Part 41

The brain will usually be found to be in an entirely satisfactory condition for study in any specimen injected with five per cent. formalin or the glycerine and formalin mixture. The brain is a little swollen, but all parts are well preserved, and the white and gray matter are clearly marked off from each other. Either the specimen used for the muscles or that employed for the blood-vessels may therefore be used,--or if the brain was removed from the specimen employed for the viscera, that will be satisfactory.

The following directions for removing the brain are designed for specimens preserved as above. For removing the _fresh_ brain the process is essentially similar, but as the brain is then very soft, care should be taken not to tear it. The fresh brain should be preserved in the alcohol-formalin mixture given below, and should be allowed to rest only on some soft substance, as absorbent cotton.

Remove the head from the body by cutting through the neck a little craniad of the first rib if this has not already been done. Remove all skin, muscles, and other soft parts from the head and cervical vertebræ, as far as possible. Remove the structures in the orbit by cutting through the zygomatic arch at each end, and removing it. The lower jaw should also be removed, if this has not already been done. (If a fresh specimen is used, and the head is to be employed for other purposes, the brain can be removed without separating the head from the body, and without taking away the lower jaw and other structures on the ventral surface of the skull.)

Have at hand dissecting-instruments and a dish containing alcohol and formalin in the following proportions (Parker and Floyd’s mixture):

95 per cent. alcohol 6 parts 2 per cent. formalin 2 parts

In the bottom of the dish should be placed a little absorbent cotton, to support the brain.

In removing the brain have at hand entire and dissected skulls and note the relations of parts on these as far as necessary before cutting the specimen.

With bone-forceps make a small opening in the parietal bone so as to expose the dura mater, but do not cut through the dura mater. With some blunt instrument free the dura mater from the bone about the opening, and continue to cut away the bone until the dorsal and lateral faces of the cerebrum are fully exposed craniad of the tentorium. The olfactory bulbs (Fig. 137, _I_) should be exposed carefully and as fully as possible. Cut away the dorsal arch of the atlas and carefully insert the forceps in the foramen magnum and, working as before, remove the squamous portion of the occipital and the parietal bones as far as the tentorium and as far ventrad as possible. Leave the dura mater intact if possible. Free the surface of the tentorium from the dura mater, carefully separate slightly the cerebellum and cerebrum; insert the bone-forceps (not too far) with the blades inclined from without ventromediad, and cut the tentorium on each side. Remove it slowly, cutting adhesions to the dura mater. That part of the dura mater which dips between the cerebral hemispheres is the falx cerebri. Cut the dura mater along both sides of the falx cerebri and remove it by turning it down at the sides and cutting it at the level of the cut edge of the bone. Remove it also from the cerebellum and notice how it dips down on both sides of the tentorium and in close contact with it. Cut the falx at the cranial end between the olfactory bulbs and cut the tentorial dura (cut its adhesions, but do not remove with it the pineal body). The falx and tentorial dura may then be removed.

Allow the head to hang sideways over the dish of alcohol-formalin in such a way that the brain will tend to fall out of the cranium. Free the olfactory bulbs from the bone. Then begin at the caudal end and tilt the brain out with the handle of a scalpel. In doing this note carefully and cut the cranial nerves. They should be left with central ends as long as possible, and those on the side which is uppermost should be cut first. In doing this refer to the foramina in the base of the skull and to Fig. 138. Take especial pains also not to break off the hypophysis, which is lodged in the sella turcica.

The brain falls out and rests with its dorsal surface on the cotton. Now remove the remainder of the dura mater, carefully cutting all adhesions to nerves. Remove also the pia mater, as far as that can be done without pulling off at the same time parts of the brain-substance. Preserve the brain in the alcohol-formalin mixture.

_Study of the Brain._--In the study of the brain demonstration specimens are to be used as much as or more than your own specimen. See everything on a demonstration preparation before attempting to expose it in your own specimen.

I. Examine the brain of a shark or of a frog. Cranial nerves may be neglected, but the divisions of the brain should be recognized in dorsal and ventral views and in longitudinal sections, and sketched.

II. Read the general description of the cat’s brain (pp. 339-343), using your own specimen and a longitudinal section. Cut nothing on your own specimen except when especially directed to do so. Study the cavities on a preparation. Compare the diagrams (Figs. 139 and 140) and the figures of the brain.

III. Study the individual parts as follows. To avoid errors make constant reference to preparations and figures.

1. The medulla (p. 344 and Figs. 138 and 141). Use your own specimen and a preparation and dissect out carefully the cranial nerves on your own specimen.

2. The cerebellum (p. 347). Study it entire, then to expose the fourth ventricle (p. 349) slice away with a very sharp scalpel one-half of the cerebellum by making a median longitudinal incision and then horizontal incisions.

3. The pons (p. 347).

4. The mesencephalon (p. 351, and Figs. 141 and 142). Study it first in a preparation. Then study the floor on your own specimen; origin of third nerves.

5. The diencephalon (Figs. 141 and 142). Study the roof and thalami and the pineal body on a preparation and on a longitudinal section; the floor on your specimen.

6. The telencephalon (p. 357). (Note that only _one_ side of this is to be dissected.)

_a._ Study it externally; sulci and gyri (Figs. 145 and 146).

_b._ Examine a preparation showing the corpus callosum (Fig. 147). Then slice away with a very sharp scalpel the top of _one_ hemisphere nearly to the corpus callosum (see the preparation). Expose the corpus callosum on this side to its cranial and caudal borders, by _tearing_ away the brain-substance at its side and above it.

_c._ Raise the corpus callosum at the side and remove it, thus exposing the lateral ventricle in which note the septum pellucidum and fornix, the corpus striatum, and choroid plexus of the lateral ventricle (Fig. 148). (These are to be exposed on _one_ side only, the other being left intact.)

_d._ Expose the anterior and inferior horns of the ventricle and find the hippocampus, the fimbria, caudal part of the fornix, the foramen of Monroe, the anterior commissure. See all these also on a preparation (Fig. 148).

_e._ Remove the occipital and parietal portions of the cerebrum, on the side already dissected, so as to expose the roof of the third ventricle and the midbrain in your specimen, and note the pineal body, choroid plexus of third ventricle, and structures on the roof of the midbrain (Fig. 141).

_f._ Remove the choroid plexus or roof of the third ventricle and study again the thalami (Fig. 141).

_g._ Make a longitudinal section of the brain, in the following manner: Use a very sharp large scalpel, or a razor. Have this wet with the alcohol mixture at the time of using. Place the brain ventral surface down on a sheet of cork or a block of soft wood, the long axis of the brain coinciding with the direction of grain of the wood. Holding the brain firmly with one hand, place the wet knife between the hemispheres with its edge resting on the corpus callosum. See that it is in the median plane and parallel with the long axis of the brain. See also that it is not inclined to one side or the other, so that it will make on cutting a median section throughout. The point of the knife should just reach the cork or wood between the olfactory bulbs. Now draw the knife caudad, keeping its point against the cork: the brain will thus be divided.

If the section is not exactly median, observe the amount of divergence by placing the two halves together and finding the median ventral line. Then on the half that has _too much_ slice away thin shavings until the cavities are exposed, showing the section to be median. Compare with a demonstration section or Fig. 143. Draw the section and compare with a section of shark’s brain (see Fig. 143).

_h._ Study a series of transverse sections, identifying parts. Observe especially in these sections the fornix, corpus callosum, and ventricles, and the distribution of white and gray matter (see Figs. 149-153).

III. PERIPHERAL NERVOUS SYSTEM.

(There are some advantages in dissecting the eye with its muscles before dissecting the nerves, as a knowledge of the eye-muscles is presupposed for dissecting some of the cranial nerves. For directions on the eye, see p. 469.)

A new specimen should be used, if possible, for the peripheral nervous system, though that used for the blood-vessels can be employed, at considerable disadvantage.

Prepare as for the blood-vessels. The arteries should be injected with red starch, to aid in tracing the nerves.

1. THE CRANIAL NERVES (p. 369) AND SYMPATHETIC SYSTEM (p. 404).

1. Reflect the skin covering the sternomastoid muscle, and make a longitudinal incision of the muscle so as to expose the carotid artery. Lying along the artery find the combined trunk of the sympathetic and vagus nerves. Follow the vagus (p. 378) first craniad; transect the muscles as necessity arises, and find its ganglion nodosum and at the same time locate the superior cervical ganglion of the sympathetic nerve (p. 404, and Fig. 156). Then find the hypoglossal nerve (Fig. 156, _b_), passing outside of the carotid artery to the tongue, and the accessory (Fig. 156, _c_), passing to the trapezius. Cut and reflect the digastric muscle and find the small glossopharyngeal nerve (Fig. 156, _a_), passing to the surface of the bulla and then beneath the carotid artery.

2. Follow the vagus (p. 378) caudad to its termination. To do this it is necessary to remove one side of the thorax, as in dissecting the blood-vessels. Do not injure the nerves of the axilla, nor the phrenic or sympathetic nerves. For the vagus in the thorax, compare Fig. 157. Find the branches of the nerve; in dissecting them, pull on them to make them tense. They are then more easily visible. To dissect the abdominal portion of the vagus, open the abdominal cavity, and compare Fig. 164 (p. 407).

3. Dissect the sympathetic (p. 404), following it and its branches to the pelvic region (Figs. 156, 157, and 164).

4. The hypoglossal (p. 383, and Fig. 156, _b_).

5. The glossopharyngeal (p. 378, and Fig. 156, _a_).

6. The accessory nerve (p. 382, and Fig. 156, _c_; Fig. 158, 1).

Cut away a portion of the tympanic bulla and the base of the skull, sufficient to follow these nerves in the jugular foramen, to the brain.

7. Locate the stylomastoid foramen and pick away overlying tissue until the facial nerve is found emerging and then follow its branches to their distribution (p. 375, and Fig. 155).

8. Expose the ventral surface of the pterygoid muscles just mediad of the angle of the jaw. Divide and reflect them, and the mandibular division of the fifth nerve (p. 373, and Fig. 154) will be found dorsad of them and of the internal maxillary artery. The chorda tympani (p. 375) passes ventrad of the artery to join the lingual. Follow out (1) the lingual branch (p. 375) (with the chorda tympani), and (2) the inferior alveolar (p. 375) by cutting away the ventral border of the mandible. Then cut the mandible near the canine tooth, and pull it to one side, and follow out the muscular branches of the mandibular nerve.

9. Remove the mandible and find the maxillary nerve (p. 371) emerging from the foramen rotundum. Follow its branches and find the sphenopalatine ganglion (p. 372).

10. Remove the zygoma so as to expose the whole ventral aspect of the orbit. Carefully pick away the fat in the orbit without injuring any nerves, so as to expose the four recti muscles and the inferior oblique (see p. 411, and Fig. 166). Find the abducens nerve (p. 375, and Fig. 154), entering the dorsal edge of the lateral rectus, and follow it back. Look on the inner surface of the inferior rectus for the branch of the third nerve (p. 369) which supplies it. Find the branch of this nerve which runs to the inferior oblique muscle, and on it the ciliary ganglion; find the branches to the ciliary ganglion from the ophthalmic nerve and follow them (p. 371). Follow also the short ciliary nerves (p. 370) to the eyeball.

11. Trace the third nerve (p. 369) to its foramen of exit and find its branches. Where it passes between the superior and lateral recti, find the ophthalmic nerve (p. 370) by its side and trace its branches.

12. Find the fourth nerve (p. 370), passing outside of the lateral rectus at its origin and entering the superior oblique.

13. Follow the third, fourth, fifth, and sixth nerves into the skull by chipping away the bone and removing the dura. Note the semilunar or Gasserian ganglion (p. 370, and Fig. 138, _k_) and the origin of the fifth nerves from it, and the relation of the ventral root of the fifth nerve to the mandibular nerve.

2. SPINAL NERVES.

The spinal nerves may be dissected on the same side used for dissecting the cranial nerves. (If an undissected specimen is used, remove the skin from the side of the neck, and cut the sternomastoid, sternohyoid, and sternothyroid muscles, as directed for the vagus and sympathetic.)

_Cervical Nerves_ (p. 383).--The ventral rami of the cervical nerves are to be sought as they pass out between the bundles of the scalenus, or between the scalenus and longus capitis, in the neck. This region has already been uncovered in dissecting the vagus and sympathetic (Fig. 156). Dissect first the _second_ cervical (p. 385). Find its ventral ramus as it emerges between the levator scapulæ ventralis and cleidomastoid (Fig. 158, 2), and follow its branches,--the auricularis magnus (5) and cutaneus colli (6). Find its _dorsal_ ramus, the great occipital nerve (p. 384), by reflecting the clavotrapezius muscle; the nerve will be found emerging from the underlying muscles close to the craniomedial angle of the clavotrapezius, near its origin. Trace the nerve in both directions.

The ventral ramus of the first cervical (p. 385) will be found emerging from beneath the wing of the atlas, a little distance craniad of the second (Fig. 156, _j_). Trace it. To find its short dorsal ramus, the suboccipital nerve (p. 384), it is necessary to dissect apart the muscles on the dorsal side of the atlas till the nerve is found passing from the atlantal foramen.

Dissect the third, fourth, and fifth nerves (p. 385, and Fig. 158).

_Brachial Plexus_ (p. 386, and Figs. 159 and 160).--The brachial plexus has been partly uncovered in dissecting the vagus and sympathetic. (If a new specimen or the opposite side is used, reflect the skin from the ventral surface of the thorax and arm, and cut the pectoral muscles, thus uncovering the vessels and nerves of the axilla.) Reflect the skin from the ventral surface of the upper arm. (Do not use scalpel, but tear the skin from the muscles. In this way the nerves will be seen passing to the skin, while if the scalpel were used the nerves might be cut.)

Tie the axillary vein or its two branches in two places, and cut the vein between the tied regions. Leave the arteries as guides for dissection, but remove the veins. Now clean thoroughly the nerves forming the brachial plexus as they pass from the thorax or neck. Be careful not to injure any of the fine nerves or the interconnections of the nerves in doing this. Find and distinguish clearly the fifth, sixth, seventh, and eighth cervical nerves and the first thoracic, as they emerge from the neck or thorax. (Compare Fig. 159 and Fig. 157, _V_-_VIII_ and _I′_.)

Follow out the branches of the plexus, noting the origin and distribution of each branch, in order to determine its name. To follow the phrenic (p. 388, and Fig. 157, _f_), remove a portion of the thoracic wall. In following the other branches of the plexus, pull back the skin wherever an exposure is to be made, and separate the muscles. The epitrochlearis may be cut near the elbow, and the clavobrachial near the shoulder. In following the interosseous branches of the median nerve the fifth head of the flexor profundus, and the extensor brevis pollicis, may be cut. As a rule it will not be necessary to cut other muscles.

_Thoracic and Lumbar Nerves._--One or two of the thoracic nerves (p. 393) should be dissected from the outside by finding the intercostal nerve along the caudal border of one of the ribs and tracing it in both directions. The nerve may be exposed by removing the external muscles covering the rib, and cutting the external intercostal muscles. The dorsal ramus should be traced after the ventral ramus has been studied.

The first lumbar nerve (p. 395) should be dissected in the same way.

The other lumbar and sacral nerves (pp. 395-400) are best dissected from within.

The alimentary canal and its appendages should be removed from the abdomen, leaving only five or six centimeters of the caudal end of the rectum. The kidneys and urogenital organs may be left, to be removed during dissection. (Compare Fig. 162.)

Turn one of the kidneys to the other side, and find the second lumbar nerve (Fig. 162, _a_) appearing at the lateral border of the iliopsoas muscle. Trace it to its origin; trace it also distad, following both branches. It will be necessary to trace the nerves through the abdominal wall, then find them from the outside, and follow them to their distribution.

The third nerve (Fig. 163, _b_) may be found by dissecting apart the fibre-bundles of the iliopsoas and psoas minor, and following in the same way. The kidneys, ureters, vena cava, and aorta may be removed as occasion arises.

The remainder of the lumbar nerves may be found in order, in a similar manner. Follow the saphenous nerve and its branches (p. 397) by removing the integument from the medial side of the leg (see Fig. 127, p. 310). To dissect the sacral nerves (p. 399) separate the innominate bones at the pubic symphysis and divaricate them. Find the nerves arising from the sacral plexus, by cutting the levator ani muscle. After N. hemorrhoidalis inferior and N. pudendus have been traced, the rectum and urogenital organs may be removed, taking great care not to remove more than is necessary. To dissect the great sciatic nerve (p. 400, and Fig. 163), separate the biceps and caudofemoralis near their proximal ends and find the large nerve-trunk (_a_). Then lift the biceps away from the nerve, cut that muscle near its middle, and reflect it. The nerve may now be followed to its terminal branches; during the process cut the muscles only when absolutely necessary. The inferior gluteal nerve (_i_) will be found on the dorsal surface of the great sciatic; by cutting the caudofemoralis and gluteus maximus muscles near their insertions and turning them back the distribution of the nerve may be followed. The superior gluteal (_j_) will be found at the cranial margin of the pyriformis by reflecting the gluteus medius in the same way; the tensor fasciæ latæ may also be cut.

SENSE-ORGANS.

I. THE EYE (p. 410).

Use any specimen on which one side of the head has been left intact. Remove the head from the body by cutting through the neck a little craniad of the first rib.

Study the eye externally. Observe the eyelids, the conjunctiva, nictitating membrane with its cartilage, and the Harderian gland (Fig. 165); the two openings of the lachrymal duct, the Meibomian glands if possible.

Remove the zygomatic arch and expose the orbit and the structures which it contains. Study the periorbita (p. 409), and find the lachrymal gland (p. 410).

Study the muscles of the eyeball (p. 411, and Fig. 166). The lateral rectus on the lateral surface will perhaps be first found, and its tendon traced beneath the inferior oblique. Use great care not to injure the levator palpebræ superioris.

After studying the muscles, find the optic nerve. Cut it and the muscles, and remove the eyeball for farther study.

_The Eyeball_ (p. 412, and Fig. 167).--For an examination of the eye it is well to have a fresh specimen and one hardened in formalin or alcohol. The hardened specimen is more essential, however. An eye from one of the specimens used in dissection is usually satisfactory. All accessory portions should be trimmed from the eyeball, leaving only the spherical ball with a short stalk formed by the optic nerve.

Observe such features of the eye as can be seen externally: the optic nerves, sclerotic, cornea, iris, and pupil. This should be done on a fresh specimen, if one is at hand. The changes in size and form of the pupil can be observed in the living cat by changing it from a light to a dark place and _vice versa_.

_Dissection._--Examine the internal structures on a preparation. Then with fine scissors and forceps remove from the eye about one-fourth of the wall, in the form of a quadrant having one point at the optic nerve, the other at the centre of the cornea. The coats of the eye can then be studied on the piece removed, while the other structures will be visible within the eyeball. No special directions are necessary for observation of the structures described, unless it be the capsule of the lens and the zonula ciliaris. The capsule of the lens may be demonstrated by tearing a bit of it off with fine forceps. The zonula ciliaris is easily seen by divaricating the edges of the cut made in removing the quadrant, so as to stretch the fine fibres of which the zonula is formed.

II. THE EAR (p. 415).

(The muscles of the external ear are of little practical importance, and will doubtless usually be omitted.)

(1) The External Ear. Read the description of the external ear (p. 415), verifying it by examination and comparison of an ear still covered with integument, in the natural condition, and of a preparation of the isolated cartilages of the ear (Fig. 168). The latter may be obtained by dissecting the skin and muscles from a fresh ear.

(2) The scutiform cartilage (p. 418) should be observed in the natural position, and as isolated.

(3) The muscles of the external ear (p. 418, and Fig. 169). Those connecting the external ear with other parts of the head have been studied in connection with the facial muscles. The remainder will be studied on an external ear removed according to the directions given on page 435.