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of the tears.
D Duct to the nose.
pulling it gently outward with the finger, a small hole which will admit a pin. This is the orifice of the canal, or drain, which leads to the nose. The two canals end in a larger one, on the side
of the nose, the lachrymal sacl and duct,” which leads down to the nasal cavity, where the fluid is not only thus conveniently got rid of, but is of further use in moistening the nose, as waste water is sometimes employed to irrigate the neighbouring ground. To prevent the lachrymal fluid from running over on the cheek, there is a beautiful provision.
The edge of each lid is square Apparatus for the secretion and removal cut, like the edge of a table ; G Lachrymal gland, and its ducts to the
the eyelashes grow along the
outer margin, and along the CC Canals for removal.
inner may be seen, even with
the naked eye, a row of from thirty to forty very fine points. These are the openings of the Meibomian glands, which secrete an oily or greasy substance. This greasing of the edges prevents the lids adhering to each other, and serves to turn the fluid and keep it within bounds, just as a greasy surface rejects water or ink. The fluid has no choice then but to flow into the drains which are open to receive it within the greasy line. When, under the influence of mental emotion, or when it is necessary to wash away some foreign substance, the gland secretes more actively, the fluid is then unable to escape by the drains, and, gathering at the inner corner of the eye, at last, like the swollen river, the tears overflow their banks, and roll down the cheek.
3. The BRAIN is divided into cerebrum and cerebellum.3 The cerebrum, or great brain, is double, consisting of right and left halves, or hemispheres, joined across the middle by commissures. Each hemisphere is composed of two parts. First, the lower or deeper, composed of a series of ganglia, or masses of grey matter, named, from before backwards : 1. The Corpus Striatum ;4 2. The Optic 1 Saccus, a bag.
? Duco, to lead.
Thalamus, these two being each about the size of a pigeon's egg,
Secondly, the outer and greater portion, composed of the convolutions. These are ridge
30 like elevations separated by valleys, or sulci,3 the
Plan of the brain as if by vertical section through one opposed sides of which side. are separated only by the blood-vessels which nour- 2 Ganglia. ish them ; so that the
B Cerebellum and its three processes.
C Upper end of spinal cord divided, showing central rounded edges of the
grey matter, and the white columns. convolutions alone come to the surface, like the edges of the leaves of a closed book. The convolutions average the third of an inch in breadth, and an inch, or often less, in depth ; thus increasing the surface for the grey matter by about six times, compared with what a smooth brain would have presented. Each convolution is composed, externally, of a layer of grey or vesicular matter, about an eighth or tenth of an inch in thickness ; forming a continuous folded sheet over the whole brain, and internally of a central stalk of white conducting matter. The convolutions cover the whole surface of the hemisphere, and many therefore lie hidden in between the two hemispheres, and in the base, as well as where Optic Bed.
2 Convolvo, to roll together; rather to wind or turn. 3 Sulcus, a groove or furrow.
they come in relation with the surface of the head, on the top, front, sides, and back. They are not isolated parts, as they appear when cut across, as in the diagrams, but, when seen on surface views, are tortuous and continually branching so as to be continuous with each other, like a network. They are not the same in different brains or even on the two sides of the same brain, as if intended simply as a means of increasing the surface, without regard to symmetry ; though in those quadrupeds in which they are shallow and few in number, and also in different brains of the same species, they correspond exactly on the two sides.
The white or conducting matter of the hemispheres, issuing from the convolutions, is arranged in three ways. 1. Connecting the right and left hemispheres, and forming the great commissure termed the corpus callosum ; thus enabling the two sides of the brain to
communicate and act with each other. 2. Connecting the convolutions, with the body generally, passing down to the ganglia, or lower brain, and thence by the crus cerebri or great stalk of the cerebrum, to the spinal cord. And 3. Probably many fibres connect together convolutions on the same side, enabling them to act upon and with each other.
The cerebellum is about an eighth Vertical section showing the rela
part of the weight of the cerebrum. It is placed below the back part of the
cerebrum, which overlaps it a little, 11 Convolutions. 3 Corpus callosum.1
the two meeting just at the bony pro2 One of the gangliaand com- minence which may be felt at the back
of the head. The cerebellum, unlike the 4 Crus cerebri.3
other parts of the nervous centre, is single, the middle line being occupied not by a commissure but by a portion of cerebellum. It has a plaited, rather than a convoluted look, but this is mainly due to the convolutions being parallel and subdivided by lesser ones, both on their edges and their sides. The result is, a proportionally much greater surface or package of grey matter, though the thickness of it is necessarily less, than in the
1 Hard, or firm, body. 2 Ganglion, a collection of grey matter, or a knot, or swelling, on a nerve. 8 Stalk, or leg, of the cerebrum.
cerebrum. Likewise, in the lower animals, as reptiles and birds, we observe that the cerebellum has become subdivided while the cerebrum is yet smooth and simple ; so that convolution, or plaiting, both begins first, and is carried to the highest extent, in the cerebellum.
The central white matter of each side of the cerebellum sends off three conducting processes, or channels of action, as seen in Fig. 10; one upwards to the cerebrum ; a second downwards to the spinal cord ; and the third and largest across to join its fellow, and form the commissure, or bridge of Varolius.
The SPINAL CORD, or Medulla? Spinalis, is about the size of an average finger above, becoming smaller below, where it ends opposite the lowest rib. Each half is composed first, of a central pillar of grey matter, which joins its fellow across the middle line, and is crescentic in form in a transverse section (Fig. 10). Secondly, on the outside, it is composed of two white columns, the posterior of which is mainly connected with the cerebellum, while the middle and fore part, or antero-lateral column, is connected with the cerebrum, and is joined across the middle to its fellow by a thin commissure.
The NERVES at their origin are forty in number. On each side, there are nine cerebral, the rest being spinal. The 1st cerebral nerve is the olfactory, or nerve of smell. The 2d, the optic. The 3d, 4th, and 6th are the motor nerves of the eye. The 5th nerve moves the muscles of the jaw, and gives feeling to the face and head. The 7th nerve gives motion to the features, by one portion ; while the other is the auditory or nerve of hearing. The 8th nerve, partly motor partly sensory, is mainly the nerve of respiration and digestion. And the 9th gives motion to the tongue.
The spinal nerves resemble each other at their origin, having two roots, the anterior motor, and the posterior sensory, as first discovered by Sir Charles Bell. The posterior root, like all other nerves of common sensation, has a swelling, or ganglion on it, formed of intermixed grey matter. Thereafter, the two roots mingle together and form a compound spinal nerve. Ultimately, however, most of the sensory filaments go to the skin, while all the motor ones are distributed to the muscles.
FUNCTIONS OF THE NERVOUS CENTRES.—The spinal cord is partly a centre for reflex actions, and partly a conductor for sensation and voluntary motion between the brain and the spinal nerves. Its upper part, by which it joins the brain, termed the Medulla Oblongata, is the most vitally important part, as it regulates the respiratory act. Here about a third or fourth part of the fibres decussate or cross the middle line, which in part explains the singular fact that one side of the brain regulates the opposite side of the body. The remaining fibres decussate either above or below
1 Medulla, marrow.
The cerebellum appears to be the organ which regulates muscular action. We are led to this conclusion by the correspondence in animals between its size and the activity of their motions ; as well as by the results of direct experiment.
The cerebral ganglia appear to be the seat of sensation, and are probably also the centres for those reflex motions which are guided by sensations.
The functions of the convolutions are but ill understood, except in a general sense. In man they are enormously developed, and form as a whole the instrument through which the mind works, including the emotions and moral feelings as well as the intellect. That each faculty has its local connexion, as held by the phrenologists, is not admitted by physiologists as proved. Whether as a whole, or in parts, however, we know that this organ is the favoured seat of the necessary union between the two parts of man's nature, the mind and body. This union could not exist without the one influencing the other; and so we find, that, while on the one hand, mental action excites and employs and fatigues the brain,— on the other hand, that the state of the brain, as a part of the body, exerts an important influence on the mind. Thus irritability, peevishness, despondency, or their opposites, may depend simply on the state of the stomach, or on the want of exercise, fresh air, or a clean skin ; so that the preservation of our bodily health is a duty which we owe to the mind, and to our neighbours, no less than to the body itself.
THE DIGESTIVE, CIRCULATORY, AND RESPIRATORY SYSTEMS.
The parts subservient to sensation, motion, and intelligence, have now been considered, and the body might have served with these only, were it not that all living action is attended by waste.