Part 41

The teeth (p. 224). These should be studied on a skull in which the roots of the teeth have been laid bare by means of the bone-forceps, as in Figs. 93 and 94.

The tongue (p. 226, and Fig. 95). Study its dorsal surface, the papillæ, etc. Then dissect its _muscles_ (p. 228) from the ventral side. The stylohyoid, digastric, and mylohyoid muscles must be removed. Then dissect (_a_) the styloglossus (Fig. 96, _e_), (_b_) the genioglossus (Fig. 96, _f_), (_c_) the hyoglossus (Fig. 96, _h_). Make a transverse section of the tongue to see the intrinsic muscle-fibres.

III. The thyroid gland (p. 254, and Fig. 96, 6). Clean the sternomastoid muscles. Cut their interdigitating portions along the median line and reflect them. Find the lateral lobe of the thyroid gland beneath the lateral borders of the sternohyoid muscles. Dissect it, being careful not to destroy the delicate isthmus.

IV. The pharynx (p. 231). Remove the large lymphatic gland between the atlantal transverse process and the larynx. Clean the outer surface of the pharynx. Dissect the muscles of the pharynx (p. 232) as follows:

(_a_) The inferior constrictor (Fig. 96, _k_).

(_b_) The middle constrictor (Fig. 96, _j_).

(_c_) The glossopharyngeus (Fig. 96, _i_).

(_d_) The stylopharyngeus (Fig. 96, _l_).

The superior constrictor cannot well be seen at this point; it will be examined later.

Disarticulate the cranial cornu of the hyoid from the bulla tympani and make an incision the length of the lateral wall of the pharynx so as to expose its cavity; study. Examine its opening into the mouth-cavity and study the general description (p. 231). Find and study the soft palate (p. 230). Dissect the tensor and levator palatini muscles (p. 230, and Fig. 66, _d_ and _e_, p. 112), using, if necessary, the specimen on which the other muscles were dissected. Then slit the soft palate lengthwise at one side of the median line to expose the nasopharynx (p. 231). Bring the choanæ into view by use of a bit of mirror-glass. Pass a bristle into the nares and out at the choanæ. Pass a bristle through the Eustachian tube into the middle ear.

V. Œsophagus (p. 234). After completing the study of the pharynx, leave the larynx and lungs in position and by displacing them toward the right follow the œsophagus to its termination. Open it to see the folds of the mucosa.

VI. Cut through the duodenum at its distal end and through the duodenal mesentery so as to separate the stomach and duodenum, with the liver, pancreas, and spleen, from the remainder of the alimentary canal. Float the parts in a dish of water.

VII. Study the stomach (p. 234, and Fig. 97). Cut out the ventral wall of the stomach and wash it out, so as to study its cavity, and then continue the cut so as to expose the pyloric valve (p. 235).

VIII. Study the duodenum (p. 236). Cut away the ventral walls of the duodenum far enough to expose the ampulla of Vater.

IX. Study the liver (p. 239, and Figs. 100 and 101). Expose the hepatic duct of the left lateral lobe near its entrance into the lobe, and follow it so as to expose it fully. If necessary, prick it and inflate with blowpipe in order to follow it. Then expose the cystic duct and other hepatic ducts and follow them to or from the common bile-duct. Trace the latter to the duodenum. If the air does not enter any duct readily, it may be made to do so by manipulating the duct so as to break up the precipitated bile which obstructs it.

X. Study the pancreas (p. 241, and Fig. 102). Expose the pancreatic duct near its entrance into the ampulla of Vater, by removing the peritoneum from the pancreas just caudad of the end of the common bile-duct, and by dissecting apart the pancreatic lobules until the duct appears. (If possible, expose also the accessory duct in the same way on the ventral side of the pancreas, two centimeters caudoventrad of the ampulla of Vater. Prick and inflate. Trace its connection with the main duct. To demonstrate its opening pass a bristle through an opening in it into the duodenum.) (The pancreatic duct may be injected, if desired.)

XI. The ventral wall of the duodenum should now be removed. Cut out a small piece, clean its mucosa with a fine brush and examine the villi with a lens. Demonstrate the coats of the duodenal wall by stripping them off with forceps.

XII. The ampulla of Vater (p. 236). Pass bristles through openings in the common bile-duct and pancreatic duct into the duodenum through the ampulla. Slice away the duodenal wall parallel to the bristles until the bristles are exposed.

XIII. The spleen (p. 242, and Fig. 102).

XIV. Cut the mesentery from the small intestine and colon and slit them both lengthwise, but do not destroy the ileocolic valve. Wash and brush the mucous membrane clean and study the villi, solitary glands, and Peyer’s patches. Study the ileocolic valve (Fig. 99) and open it to study its inner surface.

DISSECTION OF THE ORGANS OF RESPIRATION.

I. The nasal cavity (p. 243). Review the description of the nasal cavity given under the Bones (p. 59). Study the cartilaginous framework that supports the external nose (p. 243); make a cross-section of the framework near the end of the nose and verify the description (p. 244, and Fig. 103). Without injuring the larynx or tongue saw through the head in a vertical plane and a little to one side of the median line. (If desired, the brain may first be removed from the specimen and preserved for future study. For directions, see page 462. The removal of the brain does not injure the head for the study of the nasal cavity; the skull may be sawn lengthwise in the same way as before.) Wash out the cut surfaces before examining. Find the ventral conchæ (p. 40), the labyrinths of the ethmoid (p. 43), the inferior meatus of the nose (p. 243), and by bristles the lachrymal duct or canal (p. 245). The lachrymal duct is conveniently found by passing bristles into the openings of the lachrymal canals. One of these is to be found on the border of each eyelid, two or three millimeters from the inner angle of the eye (see p. 410).

Cut the œsophagus and trachea at the point where the blowpipe was inserted into the trachea. Separate the tongue, hyoid, larynx, œsophagus, and first part of the trachea from the adjacent parts and remove them.

II. The larynx (p. 246). (1) Read the description, studying the cartilages (p. 247) on a preparation (Fig. 104). Then very carefully remove the pharynx and its constrictor muscles, the sternothyroid and remains of sternohyoid muscles, and (2) dissect the muscles of the larynx (p. 249, and Fig. 105). Dissect all those of one side first. The mucosa lining the vestibule and middle portions of the laryngeal cavity must be removed. After studying the muscles, remove the muscles and entire mucosa by scraping carefully with a dull but smooth scalpel, and study the cartilages (Fig. 104).

III. The trachea and lungs (p. 251, and Fig. 106). The distal end of the trachea and lungs may now be removed with the heart from the body. The lungs may be inflated with the bellows. The bronchi should then be exposed, cleaned, and followed as far as possible into the lungs. The heart should be preserved for future study.

DISSECTION OF THE UROGENITAL ORGANS.

A. _Excretory Organs._

1. Expose the kidney (p. 255, and Fig. 108) by removal of the peritoneum and the surrounding fat, taking care not to open the capsule of the kidney. In removing the fat from the cranial end do not injure the suprarenal body (p. 257), which should be studied.

2. Open the capsule of the kidney and slice away its ventral wall to expose the sinus. After dissecting the contents of the sinus open the pelvis and study the papilla. Pass a bristle from the pelvis into the ureter.

3. Make a median section of the kidney parallel to its ventral surface and study structure (Fig. 109).

4. Trace the ureter to the bladder, being careful not to injure the vas deferens. Study the bladder and its ligaments, structure of its wall, etc. Pass a bristle through the ureter into it and then open it to see the openings of the ureter. Trace the neck of the bladder to the pubis.

B. _Male Genital Organs._

1. Study the external genital organs (p. 257).

2. Carefully remove the integument about the anus and expose the external sphincter ani muscle (p. 268, and Fig. 113, _i_). Trace it to its origin. Remove the integument of the scrotum and dissect the intercolumnar fascia, the levator scroti muscle (p. 271, and Fig. 113, _j_), the tunica vaginalis communis, and propria (p. 258).

3. The testis; the epididymis with great care, especially in uncovering the vasa efferentia; the vas deferens and spermatic cord (p. 259, and Figs. 110 and 111).

4. Follow the cord to the external inguinal ring (p. 259) and expose the ring. Dissect off in order from the inguinal canal: (_a_) The external oblique muscle; (_b_) the internal oblique muscle; (_c_) the transversus. In this way the cord is followed with its tunica communis and tunica propria to the internal inguinal ring.

5. Trace the vas deferens (p. 260, and Fig. 111) within the abdominal cavity as far as it can be seen dorsad of the neck of the bladder.

6. The penis (p. 262, and Figs. 111 and 113). Cut the skin along the dorsum of the penis and reflect. Dissect the ligamentum suspensorium penis. Then introduce a bristle into the urethra and remove the integument from the whole penis and identify the corpora cavernosa and corpus spongiosum and the urethra.

7. Remove the fat at the side of the rectum and find the levator ani muscle (p. 269, and Fig. 162, 11). Transect and reflect it. The internal sphincter ani (p. 269, and Fig. 113, _h_). Find the external opening of the anal gland (p. 239); slit and examine it.

8. Dissect the ischiocavernosus (p. 269) and bulbocavernosus (p. 271) muscles (Fig. 113), and find the bulbus of the corpus cavernosum beneath the former. Cut it free from the ischial ramus.

9. Clean the muscles from the pelvis on the side from which the crus penis is removed, and remove with bone forceps the body of the pubis and the ramus of the ischium. Then trace the neck of the bladder to its junction with the vasa deferentia, and the urethra from that point to the external opening (Fig. 111). Study the compressor urethræ muscle (p. 271, and Fig. 113, _n_). Find Cowper’s glands (p. 261) and the prostate gland (Fig. 113). Dissect the other muscles of the urogenital organs (pp. 268 to 273, and Fig. 113).

10. Slit the urethra on one side throughout its length and find the veru montanum (p. 261); the openings of the vasa deferentia (p. 261), by bristles passed into the urethra from them; the openings of the prostate gland and the openings of Cowper’s glands.

11. Study the structure of the penis (p. 262) by making a cross-section, and that of the glans by making a longitudinal section. Note, if possible, the os penis.

C. _Female Urogenital Organs_ (p. 263, and Fig. 112). The kidney and its ducts and the bladder are like those of the male (p. 255), except the neck of the bladder, which will be seen in dissecting the uterus and vagina.

_a._ Without cutting anything examine--

1. The ovaries (p. 264); the ligaments of the ovary (p. 264, and Fig. 112).

2. The uterine tube. Its ostium (p. 264).

3. The body of the uterus (p. 266) as far as exposed.

4. The cornu of the uterus.

5. The broad and round ligaments of the uterus (p. 266).

6. Slit open one Fallopian tube, cornu, and body of the uterus as far as the junction of the divisions of the body of the uterus.

_b._ Remove the ventral wall of the pelvis on one side only and find the vagina and urogenital sinus (p. 267). The levator ani muscle (p. 269, and Fig. 162, 11) must be cut and reflected.

1. Note the constrictor vestibuli, caudovaginalis, and urethralis muscles (p. 272, and Fig. 114).

2. Introduce a probe from the uterus into the vagina and feel with the finger for the hard cervix uteri (p. 266). Then open the vagina on the side, but do not cut the os uteri (p. 266).

3. Examine the cervix uteri and os uteri (p. 266).

4. Introduce a probe through the neck of the bladder and note its emergence into the urogenital sinus.

5. Find the end of the clitoris (p. 267), and its prepuce, and then dissect the integument carefully from about the external opening of the urogenital sinus and note an external sphincter of the sinus (M. levator vulvæ) (p. 272, and Fig. 114), equivalent to the levator scroti in the male and continuous with the external sphincter ani.

6. Find (if possible) the corpora cavernosa of the clitoris (p. 267) and the ischiocavernosi muscles (p. 269).

DISSECTION OF THE CIRCULATORY SYSTEM.

I. THE HEART (p. 274, and Figs. 115-117).

Use the heart from the specimen dissected for the muscles, or from the specimen on which the viscera were studied.

1. Study the outside (p. 275, and Figs. 115 and 116), and learn to recognize all parts. Find the pulmonary veins (p. 275, and Fig. 116, _g_, _h_, _i_) and cut them, thus separating the heart from the lungs.

2. The Pericardium (p. 279). In a specimen the thoracic contents of which have not been injured, dissect the mediastinal septum from the pericardial sac and remove fat about the great blood-vessels so as to expose them all fully. Study their relations. Prick and inflate the pericardium. Slit it lengthwise over the ventricles and reflect it so as to expose its contents. Study the attachment of serous and fibrous layers to the heart and their relation to one another.

3. In dissecting the heart follow the course of the blood, studying each cavity with the aid of the descriptions (pp. 275-279) as you proceed. Dissect as follows:

_a._ Remove the dorsal wall of the right auricle (p. 275) and of its appendage except that part of it to which the venæ cavæ are attached.

_b._ Introduce the probe from the right auricle into the right ventricle, and feel with the probe the line along which the ventricular wall joins the septum. Cut along this line so as to turn back the ventricular wall as a flap, which remains attached at the base of the ventricle.

_c._ Introduce a probe through the conus arteriosus into the pulmonary artery and cut along the probe.

_d._ Remove the dorsal wall of the left auricle and its appendage, _but do not_ remove that part to which the pulmonary veins are attached.

_e._ Make a longitudinal incision beginning at the apex and divaricate the lips of the cut as you pass toward the base, thus avoiding injury to the lateral flap of the bicuspid valve. Without injury to the flap or the columnæ carneæ, remove the heart-wall at the sides of this incision near the base of the ventricle, as much as necessary to expose the cavity.

_f._ Pass a probe into the aorta. Introduce scissors behind the septal flap of the bicuspid valve and slit the aorta without injury to the bicuspid valve.

II. DISSECTION OF THE BLOOD-VESSELS.

_Preparation and Injection._--A new specimen must be prepared for the dissection of the blood-vessels. The same specimen may be used, if necessary, for the dissection of the peripheral nervous system,--one side being used for the blood-vessels, the other for the nerves,--but it is much better to use separate specimens for the two systems.

Inject the femoral artery of the specimen with either five per cent. formalin, or with the mixture of five per cent. formalin and glycerine, exactly as for the muscles. Then, without removing the canula or waiting, inject the artery with red starch. This is prepared as follows:

Mix together equal volumes of vermilion (or red lead), glycerine, and five per cent. formalin. Grind these together in a mortar, so as to destroy the lumps; strain the resulting mixture through fine muslin. This color mixture can be preserved in a closed bottle till it is to be used. Then mix together one volume of dry starch, one and one-fourth volumes of five per cent. formalin, and one-fourth volume of the color mixture. See that no lumps are present.

Inject the above red starch mass into the femoral artery. It will drive into the veins the formalin already injected, and the arteries will be filled with the red starch.

The veins need not be injected, as they will be found to be filled with the formalin mixture, colored by the blood, so that they can be traced without great difficulty.

_Dissection of Blood-vessels._--In general the arteries and veins will be traced together. It is an excellent plan to make a sketch of each vessel as it is dissected, showing its origin, branches, and name. This will be a great aid to the memory and will be of much assistance in reviewing the vessels. The sketches can later be combined into a well-ordered drawing. The description of each vessel must of course be studied as the vessel is dissected.

In tracing blood-vessels _do not_ grasp them between the forceps-blades, but handle them by taking hold with the forceps of the connective-tissue coats of the vessels. _Do not_ use the scalpel, but _pull_ away the connective tissue and fat with fine forceps, using two pairs of forceps or one forceps and the tracer.

Variations are especially common in the blood-vessels, and the student must not be surprised to find considerable deviations from the conditions described and figured in the text. These variations usually present nothing new in principle and are easily understood by comparing them with the structures described.

_Directions for Dissecting the Blood-vessels._--Make a median longitudinal incision through the skin from the symphysis of the jaw to the caudal end of the xiphoid process. About three or four centimeters caudad of the cranial end of the sternum make an incision at nearly right angles to this, passing from the first incision on the ventral side of the left arm about to the elbow. Reflect the flaps of skin, so as to uncover the left side of the thorax and the under surface of the arm, exposing the pectoral muscles. Isolate and transect the pectoral muscles one at a time, cutting each near its thoracic attachment. (The muscles (p. 145) should be reviewed at the same time.) In this way the nerves and blood-vessels of the axilla are exposed (Fig. 122, p. 295).

Find the axillary artery and vein (Fig. 122, _f_ and _g_) emerging from the thorax just craniad of the first rib, along with the nerves of the brachial plexus. Remove connective tissue, etc., so that the vessels and nerves are well isolated as they pass out of the thorax. Take great pains not to puncture the vessels, particularly the veins.

Then remove the left side of the thorax by cutting through the first rib near its sternal end and then near its dorsal end, without injury to the vessels and nerves, cut the other ribs in the same way, and take out the thoracic wall.

Now find with tracer and forceps the great blood-vessels leaving the cranial end of the heart (see Fig. 118). Take the greatest pains not to injure them. Find the aorta and aortic arch (p. 281); the left subclavian artery (p. 283) (continuous with the axillary); the innominate artery (p. 282), and the beginnings of its three branches (see Fig. 115). Find also the superior vena cava, the innominate veins, and the subclavian vein, continuous with the axillary vein.

I. Study the smaller branches of the thoracic aorta (p. 283, and Fig. 118)--the intercostals, the bronchial and œsophageal arteries, and the first pair of lumbar arteries. (The coronary arteries will be examined later.)

II. Dissect the subclavian and its branches (p. 290) as follows:

1. The internal mammary (p. 292). Follow it onto the ventral wall of the abdomen. Follow the vein at the same time (p. 318).

2. The vertebral artery (p. 291). Find its beginning, but do not trace it at present.

3. The costocervical axis (p. 292). Find its beginning, and trace the superior intercostal branch some distance The other branches are not to be followed at present.

4. The thyrocervical axis (p. 293). Find its beginning, but do not trace it at present.

5. The axillary artery (continuation of the subclavian) (p. 294). Follow its branches, tracing at the same time the axillary vein (p. 318). (Consult Fig. 122.) In tracing the blood-vessels, separate the muscles, but do not cut them except where absolutely necessary. (The muscles should be reviewed as the vessels are traced.)

The following notes may be of assistance in following the different branches:

(_a_) The anterior thoracic was probably cut in dissecting the pectoral muscles; it may be found, but its distal end is probably cut off.

(_b_) The long thoracic is easily followed.

(_c_) The subscapular. Follow the main artery before dissecting its branches. Where the subscapular disappears between the long head of the triceps, the latissimus dorsi, and the scapula, it may be traced and found again as follows: Remove the skin from the outer side of the shoulder,--taking great pains to remove _only_ the skin and not to injure the vena cephalica (p. 319), a large vein that lies just beneath the skin on the lateral surface of the shoulder, coming from the elbow. The branches of the subscapular will be found appearing on the lateral surface of the arm in the angle between the spinotrapezius, the long head of the triceps, and the infraspinatus. The distal branches may then be followed.

(_d_) The posterior circumflex (p. 296) may be traced distad in a similar manner, by seeking it beneath the caudal border of the spinotrapezius. (Do not injure the vena cephalica.)

(_e_) The other branches of the brachial artery and vein present no difficulty till we come to the collateralis radialis superior (Fig. 122, _x_). This must be traced with great care, along with the vena mediana cubiti (p. 319, and Fig. 122, _y_). Remove the skin from the extensor side of the forearm, taking great pains not to remove anything more than the skin. The artery and vein lie beneath the skin and should be traced to the hand (see Fig. 130).

Along with the collateral radial artery trace the vena cephalica (p. 319, and Fig. 130, _a_ and _c_). Follow it across the shoulder, noting the branch to the posterior circumflex vein.

The remainder of the blood-vessels of the arm present no special difficulty (see Figs. 123 and 124).

Make a diagram of the subclavian artery, as far as dissected.

III. Remove the skin from the sides of the neck, exposing the sternomastoid muscles and the external jugular veins crossing them (see Fig. 131). Clean the surface of the sternomastoid muscles, without injuring the vein; separate the two muscles caudad, and cut each close to the attachment to the sternum. Find the sternohyoid and sternothyroid muscles, and cut them close to their attachment to the first rib. Uncover the right side of the thorax in the same way as the left, cut the ribs without injury to the nerves and vessels of the right axilla, cut the internal mammary artery and vein (after tying the latter), and thus remove the sternum with nearly the entire thoracic wall.

The blood-vessels of the thorax may now be more completely exposed. (If the nerves are to be dissected on the same specimen, find the phrenic, vagus, and sympathetic nerves (Fig. 157), and take the greatest pains not to injure them.)

1. The coronary arteries and the veins of the heart (pp. 281 and 316).

2. The superior vena cava (p. 316). Find its branches. Trace the azygos as far back as the diaphragm without dissecting it at all.

Find the division of the innominate (p. 318) into subclavian and external jugular.

Trace next--

3. The external jugular (p. 319). Remove the skin from the side of the face and trace its branches (Fig. 131). The internal jugular, vena facialis profunda, the submentalis, and the deep terminal branches of the posterior facial cannot be followed at this time; veins shown on Fig. 131 should all be found, however.

4. Trace the thyrocervical axis and its branches (p. 293). Add them to your diagram of the subclavian.