Fig. 119.
Fig. 119. A section of the end of the finger and nail. 4, Section of the last bone of the finger. 5, Fat, forming the cushion at the end of the finger. 2, The nail. 1, 1, The cuticle continued under and around the root of the nail, at 3, 3, 3.
722. The NAILS are hard, elastic, flexible, semi-transparent scales, and present the appearance of a layer of horn. The nail is divided into the root, the body, and the free portion. The root is that part which is covered on both surfaces; the body is that portion which has one surface free; the free portion projects beyond the end of the finger.
723. The nail is formed of several laminæ, or plates, that are fitted the one to the other; the deepest is that which is last formed. The nails, as well as the hoofs of animals and 325 the cuticle, are products of secretion. They receive no blood-vessels or nerves. If the cuticle be removed in severe scalds they will separate with it, as the hoofs of animals are removed by the agency of hot water. The nails increase in length and thickness, by the deposition of albumen upon their under surface, and at their roots, in a manner similar to the growth of the cuticle, of which they constitute a part.
How can it be spread along the hairs? Why should soap not be used in washing the hair? 721. Of what use is the hair upon the head? About the flexions of the joints? In the nasal and ear passages? Upon the eyebrows and eyelids? 722. Describe the nails. 723. How are they formed?
Observations. 1st. The nail upon its under surface is fashioned into thin vertical plates, which are received between the folds of the sensitive skin. In this manner, the two kinds of laminæ reciprocally embrace each other, and the firmness of connection of the nail is maintained. If we look on the surface of the nail, we see an indication of this structure in the alternate red and white lines which are there observed. The former of these correspond with the sensitive laminæ; the latter with the horny plates. The ribbed appearance of the nail is due to the same circumstance. These sensitive laminæ are provided with an unusual number of capillary vessels for the formation of the nail, and hence they give a red tint to the portion under which they lie.
2d. Near the root of the nail there is a part that is not laminated, and it is less abundantly supplied with blood-vessels. This portion consequently looks pale compared with the laminated portion, and from its half-moon shape is technically termed lunula. Beyond the lunula, the root of the nail is imbedded in the fold of the sensitive skin, and has the same relation to that structure that any single one of the thin horny plates of its under surface has to its corresponding pair of sensitive laminæ.
724. The nails, from their position, are continually receiving knocks, which produce a momentary disturbance of their cell formation, followed by a white spot. The care of the nails should be strictly limited to the knife or scissors, to 326 their free border, and an ivory presser, to prevent adhesion of the free margin of the scarf-skin to the surface of the nail. This edge of the cuticle should never be pared, the surface of the nail never scraped, nor the nails cleaned with any instrument whatever, except the nail-brush, aided by water and soap. An observance of these suggestions, will prevent irregularities and disorders of the nails.
Give observation 1st. Observation 2d. 724. How should the nails be treated to prevent irregularities and disease?
Observations. 1st. When we wear a shoe that is too short for the foot, the edge of the nail is brought against the leather. This interrupts the forward growth of the nail, and it spreads out on the sides, and becomes unusually thick. It then presses upon the soft parts, and is said to “grow into the flesh.” The prevention is, to wear shoes of ample size.
2d. Instances are by no means unfrequent in which the power of production of the nail at the root becomes entirely destroyed, and it then grows in thickness only. When this affection occurs, it is often remarkable what a mass the nail presents. Instances are on record, where the nail is regularly shed; and, whenever the old nail falls off, a new one is found beneath it, perfectly formed. Sometimes the growth in length is not entirely checked, although growth in thickness is induced; the nail then presents a peculiar appearance.
What causes the edge of the nail “to grow into the flesh” of the toe? How prevented?
725. In the preceding chapters, we have seen how various and complex are many of the motions necessary to maintain the life of an animal whose organization is superior to all others. We have noted the wonderful mechanism of the muscular system, in producing the varied movements of the body, the different processes by which the food is converted into chyle and mixed with the blood, and the circulation of this fluid to every organ and tissue of the system, that each may select from it the very principles which it requires for its growth.
726. Lymphatic absorption commences as soon as nutrition is completed, and conveys the useless, worn-out particles of different tissues back into the circulating fluid; while the respiratory organs and secretory glands perform the work of preparing the waste products to be eliminated from the body. Each of these processes effects a single object, and is performed in a regular manner.
727. “They must succeed each other in proper order in propelling every particle to its proper destination, or life would be sacrificed almost at the moment of its commencement. There is, therefore, a mutual dependence of all portions of the machinery of organic life upon each other, and a necessity for some medium of communication from one organ to another, by which they may convey mutual information of 328 their several conditions, if we may be permitted to employ a figurative expression. Were there no such medium, how would the stomach notify the heart that additional exertion on its part is required, because the stomach is busy in digesting food?
725. What has been noted in the preceding chapters? 726. Show the manner in which the several processes are performed. 727. How must they succeed each other?
728. “When we are exerting the muscular system for a long time in some laborious employment, how else are our members to inform the stomach that they are too much occupied with their duties to spare the blood necessary in digestion; that it is requisite that the appetite should decline; and that digestion should cease for the time, even if the stomach should be oppressed with its contents? When we are thinking, how else are the blood-vessels to be told that an unusual supply of their contents is wanting in the head? or when the whole frame is weary with exertion, how, without some regular line of intelligence between all the organs, is the brain to be instructed that circumstances require that it should go to sleep? To supply the necessary medium of communication, Providence has furnished all the animals that possess distinct organs, with a peculiar apparatus called the Nervous System.”
729. The NERVOUS SYSTEM consists of the Cer´e-bro-spi´nal Cen´tre, and of numerous rounded and flattened white cords, called nerves, which are connected at one extremity with the cerebro-spinal centre, and at the other, distributed to all the textures of the body. The sympathetic nerve is an exception to this description; for, instead of one, it has many small centres, which are called gan´gli-a, and which communicate very freely with the cerebro-spinal centre, and with its nerves.
728. What is the medium of communication from one organ to another? 729–754. Give the anatomy of the brain and cranial nerves. 729. Of what does the nervous system consist? What constitutes an exception to this?
730. The CEREBRO-SPINAL CENTRE consists of two portions: The brain, and the spinal cord. For convenience of description, the nervous system may be divided into the Brain, Cranial Nerves, Spinal Cord, Spinal Nerves, and the Sympathetic Nerve.
731. The term BRAIN designates those parts of the nervous system, exclusive of the nerves themselves, which are contained within the cranium, or skull-bones; they are the Cer´e-brum, Cer-e-bel´lum, and Me-dul´la Ob-lon-ga´ta. These are invested and protected by the membranes of the brain, which are called the Du´ra Ma´ter, A-rach´noid, and Pi´a Ma´ter.
Fig. 120.
Fig. 120. 1, 1, The scalp turned down. 2, 2, 2, The cut edge of the bones of the skull. 3, The external strong membrane of the brain (dura mater,) suspended by a hook. 4, The left hemisphere of the brain, showing its convolutions.
730. Of what does the cerebro-spinal centre consist? How is the nervous system divided? 731. What does the term brain designate? Name them. How are they protected? Describe fig. 120.
732. The CEREBRUM IS divided into two hemispheres, by a cleft, or fissure. Into this cleft dips a portion of the dura mater, called the falx cer´e-bri, from its resembling a sickle. The apparent design of this membrane is to relieve the one side from the pressure of the other, when the head is reclining to either side. Upon the superior surface of the cerebrum are seen undulating windings, called con-vo-lu´tions. Upon its inferior, or lower surface, each hemisphere admits of a division into three lobes—the anterior, middle, and posterior. (Fig. 122, 123)
Fig. 121
Fig. 121. A section of the skull-bones and cerebrum. 1, 1, The skull. 2, 2, the dura mater. 3, 3, The cineritious portion of the cerebrum. 4, 4, The medullary portion. The dark points indicate the position of divided blood-vessels. 5, 5, The lateral ventricles.
732. How is the cerebrum divided? What is the use of the falx cerebri? What is seen upon the superior surface of the brain? Its inferior?
733. When the upper part of the hemispheres is removed horizontally with a sharp knife, a centre of white substance is brought to view. This is surrounded by a border of gray, which follows the depressions of the convolutions, and presents a zigzag outline. The divided surface will be seen studded with numerous small red points, which are produced by the escape of blood from the division of the minute arteries and veins. The gray border is called the cortical, or cineritious portion, while the white central portion is called the medullary. The two hemispheres are connected by a dense layer of transverse fibres, called cor´pus cal-lo´sum.
734. In the interior of the brain there are several cavities, two of which are of considerable size, and are called the lateral ventricles. They extend from the anterior to the posterior part of the brain, and wind their way into other parts of the cerebral organ.
Observation. In the disease called “dropsy of the brain,” (hydrocephalus internus,) the serum, or water, is usually deposited in these ventricles. This is effused from the many small blood-vessels of the membrane in these cavities.
735. The brain is of a pulpy character, quite soft in infancy and childhood; but it gradually becomes more and more consistent, and in middle age it assumes the form of determinate structure and arrangement. It is more abundantly supplied with blood than any organ of the system. No lymphatics have been detected, but it is to be presumed that they exist in this organ.
736. The CEREBELLUM is about seven times smaller than the cerebrum. Like that organ, it is composed of white and 332 gray matter, but the gray constitutes the larger portion. Its surface is formed of parallel plates separated by fissures. The white matter is so arranged, that when cut vertically, the appearance of the trunk and branches of a tree (ar´bor vi´tæ) is presented. It is situated under the posterior lobe of the cerebrum, from which it is separated by a process of the dura mater, called the ten-to´ri-um.
733. Describe the appearance of the brain when a horizontal section has been made. What is the gray border often called? What connects the hemispheres? 734. Describe the ventricles of the brain. In the disease called “dropsy of the brain,” where is the water deposited? 735. What is the character of the brain in childhood? In adults? 736. How does the cerebellum compare in size with the cerebrum?
Fig. 122.
Fig. 122. The under surface, or base, of the brain and origin of the cranial nerves. 1, 1, The anterior lobes of the cerebrum. 2, 2, The middle lobes. 3, 3, The posterior lobes, almost concealed by the cerebellum. 4, 4, The cerebellum. 7, 7, The longitudinal fissure that divides the brain into two hemispheres. 8, The first pair of nerves. 9, 9, The second pair of nerves. 10, The decussation, or crossing, of its fibres. 13, 13, The third pair of nerves. 14, The pons varolii. 15, 15, The fourth pair of nerves. 16, 16, The fifth pair of nerves. 17, The sixth pair of nerves. 18, 18, The seventh and eighth pair of nerves. 19, The medulla oblongata, with the crossing of some of its fibres exhibited. 20, The ninth pair of nerves. 21, The tenth pair of nerves, 22, The eleventh pair of nerves. 23, The twelfth pair of nerves.
Describe this portion of the brain. Explain fig. 122.
737. The MEDULLA OBLONGATA, or that portion of the spinal cord which is within the skull, consists of three pairs of bodies, (cor´pus py-ram-i-da´le, res-ti-for´me, and ol-i-va´re,) united in a single bulb.
Fig. 123.
Fig. 123. The base of the skull and the openings through which the cranial nerves pass. 1, 1, The first pair of nerves. 2, 2, The cribriform plate of the ethmoid bone through which this nerve passes. 3, 3, The second pair of nerves. 4, 4, The optic foramen in the sphenoid bone; through which passes the second pair of nerves. 5, 5, The sphenoidal fissure. 6, 6, The third pair of nerves. 7, 7, The fifth pair of nerves. 8, 8, The ophthalmic branch of the fifth nerve. The third, the ophthalmic branch of the fifth and the sixth nerve pass from the brain through the sphenoidal fissure to the eye. 9, 9, The superior maxillary branch of the fifth nerve. 10, 10, The foramen rotundum, (round opening,) through which the nerve 9, 9, passes to the upper jaw. 11, 11, The inferior maxillary branch of the fifth pair. 12, 12, The foramen ovale, (oval opening,) through which the nerve 11, 11, passes to the lower jaw. 13, 13, The sixth pair of nerves. 14, 14, The seventh and eighth pair of nerves. 15, 15, The opening in the temporal bone, through which the seventh and eighth nerves pass to the face and ear. 16, 16, The ninth pair of nerves. 17, The tenth pair of nerves. 18, 18, The eleventh pair of nerves. 19, 19, The foramen lacerum (rough opening.) The ninth, tenth, and eleventh nerves pass from the brain through this opening. 20, The spinal cord. 21, The foramen spinalis, through which the spinal cord passes. 22, 22, The position of the anterior lobe of the brain. 23, 23, The middle lobe. 24, 24, The posterior lobe. 25, 25, A section of the skull-bones.
737. Describe the medulla oblongata. Explain fig. 123.
738. The DURA MATER is a firm, fibrous membrane, which is exposed on the removal of a section of the skull-bones. This lines the interior of the skull and spinal column, and likewise sends processes inward, for the support and protection of the different parts of the brain. It also sends processes externally, which form the sheaths for the nerves, as they quit the skull and spinal column. The dura mater is supplied with arteries and nerves.
Fig. 124.
Fig. 124. A vertical section of the cerebrum, cerebellum, and medulla oblongata, showing the relation of the cranial nerves at their origin. 1, The cerebrum. 2, The cerebellum, with its arbor vitæ represented. 3, The medulla oblongata. 4, The spinal cord. 5, The corpus callosum. 6, The first pair of nerves. 7, The second pair. 8, The eye. 9, The third pair of nerves. 10, The fourth pair. 11, The fifth pair. 12, The sixth pair. 13, The seventh pair. 14, The eighth pair. 15, The ninth pair. 16, The tenth pair. 10, The eleventh pair. 18, The twelfth pair. 20, Spinal nerves. 21, The tentorium.
738. Describe the dura mater. What is its use? Explain fig. 124.
739. The ARACHNOID, so called from its extreme tenuity, is the serous membrane of the brain and spinal cord, and is, like other serous membranes, a closed sac. It envelops these organs, and is reflected upon the inner surface of the dura mater, giving to that membrane its serous investment.
740. The PIA MATER is a vascular membrane, composed of innumerable vessels, held together by cellular membrane. It invests the whole surface of the brain, and dips into its convolutions. The pia mater is the nutrient membrane of the brain, and receives its blood from the carotid and vertebral arteries. Its nerves are minute branches of the sympathetic, which accompany the branches of the arteries.
741. The CRANIAL NERVES, that connect with the brain, are arranged in twelve pairs. They are called: 1st. The Ol-fact´o-ry. 2d. The Op´tic. 3d. The Mo-to´res Oc-u-lo´rum. 4th. The Pa-thet´i-cus. 5th. The Tri-fa´cial. 6th. The Ab-du-cen´tes. 7th. The Por´ti-o Du´ra. 8th. The Por´ti-o Mol´lis. 9th. The Glos´so-pha-ryn´gi-al. 10th. The Pneu-mo-gas´tric. 11th. The Spi´nal Ac´ces-so-ry. 12th. The Hy´po-glos´sal.
742. The OLFACTORY NERVE (first pair) passes from the cavity of the skull through many small openings in a plate of the eth´moid bone. (This plate is called crib´ri-form, from its resemblance to a sieve.) This nerve ramifies upon the membrane that lines the nasal passages. It is the softest nerve of the body. (Fig. 136.)
743. The OPTIC NERVE (second pair) passes from the interior of the cranium, through an opening in the base of the skull, (fo-ra´men op´ti-cum,) to the cavity for the eye. It pierces the coats of the eye, and expands in the retina.
744. The MOTORES OCULORUM (third pair) pass from the 336 brain, through an opening of the sphe´noid bone, (sphe-noid´al fis´sure,) to the muscles of the eye.
739. Describe the arachnoid membrane. 740. What is said respecting the pia mater? 741. How many pairs of cranial nerves? Name them. 742. Describe the olfactory nerve. 743. The optic nerve. 744. Describe the motores oculorum.
745. The PATHETICUS (fourth pair) passes from the brain, through the sphenoidal fissure, to the superior oblique muscle of the eye.
Fig. 125.
Fig. 125. The distribution of the third, fourth, and sixth pairs of nerves, to the muscles of the eye. 1, The ball of the eye and rectus externus muscle. 2, The upper jaw. 3, The third pair, distributed to all the muscles of the eye, except the superior oblique, and external rectus. 4, The fourth pair passes to the superior oblique muscle. 6, The sixth pair, is distributed to the external rectus muscle.
746. The TRIFACIAL NERVE (fifth pair) is analogous to the spinal nerves in its origin by two roots, from the anterior and posterior columns of the spinal cord. It has a ganglion, like the spinal nerves upon its posterior root. For these reasons, it ranges with the spinal nerves, and is considered the cranial spinal nerve. This nerve divides into three branches:—The oph-thal´mic, superior max´il-la-ry, and inferior max´il-la-ry.
745. The patheticus. What does fig. 125 represent? 746. What is the trifacial nerve sometimes called? Why is it classed with the cranial spinal nerves? Give the names of its branches.
747. The ophthalmic nerve passes from the cranial cavity through the sphenoidal fissure. It sends branches to the forehead, eye, and nose. The superior maxillary nerve passes through an opening in the base of the skull, (foramen ro-tund´dum,) and sends branches to the eye, the teeth of the upper jaw, and the muscles of the face. The inferior maxillary nerve escapes from the cranial cavity through an opening called foramen o-va´le. It sends branches to the muscles of the lower jaw, the ear, the tongue, and the teeth of the lower jaw.
Fig. 126.
Fig. 126. The distribution of the fifth pair of nerves. 1, The orbit for the eye. 2, The upper jaw. 3, The tongue. 4, The lower jaw. 5, The fifth pair of nerves. 6, The first branch of this nerve, that passes to the eye. 9, 10, 11, 12, 13, 14, Divisions of this branch. 7, The second branch of the fifth pair of nerves is distributed to the teeth of the upper jaw. 15, 16, 17, 18, 19, 20, Divisions of this branch. 8, The third branch of the fifth pair, that passes to the tongue and teeth of the lower jaw. 23. The division of this branch that passes to the tongue, called the gust´a-to-ry. 24. The division that is distributed to the teeth of the lower jaw.
747. Where do the filaments of the ophthalmic branch ramify? The superior maxillary? The inferior maxillary? Explain fig. 126.
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, (mas´toid 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 pneumogastric nerve. 753. The spinal accessory nerve. 754. The hypo-glossal nerve.
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. 341
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.
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 342 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.
758. How many pairs of nerves issue from the spinal cord? Explain fig. 128. Fig. 129.
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 roots. 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.
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?
761. The spinal nerves are divided into—
| Cervical, | 8 | pairs, |
| Dorsal, | 12 | “ |
| Lumbar, | 5 | “ |
| Sacral, | 6 | “ |
762. The four lower cervical and upper dorsal pass into each other and then separate to reunite. This is called the brach´i-al plex´us. From this plexus six nerves proceed, which ramify upon the muscles and skin of the upper extremities.
763. The last dorsal and the five lumbar nerves form a plexus called the lumbar, similar to that of the cervical. Six nerves pass from this plexus, which ramify upon the muscles and skin of the lower extremities.
764. The last lumbar and the four upper sacral unite to form the sacral plexus. From this plexus five nerves proceed, that are distributed to the muscles and skin of the hip and lower extremities.
765. The SYMPATHETIC NERVE[19] consists of a series of Gan´gli-a, or knots, extending each side of the spinal column, forming a chain its whole length. It communicates with both the cranial and spinal nerves. With the exception of the neck, there is a ganglion for each intervertebral space. These ganglia are composed of a mixture of cineritious and medullary matter, and are supposed to be productive of peculiar nervous power.
761. Give the division of the spinal nerves. 762. What nerves constitute the brachial plexus? How many nerves pass from this plexus? 763. How many nerves from the lumbar plexus, and where do they ramify? 764. How is the sacral plexus formed? 765. Of what does the sympathetic nerve consist? How is the sympathetic nerve distributed? What exception? Of what are the ganglia composed?
Fig. 131.
Fig. 131. A beautiful representation of the sympathetic ganglia and their connection with other nerves. It is from the grand engraving of Manec, reduced in size. A, A, A, The semilunar ganglion and solar plexus, situated below the diaphragm and behind the stomach. This ganglion is situated in the region (pit of the stomach) where a blow gives severe suffering. D, D, D, The thoracic ganglia, ten or eleven in number. E, E, The external and internal branches of the thoracic ganglia. G, H, The right and left coronary plexus, situated upon the heart. I, N, Q, The inferior, middle, and superior cervical ganglia. 1, The renal plexus of nerves that surrounds the kidneys. 2, The lumbar ganglion. 3, Their internal branches. 4, Their external branches. 5, The aortic plexus of nerves that lies upon the aorta. The other letters and figures represent nerves that connect important organs and nerves with the sympathetic ganglia.
766. The GANGLIA may be considered as distinct centres, giving off branches in four directions; namely, the superior, or ascending, to communicate with the ganglion above; the inferior, or descending, to communicate with the ganglion below; the external, to communicate with the spinal nerves; and the internal, to communicate with the sympathetic filaments. It is generally admitted that the nerves that pass from the ganglia are larger than those that entered them; as if they imparted to the nerve some additional power.
767. The branches of distribution accompany the arteries which supply the different organs, and form communications around them, which are called plexuses, and take the name of the artery with which they are associated. Thus we have the mesenteric plexus, hepatic plexus, splenic plexus, &c. All the internal organs of the head, neck, and trunk, are supplied with branches from the sympathetic, and some of them exclusively; for this reason, it is considered a nerve of organic life.
What is the design of fig, 131? 766. How may the ganglia be considered? 767. What is said of the branches of the sympathetic nerve?
768. The brain is regarded by physiologists and philosophers as the organ of the mind. Most writers consider it as an aggregate of parts, each charged with specific functions, and that these functions are the highest and most important in the animal economy. To the large brain, or cerebral lobes, they ascribe the seat of the faculties of thinking, memory, and the will. In man, this lobe extends so far backward as to cover the whole of the cerebellum. To the cerebellum, or little brain, is ascribed the seat of the animal, or lower propensities.
769. “The constant relation between mental power and development of brain, explains why capacities and dispositions are so different. In infancy, for example, the intellectual powers are feeble and inactive. This arises partly from the inaptitude of a still imperfect brain; but in proportion as the latter advances toward its mature state, the mental faculties also become vigorous and active.”
770. We are able, in most instances, at least, to trace a correspondence between the development of the cerebral lobes and the amount of intelligence possessed by the person. The weight of the brain in man to that of the whole body varies 347 in different individuals. The heaviest brain on record was that of Cuvier, which weighed 4 pounds and 13½ ounces.
768–772. Give the physiology of the nervous system. 768. How is the brain regarded by physiologists and philosophers? What do they ascribe to the cerebrum? To the cerebellum? 769. What does the relation between mental powers and development of brain explain? 770. What is said respecting the correspondence between the development of the brain and the amount of intelligence possessed by the person? What is said of the weight of the brain?
771. The brain likewise holds an important relation to all the other organs of the system. To the muscular system it imparts an influence which induces contraction of the fibres. By this relation they are brought under the control of the will. To the skin, eye, and ear, it imparts an influence that gives sensibility, or the power of feeling, seeing, hearing, &c.
772. Again, the involuntary functions of the different portions of the system are more or less influenced by the brain. If the action of this central organ of the nervous system is destroyed, the functions of the digestive, respiratory, and circulatory apparatuses will be much disturbed or entirely suppressed.
773. The brain is the seat of sensation. It receives the impressions made on all parts of the body, through the medium of the sensitive nerves. That the impressions of external objects, made on these nerves, be communicated to the brain, where sensation is perceived, it is necessary that they be not diseased or injured.
Observation. There is a plain distinction between sensations and impressions; the latter are the changes produced in the extremities of the nerve; the former, the changes produced in the brain and communicated to the mind.
774. What part of the brain receives the impressions or has the most intimate relation with the intellectual faculties is unknown. Some portions, however, are of less importance than others. Large portions of the cortical, or outer part, are frequently removed without affecting the functions of this 348 organ. Pieces of the medullary, or central parts, have been removed by injuries without impairing the intellect or destroying life. This organ, although it takes cognizance of every sensation, is, of itself, but slightly sensible. It may be cut, or parts may be removed without pain, and the individual, at the same times retain his consciousness.
771. What is said of the relation of the brain to all of the organs of the body? 772. Are the involuntary functions of different parts of the system influenced by the brain? 773. Where is sensation perceived? By what agency are the impressions of external objects conveyed to the brain? What is the difference between sensations and impressions? 774. Is it known what part of the brain has the most intimate relation with the intellectual faculties?
775. The brain is the seat of the will. It superintends the physical as well as the mental movements, and the medium of communication from this organ to the muscles, or the parts to be moved, is the motor nerves. If the brain is in a quiescent state, the muscles are at rest; if, by an act of the will, the brain sends a portion of nervous influence to a voluntary muscle, it immediately contracts, and those parts to which the muscle is attached move. There is no perceptible interval between the act of the will and the motion of the part.
776. Some physiologists assert, that the medulla oblongata is the point at which excitement to motion commences, and sensation terminates; and also, that it possesses the power of originating motion in itself.
Observation. The medulla oblongata, unlike the brain, is highly sensitive; if slightly punctured, convulsions follow; if much injured, respiration, or breathing, immediately ceases.
777. It is remarkable that the nerves which arise from the right side of the spinal cord communicate with the left hemisphere of the cerebrum, and vice versa; this results from the crossing of the fibres in the medulla oblongata. It follows from this, that if the right side of the brain receives an injury, the parts of the opposite side of the body lose their sensibility and motion.
Observations. 1st. If the cranial nerves which are connected 349 by a single root are divided, only the sensation of the part to which they are distributed is lost. Thus, if the optic nerve is divided, the sense of vision disappears, but the motions of the eye are performed as readily as before. But, if the spinal nerves are divided, both sensation and motion of the part to which they lead are destroyed.
What portions have been removed without impairing the intellect? What is remarkable of the brain? 775. What is the influence of the brain upon the muscles? 776. What do some physiologists assert of the medulla oblongata? 777. What is remarkable of the nerves? Give the 1st observation relative to the cranial nerves.
2d. When the spinal cord is divided or compressed, as in fractures of the spinal column, all parts below the fracture are paralyzed, though the nerves leading to these parts may be uninjured.
3d. Again, one side of the body or one limb may become insensible, and the power to move it, be perfectly retained; or the reverse of this may happen—the power of motion will be lost while sensation remains. In the former instance, the function of the posterior, or sensitive column of the spinal cord on one side is destroyed; in the latter, the anterior, or motor column is affected.
4th. In some cases, both sensation and motion of one side of the body or one limb are destroyed. In such instances, both the anterior and the posterior columns of one side of the spinal cord are diseased.
778. Vigorous and controllable muscular contraction requires a sound and well-developed brain. If this organ is defective in these particulars, the movements will be inefficient, and may be irregular. The central organ of the nervous system must, likewise, be in an active condition, to induce regular, steady, and controllable muscular movements.
Observations. 1st. Persons who have suffered from apoplexy and other severe diseases of the brain, have an involuntary trembling of the limbs, which results from a weakened state of the nervous system.
To the spinal nerves. What is said of the compression of the spinal cord? Give the 3d observation relative to the spinal nerves. The 4th observation. 778. Upon what does vigorous controllable muscular contraction depend? What causes the involuntary trembling of the limbs in persons who have suffered from apoplexy?