vertebræ, generally of a fusiform shape, may be taken as the focus of this gangliform plexus. At this point the great sympathetic establishes connexions with every one of the cerebral nerves, by filaments which may be traced to each, in several of which ganglions are formed,—as the lenticular in the orbit, the cavernous within the skull, and the spheno-palatine, otic, and submaxillary connected with the fifth. It anastomoses freely with the facial and eighth, with the ninth, and with three or four of the superior cervical nerves. Reinforced by filaments from the facial and vagans, its twigs extend along the ramifications of the external and internal carotid artery, forming sheaths. which may be traced accompanying the larger divisions, but becoming too minute for detection by the eye, even when aided by the microscope, though there is no reason to suppose that their distribution is limited to the larger trunks. According to this view, therefore, their distribution must be co-extensive with that of the arteries; and consequently the brain itself is supplied with nerves, as are likewise the nerves themselves, just as we have blood-vessels ramified upon the heart and coats of the blood-vessels. From this nervous centre we therefore perceive connexions established and filaments extended to the brain, the organs of sense, and those of the secretions connected with them, to the organ of voice and deglutition, and the different muscles which put them in motion, and to those subservient to the movements of the neck. Lastly, a branch or branches run down behind the sheath of the common carotid and internal jugular vein, considered as the continuation of its trunk.

Thoracic Ganglionic Centre.-At the lower part of the neck, opposite the fifth, sixth, and seventh cervical vertebræ, are placed two or three ganglions, the superior termed middle, and the lower inferior cervical ganglions. They form numerous connexions with the vagans, phrenic, and nerves of the axillary plexus. The frequent interlacements surround the blood-vessels proceeding from and

entering the chest. Nervous sheaths accompany the common carotid artery upwards, to unite with those from the superior ganglion, along the vertebral artery to the spinal marrow and brain, and along the artery of the arm throughout all its divisions and subdivisions to their ultimate terminations. Others run along the airtubes to the lungs associated with the branches of the vagans to the heart, and along the gullet; so that from this centre we have emanating the filaments which are connected with the arm and upper part of the walls of the chest, the organs of respiration, the organs of circulation, and the gullet. The great sympathetic, proceeding down the back part of the chest behind the investing membrane, establishes connexions with the twelve intercostal nerves, forming a ganglion opposite to each. Springing from four to seven branches, there are two nerves which enter the abdomen through the diaphragm by separate passages, named from their distribution on the viscera the great and small splanchnic nerves.

Abdominal Ganglionic Centre.-By the two splanchnics, reinforced by branches from the vagans and a few filaments from the phrenic, an exceedingly intricate nervous centre is formed of an assemblage of ganglions and plexuses at the upper part of the abdomen, lying upon the diaphragm behind the stomach. On each side there is a ganglion larger than the rest, which has received the name of semilunar, from the shape it usually presents. Each semilunar ganglion forms connexions with its fellow of the opposite side, surrounding one of the principal trunks from the aorta; and as we have got two moonshaped ganglions, so have we here a solar plexus. The nerves of the right and left form here, by numerous intricate interlacements with an assemblage of ganglions, a great nervous centre, from which issue filaments that sheath the arteries arising from the abdominal aorta, and accompany them in their distribution to the stomach and intestinal canal, to the spleen, pancreas, and liver, to the

kidneys, to the organs of reproduction, and lastly, along the bifurcations of the aorta to the lower limbs.

Pelvic Ganglionic Centre.-Filaments of continuation pass from the thorax into the abdomen behind the diaphragm, and run down by the side of the vertebræ of the loins, sending filaments to the plexuses from the abdominal centre. Entering the pelvis, they pursue their course in the front of the sacrum, at the extremity of which the sympathetics of each side unite together in the formation of a single ganglion, named ganglion impar. Throughout the course of the filaments of continuation, connexions are established and ganglions formed, as in the regions of the neck and chest, with the spinal nerves. the pelvis, twigs are sent off to the extremity of the intestinal canal, to the urinary and reproductive organs, along the arteries ramified within the pelvis, and to accompany those which pass out to be distributed externally.

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CHAPTER VIII.

SIGHT.

Arrangement of the different parts of the Apparatus of Sight-Appendages of the Eye-The Orbit-The Eyebrows-The Eyelashes-The Eyelids and their Motions-Lachrymal Apparatus-Motions of the Eyeball-The Globe of the Eye capable of Motion in every direction-The Muscles by which the movements of the Eye are effected, as under the command of the Will, and by Instinctive Feelings-How we see an object single with two Eyes-Regulation of the Sphere of Vision-Compound Eyes of Insects-Some species of Butterflies furnished with 34,650 Eyes-Squinting-Various Causes of-The Globe of the Eye Sclerotic Coat gives protection, and affords insertion to other partsChoroid Coat-Furnished with a Dark Secretion to absorb Light-Its intimate connexion with the Retina-The Iris, or Variegated Curtain-The hole in its centre, the Pupil-Regulates the quantity of Light admitted in the Posterior Chamber-A single object held close to one Eye, several Images produced, and the object appears multiplied-Transparent parts of the Eye-Aqueous Humour very rapidly renewed-Adaptation of the Eye to near and distant, to large and minute objects-Crystalline Lens-Its beautiful organization-Vitreous Humour -The Retina the immediate Organ of Sight-Manner in which the Retina may see its own Blood-vessels-Insensibility to Light at the entrance of the Optic Nerve Seat of distinct Vision-Curious effects from indirect Vision-Continuance of the Impression of Light-Cause of the appearance of Light when the Eye is struck-Refraction of Light-Short-sightedness-Long-sightedness--Sunbeam decomposed into seven coloured rays-Means by which we estimate the size and distance of objects, by the Visual Angle, the Intensity of Light, Shade, and Colour, and by contrast with other known objects.

LIGHT has a considerable effect upon the whole of animated nature; it promotes the growth and vigour of vegetables, and produces powerful and beneficial effects on the health and comfort of animals, from the highest to the lowest. The more perfect animals are furnished with instruments by which they are enabled to appreciate several of the properties of light, whereby they become

acquainted with near and distant objects, obtaining from this source conceptions which vastly extend the sphere of their knowledge, and contribute to their gratification and delight.

The eyes are the material instruments by which we receive our impressions of the different modifications of light. The brilliancy and beauty of these noble organs cannot escape the attention of the most heedless, and their admirable adaptation to the purposes for which they have been furnished us, must call forth the admiration and gratitude of every one who gives the subject even a superficial consideration.

The different parts of the apparatus of sight may be conveniently considered under three divisions; first, The appendages of the eye, by which it is protected from external injury, preserved in a condition adequate for its duties, and whereby its motions are effected; secondly, The various coats, which perform different functions according to the structure of each, and the humours through which the rays of light pass to be pictured upon the immediate seat of vision; and thirdly, The nervous screen which receives the image, and transmits the impression to the seat of perception in the brain, where the mind takes cognizance of it.

Appendages of the Eye.-The orbit is a four-sided vault of a pyramidal shape, composed of seven bones; the margin is rounded, and projects at the upper part over the anterior surface of the eye, whereby that organ is protected from blows or other external causes of injury. It is more complete in man than in other mammalia. The axes of the two orbits, if continued backwards, cross each other, since they diverge somewhat towards the temples, but the divergence is less in man than even in apes, which approach him so nearly in their structure.

The Eyebrows are composed of loose cellular substance, covered with skin, from which spring short bristly hairs, projecting outwards. Like all hairs, they penetrate