147, 148.)

Fig. 127.

Fig. 127. A representation of the distribution of the eighth pair of nerves with some branches of the fifth. 1, 2, 3, 5, 7, 9, Are branches of the eighth pair. They are distributed over the face in a radiated manner, which constitutes the pes anserinus, (foot of a goose.) The nerves 4, 6, 8, are branches of the fifth pair. 10, 11, 12, 13, 14, 15, 16, Are branches of nerves from the upper part of the spinal cord, (cervical.)

750. The FACIAL NERVE (eighth pair) passes from the skull through an opening situated below the ear, (mastoid foramen.) It is distributed over the face, supplying the muscles with nervous filaments.

748. What is said of the abducentes, or sixth pair of nerves? 749. Of the portio mollis? Explain fig. 127. 750. Of the facial nerve?

751. The GLOSSO-PHARYNGEAL NERVE (ninth pair) passes from the brain, through an opening with the jugular vein, (foramen lac'e-rum.) It is distributed to the mucous membrane of the tongue and throat, and also to the mucous glands of the mouth.

752. The PNEUMOGastric nerve (tenth pair) escapes from the brain through the foramen lacerum. It sends branches to the larynx, pharynx, œsophagus, lungs, spleen, pancreas, liver, stomach, and intestines. (Fig. 132.)

753. The SPINAL ACCESSORY NERVE (eleventh pair) has its origin in the respiratory tract of the spinal cord. It connects with the ninth and tenth pairs of nerves, and is distributed to the muscles about the neck.

754. The HYPO-GLOSSAL NERVE (twelfth pair) passes from the brain, through a small opening, (con'dy-loid foramen.) It ramifies upon the muscles of the tongue, and is its motor

nerve.

Observation. The cranial nerves, with the exception of the olfactory, optic, and auditory, connect with each other by means of filaments. They also send connecting nervous filaments to the upper spinal nerves, (cervical,) and the sympathetic nerve.

751. Describe the glosso-pharyngeal nerve. 752. The pneumogastie nerve. 753. The spinal accessory nerve. 754. The hypo-glossal nerve.

CHAPTER XXXVII.

ANATOMY OF THE NERVOUS SYSTEM, CONTINUED.

755. THE spinal column contains the spinal cord, the roots of the spinal nerves, and the membranes of the cord.

756. The SPINAL CORD extends from the medulla oblongata to the second lumbar vertebra, where it terminates in a rounded point. It presents a difference of diameter in different parts of its extent, and exhibits three enlargements. The uppermost of these is the medulla oblongata. There is no distinct demarkation between this enlargement and the spinal cord. The next corresponds with the origin of the nerves distributed to the upper extremities; the third enlargement is situated near the termination of the cord, and corresponds with the attachment of the nerves which are intended for the supply of the lower extremities.

757. An anterior and posterior fissure divides the spinal cord into two lateral cords. These are united by a thin layer of white substance. The lateral cords are each divided by furrows into three distinct sets of fibres, or columns; namely the anterior, lateral, and posterior columns. The anterior are the motor columns; the posterior are the columns of sensation; the lateral columns are divided in their function between motion and sensation. They contain the fasciculus described, by Sir Charles Bell, as the respiratory tract.

755-767. Give the anatomy of the spinal cord, spinal nerves, and the sympathetic nerve. 755. What does the spinal column contain? 756. Give the extent of the spinal cord. How many enlargements has this cord? What is said of each enlargement? 757. Into how many parts is the spinal cord divided? Give the function of these columns.

758. The SPINAL NERVES, that connect with the spinal cord, are arranged in thirty-one pairs, each arising by two

Fig. 128. A section of the brain and spinal column. 1, The cerebrum. 2, The cerebellum. 3, The medulla oblongata. 4, 4, The spinal cord in its canal.

Fig. 129. Anterior view of the brain and spinal cord. 1, 1, The two hemispheres of the cerebrum. 3, 3, The cerebellum. 4, The olfactory nerve. 5, The optic nerve. 7, The third pair of nerves. 8, The pons varolii. 9, The fourth pair of nerves. 10, The lower portion of the medulla oblongata. 11, 11, The spinal cord. 12, 12, Spinal nerves. 13, 13, The brachial plexus. 14, 14, The lumbar and sacral plexus.

roots; an anterior, or motor root, and a posterior, or sensitive root. Each nerve, when minutely examined, is found to consist of an aggregate of very delicate filaments, enclosed in a common cellular envelope.

759. The anterior roots arise from a narrow white line upon the anterior columns of the spinal cord. The posterior roots arise from a narrow gray band formed by the internal gray substance of the cord. They are larger, and the filaments of origin more numerous than those of the anterior A ganglion is found upon each of the posterior roots in the openings between the bones of the spinal column through which the nerve passes.

Fig. 130. A section of the spinal cord, surrounded by its sheath. B, A spinal nerve, formed by the union of the motor root (C) and the sensitive root (D.) At D, the ganglion upon this root is seen.

760. After the formation of the ganglion, the two roots unite, and constitute a spinal nerve, which passes through the opening between the vertebræ on the sides of the spinal column. The nerves divide and subdivide, until their minute filaments ramify on the tissues of the different organs.

759. Give the origin of the anterior roots. Of the posterior roots. In what respect do the posterior roots differ from the anterior? 760. When do the two roots unite, and where do they pass?