Part 42

5. Follow the common carotid artery (p. 283) and internal jugular vein (p. 320). Find the division of the common carotid into its terminal branches and then dissect its lateral branches and those of the internal jugular (see Fig. 119).

6. The external carotid (p. 285, and Fig. 119). Follow its branches with the exception of the internal maxillary.

7. The internal maxillary (p. 287). Find its inferior alveolar branch first and follow it by cutting away with bone-forceps the ventral border of the lower jaw. To follow its other branches and those of the carotid plexus, remove the zygomatic arch, cut the temporal, masseter, and pterygoid muscles, and cut the mandible behind the incisor teeth and remove it. The branches which pass into the skull are not to be followed at present. The posterior facial vein (p. 323), the vena facialis profunda (p. 323), and the submental vein (p. 323) may be followed at the same time.

8. The internal carotid (p. 285). Follow it to the point where it enters the cranium.

9. Trace the other branches of the costocervical axis (p. 292). To do this, cut the arteries and nerves of the axilla on the side on which they have been dissected, allowing the arm to fall backward. Then trace the branches of the costocervical axis with tracer, scalpel, and bone-forceps, taking care not to injure the vertebral artery. Add these branches to your diagram of the subclavian.

10. The vertebral artery (p. 291). Trace it to the foramen transversarium of the sixth cervical vertebra. Then with bone-forceps follow it to the atlas and into the atlantal foramen. Add this to your diagram of the subclavian.

11. The basilar artery (p. 291) and the other arteries of the brain (p. 289) are best studied on a preparation, similar to that shown in Fig. 121. To obtain such a preparation it is only necessary to remove the brain (for directions, see p. 462) of a specimen in which the arteries have been injected.

(12. Veins of the brain and dura mater (p. 324). These can be worked out only with much difficulty, except on specimens injected with gelatine. The skull must be chipped away and the veins followed without destroying them.)

13. Trace the pulmonary veins (p. 315) (filled with red injection) and the pulmonary artery (p. 280).

IV. Vessels in the abdominal cavity.

1. Open the abdominal cavity; find the superior mesenteric vein (p. 326, and Fig. 132) in the duodenal mesentery near the border of the pancreas. Inject this in both directions with white starch and then dissect the portal vein and its tributaries without injuring any of the structures in the abdomen (p. 326, and Fig. 132).

2. Follow the inferior vena cava (p. 325) from the heart to the diaphragm and then follow it to its tributaries in the abdominal cavity.

3. Dissect the branches of the abdominal aorta (p. 301) and of the inferior vena cava (Fig. 126). Make diagrams of the vessels dissected and review as far as necessary the viscera concerned.

V. The external iliac and its branches (vessels of the hind limbs) (pp. 309 and 329, and Figs. 127, 128, and 163).

Follow the branches of the external iliac arteries and the corresponding veins in the same manner as the vessels of the arm were traced, cutting the muscles only so far as absolutely necessary. Make diagrams of the vessels dissected.

Make a diagram (_a_) of the arterial system as a whole; (_b_) of the venous system as a whole.

THE LYMPHATIC SYSTEM (p. 330).

It will hardly be found practicable to have each student make a dissection of the lymphatic system, and such parts of it as are to be studied may best be shown on a specimen prepared for demonstration purposes.

The thoracic duct and the receptaculum chyli may be demonstrated by the following well-known method: A lean cat is fed with milk about two hours before killing it. An egg may be beaten up with the milk to advantage. Kill the cat with chloroform, and inject the arteries with colored starch through the femoral, in the usual way. The thoracic duct, the receptaculum chyli, and the lymphatics leading to the receptaculum chyli will be colored white by the milk, and can therefore be easily followed. For this purpose the abdomen should be opened, and the left side of the thorax removed, as in the dissection of the blood-vessels. The thoracic duct will be found at the left side of the aorta and may then be traced in both directions.

For a more complete study of the lymphatics they should be injected. This is done as follows: Make a glass canula with a small point, and leave the point sharp. Connect this to the syringe by means of a rubber tube. Use a saturated solution of soluble Prussian blue as injecting fluid. Employ a freshly killed animal.

For injecting the lymphatics of the limbs, make with some pointed instrument, as the tracer, a small hole in one of the pads on the sole of the foot. Introduce the point of the canula into this opening and inject the fluid. This will pass into the spaces in the connective tissue of the pad, which will swell up, and the colored fluid will pass from the connective-tissue spaces into the lymphatics. Pressure must be maintained with the syringe for a considerable time,--fifteen minutes to a half-hour for a good injection of the main trunks of the lymphatics of the limbs. The movement of the fluid should be facilitated by pressing and manipulating the limb at the same time with the hand,--in such a way as will tend to drive the fluid proximad.

The lymphatics of the head may be injected in a similar manner, the canula being introduced into the upper and lower lip, or into the bare surface at the end of the nose.

The internal lymphatic vessels may be injected by injecting the lymphatic glands with which they are connected. This may conveniently be done as follows: Draw out to a fine point the tip of an ordinary pipette or medicine-dropper. The point should be fine, but should taper rapidly in a conical fashion, so that when the point is inserted the part of the glass tube behind it will close up the opening.

Fill the pipette with soluble Prussian blue; insert the point into the gland, and inject the fluid slowly. The lymphatic vessels passing from the glands will be filled. By injecting thus the large lymphatic gland (“pancreas Aselli”) in the mesentery, the abdominal lymphatics, the receptaculum chyli, and the thoracic duct may be injected.

By using thin gelatine colored with Prussian blue as an injecting fluid permanent preparations may be obtained; of course the process of injection is then less simple, and should be looked up in some manual of methods.

NERVOUS SYSTEM.

I. THE SPINAL CORD (p. 335).

Use the specimen on which the muscles were dissected. (Or if the peripheral nerves are not to be dissected on the specimen used for the blood-vessels, that may be employed.)

Make a longitudinal dorsal median incision of the skin, between the back of the head and root of the tail. Reflect the skin for one or two inches on each side of the incision and cut away the muscles covering the neural arches of the vertebræ from the third cervical to the seventh or eighth thoracic inclusive.

Remove with bone-forceps the neural arch of one of the last cervical vertebræ and find the spinal nerve emerging from the intervertebral foramen. Isolate the nerve for a short distance, then proceed craniad, removing the neural arches on one side and isolating the nerves until the third has been uncovered. The ganglion of the second nerve should be sought among the muscles on the dorsal surface between the atlas and axis, and after it has been isolated the arch of the axis may be removed. (The nerve may be found beneath the clavotrapezius and traced to the ganglion.)

The ganglion of the second nerve should be isolated in or near the atlantal foramen, the muscles to which it passes turned aside, and the arch of the atlas removed. Having thus uncovered the first two or more spinal ganglia, proceed caudad, removing the vertebral arches, until the whole cord and its nerves are exposed. Then--

1. Study the cord, enlargements, filum terminale, etc. (p. 334, and Figs. 133 and 136).

2. Slit open and reflect the dura mater (p. 337) for an inch or two.

3. Demonstrate the arachnoid by pulling it off with forceps.

4. Reflect the pia mater in the same way as the dura mater.

5. Study the fissures and grooves of the cord.

6. Cut across the cord with fine scissors at the point where it is freed from its membranes and examine the section. Note the arrangement of gray and white matter and the fissures and grooves, particularly the anterior or ventral. Demonstrate the central canal with the blowpipe.

7. Study the origin of the spinal nerves (p. 337). Count them. Direction of exit? Carefully clean one in the thoracic region from dura mater and connective tissue, with fine scissors, and study dorsal and ventral roots and ganglion (see Fig. 135). Then follow it out and find its dorsal ramus and ventral ramus and the communicating branch of the latter with the sympathetic system. Do not trace the peripheral branches of the nerve at present.

II. THE BRAIN (p. 339).

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.