A Laboratory Manual of the Anatomy of the Rat/The Nervous System
THE NERVOUS SYSTEM
The nervous system controls most of the activities of the rest of the organism. It receives through the sense organs the stimuli from the organs of the body and the external world, transmits them as nervous impulses, coordinates them, and originates impulses which it carries to the muscles, glands, etc., stimulating these structures to action. It comprises the following parts: (1) central nervous system (brain and spinal cord), (2) peripheral nervous system (spinal and cranial nerves, and the sympathetic nervous system), and (3) the sense organs.
THE CENTRAL NERVOUS SYSTEM
The early embryonic central nervous system of vertebrates is a comparatively simple tube (neural tube). The anterior portion develops into the brain, the remainder into the spinal cord. The embryonic brain at first comprises three divisions — in front the prosencephalon, then the mesencephalon, and posteriorly the rhombencephalon. The prosencephalon further differentiates into the telencephalon (which includes the cerebral hemispheres) and the diencephalon. The mesencephalon forms dorsally the optic lobes (corpora quadrigemina), while the rhombencephalon is transformed into the metencephalon (cerebellum and pons) anteriorly and the myelencephalon (medulla oblongata) posteriorly. The cavities of the brain are derivatives of the lumen of the neural tube.
The brain fills the cranial cavity. It has already been adequately preserved by chipping away a part of the roof of the cranium and immersing the animal in the formalin solution. Dissect away the muscles on one side of the head, preserving all nerves found. Remove the dorsal and posterior walls, and one lateral wall of the cranium, taking care not to tear the cranial nerves from the brain. Observe the connective tissue coverings (meninges) of the brain. The dura mater is the tough membrane just beneath the bone. Remove it. The pia mater is the delicate membrane, containing blood vessels, resting directly upon the brain. Between these is the arachnoid layer.
The olfactory lobes are a pair of elongated structures lying between the eyes at the anterior end of the brain. The cerebrum is the large heart-shaped section lying immediately behind the olfactory lobes. It consists mostly of two lateral hemispheres separated by the longitudinal fissure. The cerebellum is the ovoid structure posterior to the cerebrum. It includes the median vermis and the right and left hemispheres. Observe the transverse furrows on both vermis and lobes. The medulla oblongata succeeds the cerebellum and joins the spinal cord posteriorly. Compare the cerebral hemispheres of the rat with those of the dog, cat, man, or other higher mammal. The rat's cerebrum lacks the grooves (sulci) and ridges (gyri) found in these other forms.
Clear away the blood vessels on the surface of the brain. The longitudinal fissure widens posteriorly, exposing the corpora quadrigemina (optic lobes). The pineal body (epiphysis) is the globular structure at the posterior end of the longitudinal fissure. Carefully press aside the hemispheres and observe that of the four lobes of the corpora quadrigemina, two are on the right, and two are on the left. The anterior pair are the superior colliculi, the posterior are the inferior colliculi. The cerebral hemispheres are connected with each other by the corpus callosum, a broad white sheet which will be observed in front of the corpora quadrigemina when the hemispheres are pressed apart.
The paraflocculus projects laterally from the lateral lobe of the cerebellum. Cautiously remove the petrotympanic bone, a bit at a time, and note that the paraflocculus occupies the floccular fossa (see the petrotympanic bone). The paraflocculus resembles a toadstool, the enlarged distal end lying in the fossa. The flattened floccus proper is anteroventral to, and in contact with, the paraflocculus.
Exercise XXVII. Sketch the dorsal view of the brain.
Remove the brain from the skull, being careful to preserve the roots of the cranial nerves. Care should be taken to keep the paraflocculus intact. The connections between the cranial nerves and the brain are easily broken. If considerable care is used in dissecting away the bone around the foramina of the nerves, many of them may be removed almost entire on that side of the head where the muscles were taken away. Separate the brain from the spinal cord and immerse it in a weak formalin solution, where it may be studied.
Each olfactory lobe will now be seen to contain two parts — the olfactory bulb, and the olfactory tract. Each bulb is a swelling at the anterior end of the tract, and in a preserved specimen the former is considerably darker than the latter. The tract rests in a depression on the oblique anterior surface of the cerebral hemisphere. By its distinct whiteness each tract may be followed backward and outward to the level of the optic chiasma, where it dis- appears. Two pear-shaped areas darker than the olfactory tracts extend from the olfactory lobes back to the optic chiasma. They meet in the median plane, and each is limited laterally by the olfactory tract.
The optic nerves of mammals cross at the optic chiasma, and continue on into the brain as the optic tracts. Thus light stimuli received by the right eye may pass to the left side of the brain, and vice versa. The two white and conspicuous optic nerves run forward a short distance from the chiasma before turning outward to enter the orbits. Observe that the optic tracts diverge behind the chiasma, and are there connected with each other by a whitish band.
The infundibulum is the prominence just behind the optic chiasma. It is in contact with the pinkish hypophysis or pituitary body, which is covered by a meningeal fold, so that the hypophysis remains in the cranium when the brain is removed. The pedunculi cerebri flank the posterior part of the infundibulum, joining the cerebrum to the medulla oblongata.
The medulla is wedge-shaped, the expanded anterior end running forward beneath the cerebellum. Just behind the infundibulum a band of nervous tissue, the pons, crosses the ventral surface of the medulla, connecting the cerebellar hemispheres. Two pyramids run back from the pons along the midventral surface of the medulla.
The distribution of many of the cranial nerves can be worked out on that side of the head from which the mus- cles have not been removed. There are twelve pairs of these nerves, as follows.
The first cranial nerves (olfactory) consist of a number of filaments proceeding from the anterior ends of the olfactory lobes, through the cribriform plate of the ethmoid bone, to the mucous membrane of the nasal cavity. They will probably not be seen.
The second cranial nerves (optic) have already been described.
The third nerves (oculomotor) arise from the cerebral peduncles and extend anteriorly beneath the dura mater on the under side of the brain. They enter the orbits through the anterior lacerated foramina and innervate the superior, inferior, and internal rectus muscles, and the inferior obliques. These muscles move the eye.
The fourth nerves (trochlear) emerge from the dorsal side of the brain behind the posterior coLLiculus, run forward along the under surface of the cerebral hemispheres lateral to the second and sixth nerves, and enter the orbits through the anterior lacerated foramina. They innervate the superior oblique muscles of the eye.
Each of the fifth nerves (trigeminal) leaves the pons as two closely connected roots. It then divides into three branches. (1) The ophthalmic branch passes through the anterior lacerated foramen into the orbito-temporal fossa, where it sends branches to the adjacent integument, and a small nerve back into the cranium through a foramen which is anterodorsal to the optic foramen. (2) The superior maxillary is a large branch which also enters the orbito-temporal fossa through the anterior lacerated foramen, runs along the dorsal side of the alveolar process of the maxillary bone, traverses the infraorbital fissure, and terminates at the roots of the vibrissae and in the skin of the face. (3) The inferior maxillary nerve passes from the cranial cavity through the foramen ovale. Immediately after emerging from the cranium it gives off several branches to the jaw muscles, a lingual ramus to the tongue, while a large trunk enters the mandibular foramen of the mandible. This trunk traverses the dental canal, emerging by the mental foramen to pass to the skin of the lower jaw and the chin. If time permits, trace the nerve through the mandible.
The sixth nerves (abducens) start from the anterior region of the medulla oblongata, course along under the brain beside the third cranial nerves, emerge into the orbits through the anterior lacerated foramen and innervate the external rectus muscles of the eyes.
The seventh cranial nerves (facial) emerge from the side of the medulla behind the fifth, and leave the cranium through the facial canals just behind the external auditory meatuses. They send branches to the lateral surfaces of the muscles of mastication, where they may easily be seen when the skin is removed from the head.
The eighth nerves (acoustic) arise from the medulla oblongata behind the seventh, penetrate the auditory capsules, and reach the inner ears.
The ninth nerves (glossopharyngeal) arise from the medulla oblongata close to the tenth. Leaving the skull by the posterior lacerated foramen, they send branches to the pharynx and tongue. They are anterior to, and deeper than, the twelfth nerves, and lie close to the skull.
The tenth nerves (vagus or pneumogastric) arise from the side of the medulla oblongata posterior to the eighth, and leave the cranium through the posterior lacerated foramen. Shortly after emerging they give off the superior laryngeal nerves to the larynx, then proceed back to the thoracic cavity along the common carotid artery. The recurrent laryngeal nerve is given off near the entrance of the vagus into the thorax. Trace it forward, on both sides, along the esophagus to the larynx. On the right it turns forward from the vagus dorsal to the subclavian artery, on the left side dorsal to the aortic arch. The tenth nerves in the thorax lie dorsal to the heart and lungs, which they are said to innervate. Passing backward along the esophagus, they penetrate the diaphragm, then innervate the stomach.
The eleventh nerves (accessory) arise from the anterior region of the spinal cord, and from the medulla enter the skull through the foramen magnum, unite with the vagus nerves, and with them escape through the posterior lacerated foramen. Trace them to the muscles of the neck.
The twelfth nerves (hypoglossal) leave the medulla behind the first ten cranial nerves, escape from the skull through the hypoglossal canals, and may be traced easily to the ventral part of the tongue.
Exercise XXVIII. Draw a ventral view of the brain, showing the origins of the cranial nerves.
Using a very sharp razor cut the brain in two along a plane parallel to, and a millimeter or two at the left of, the longitudinal fissure. Place the larger piece in water and cautiously dissect away the remnants of the left half of the brain, exposing the structures described below.
The corpus callosum is the thick commissure connecting the cerebral hemispheres. Posteriorly it takes the form of an oval mass (splenium); anteriorly it bends downward (genu), forming the rostrum. This section exposes a region of the brain not observed on the dorsal side of the organ—the thalamencephalon, or diencephalon. The diencephalon of the dogfish, frog and other lower vertebrates is visible dorsally, but in mammals the backward extension of the cerebral hemispheres, which is in general coincident with the development of higher intelligence, conceals the diencephalon.
The third ventricle is the cavity within the diencephalon. The fornix is dorsal to this ventricle. From its union, posteriorly, with the corpus callosum, the fornix runs forward and downward anterior to the third ventricle. Observe the anterior commissure at the lower end of the fornix. The lamina terminalis is the anterior boundary of the third ventricle. The conspicuous intermediate mass, or middle commissure, extends from one lateral wall to the diencephalon to the other through the third ventricle. The infundibular recess is the extension of the ventricle into the infundibulum. Posteriorly the ventricle opens into the cerebral aqueduct (aqueduct of Sylvius), which leads back through the mesencephalon to the fourth ventricle, the cavity under the cerebellum and in the medulla oblongata. The aqueduct may be seen best in a transverse section through the corpora quadrigemina, where it will be seen as a narrow vertical slit.
The corpora quadrigemina will be seen above the aqueduct. (Compare them with the optic lobes of the dog-fish.) Observe that the superior coUiculi are longer than the inferior. The isthmus is the deep dorsal fissure that separates the corpora quadrigemina from the cerebellum.
In the sectioned cerebellum note the deep fissures (sulci) which separate the prominent folds (gyri). The gray matter of the cerebellum is on the surface of the gyri, and so completely penetrates the white matter that the latter resembles the branches of a tree, and is therefore called the "arbor vitae."
The fourth ventricle is covered dorsally by the anterior, and the posterior medullary velum, both thin membranes underlying the cerebellum. The former extends from the base of the arbor vitae forward to the inferior colliculus, while the latter spreads backward.
The cerebellum is strongly attached by the peduncles to the medulla on each side. Locate these attachments by dissecting away the peripheral tissue on the under side of the cerebellum. The lateral fibrous tract (brachium pontis or middle peduncle), which runs anteroventrally along the lateral surface of the medulla, enters the pons. The median tract (brachium conjunctivum or anterior peduncle) goes forward toward the corpora quadrigemina. The posterior peduncle appears as a ridge passing back to the dorsal side of the medulla. These tracts may be seen more clearly if they be severed at their union with the cerebellum by a horizontal stroke with a sharp razor.
Clear away the tissue on the dorsal and lateral side of the cerebral hemisphere, exposing the corpus callosum. Trace it into the hemisphere. Cut through the corpus callosum, expose the cavity of the hemisphere (lateral ventricle), and open this ventricle throughout its extent. The anterior horn of the ventricle runs ventrally, bounded medially by the septum pellucidum, and laterally by the convex corpus striatum. The remainder of the cavity follows a curvilinear course, at first posteriorly and laterally, and finally ventrally, terminating near the infundibulum. From the corpus striatum backward the hippocampus forms the median wall of the ventricle. The lateral surface of the hippocampus is convex. Viewed from above it forms an angle of about forty-five degrees with the long axis of the brain; from the side it is approximately U-shaped. The lateral and third ventricles communicate through the interventricular foramina (foramina of Munro).
Exercise XXIX. Sketch a sagittal section of the brain.
THE SYMPATHETIC NERVOUS SYSTEM
The sympathetic nervous system regulates the distribution of blood in the body through its control over the nonstriated muscle tissue in the walls of the blood vessels. This regulation is accomplished through variation in the calibre of the vessels, brought about by the contraction and relaxation of these muscles. Sympathetic fibers innervate also the respiratory, reproductive, digestive, and other organs, together with their glands. The motor activities of the system are beyond the control of the will, and the stimulation of its sensory structures does not necessarily induce conscious states.
The sympathetic system of mammals is centralized in a pair of ganglion-bearing, longitudinal sympathetic trunks, one on each side of the vertebral column. This trunk connects with the spinal cord through the spinal nerves, and also sends fibers to sympathetic plexuses in various parts of the body. These plexuses in turn send nerves to the viscera. Locate the two longitudinal sympathetic trunks in the rat, one on each side of the vertebral column. Trace one of these anteriorly to the head and posteriorly to the pelvic region. The superior cervical ganglion is a spindle-shaped enlargement near the bifurcation of the common carotid artery. The cervical portion of the trunk is dorsal to the tenth cranial nerve. The inferior cervical ganglion is the swelling of the trunk at the base of the neck. The thoracic section lies ventral to the ribs and presents ganglionic enlargements. The lumbar portion is median to the dorsal muscle mass in the lumbar region. Locate if possible the splanchnic nerves, which leave the thoracic ganglia in the vicinity of the eleventh and twelfth ribs, enter the abdomen, and go to the solar plexus. Find other rami of the sympathetic trunk.
THE SPINAL NERVES
The study of the spinal cord and the spinal nerves leading from it should be postponed until dissections and drawings of all other parts of the body, including the sympathetic system, have been completed. Expose the spinal nerves by carefully dissecting away the muscles ventral and lateral to the backbone, from the occipital region of the head to the base of the tail. The spinal nerves are designated according to the region in which they are found, cervical, thoracic, lumbar, sacral, and coccygeal.
The cervical spinal nerves, eight pairs in all, are in the neck. The first pair leaves the spinal canal through a foramen in the neural arch of the atlas. The other seven pairs escape through the intervertebral foramina. The first, second, third, and part of the fourth pairs of nerves go to the neck muscles. The phrenic nerve leaves the fourth cervical nerve. Trace it caudad to the diaphragm.
Dissect the brachial plexus, the conspicuous bundle of intercommunicating nerve trunks in the axillary region. It is formed by the fourth, fifth, sixth, seventh, and eighth cervical, and the first dorsal nerves. Note the size of these nerves. The brachial plexus sends the following branches to the arm, (1) The ulnar nerve goes to the posterior region of the forearm. (2) The median nerve runs across the anterior surface of the elbow joint, supplying the muscles of the inner side of the arm. (3) The axillary nerve passes around the posterior side of the humerus, crosses the elbow joint, and as the radial nerve proceeds to the outer side of the arm. Locate, also, the branches going from the plexus to the thorax.
There are thirteen pairs of thoracic spinal nerves. The first enters the brachial plexus. The second to the twelfth inclusive run in the muscle tissue between the ribs. The thoracic nerves leave the spinal canal through the inter-vertebral foramina. Except for the first, they innervate the walls of the thorax and anterior part of the abdomen.
The six pairs of lumbar nerves are associated with the thick muscle masses in the lumbar and sacral regions. Carefully remove these muscles without severing the nerves or their connections. The first two pairs of lumbar nerves innervate the abdominal walls, by way of the lumbar plexus. Find the lumbar plexus, which is formed on each side by part of the first, second, third, fourth, and fifth lumbar nerves. The three following nerves arise from it. The femoral nerve proceeds caudally to the inner aspect of the hind limb. The obturator nerve passes through the obturator foramen of the coxal bone to the adjacent muscles. The sciatic nerve may be traced from the lumbar plexus to the deeper posterior muscles of the thigh.
The remainder of the fifth lumbar nerve, and the sixth lumbar nerve, together with part of the first and second sacral nerves, form the sacral plexus. There are four pairs of sacral nerves. Locate the caudal nerve in the tail and trace it forward to its connections with the sacral nerves.
Exercise XXX. Sketch the spinal and sympathetic nerves of one side of the body.
THE SPINAL CORD (SPINAL MEDULLA)
Clear away the muscles surrounding the backbone and sever the head from the body at the articulations between the cranium and the atlas. Carefully expose the spinal cord throughout its length by removing the roof of each neural arch. The cord is surrounded by tough membranes, the meninges.
Observe the connections of several of the spinal nerves with the cord. Each nerve proceeds from the cord as a pair of roots, a dorsal and ventral. The former bears a swelling, or ganglion. Trace a pair of roots from their origin to the point where they unite to form the spinal nerve. The dorsal ganglia in mammals contain the cell bodies of sensory neurons which send some of their fibers through the dorsal root to the spinal cord, and other fibers distally to the spinal nerve. The dorsal root is, therefore, sensory in function. The ventral root carries motor fibers from nerve cells located in the cord itself. Thus each spinal nerve carries nervous impulses both to and from the spinal cord.
Observe the cervical and lumbar enlargements of the cord in the neck and lumbar regions, respectively. The former is the source of the nerves entering the brachial plexus, the latter of the fibers going to the sciatic plexus.
Remove the cord from the neural canal, strip off the meninges, and note that the cord rapidly diminishes in size in the posterior lumbar region, finally terminating in the slender thread-like filum termmale. The posterior spinal nerve roots proceed caudally, accompanied by the filum terminale, for a considerable distance from their attachment to the cord. These roots constitute the cauda equina, so called from its resemblance to a horse's tail.
Exercise XXXI. Draw a cross section of the spinal cord to show the origins of the spinal nerves, the spinal ganglia, etc.