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Unit 4The Digestive SystemSecure the skinned rabbit on its dorsal side inthe dissection tray. Following the incision linesdepicted in the diagram, cut through themuscular wall of the abdomen from the pubicarch to the most posterior rib. Cut two additionalincisions from the linea alba to the dorsal side ofthe rabbit, almost to the vertebral column.Carefully fold the flaps of skin and musclelaterally. Do not disturb the underlying organs.Pin the flaps to the tray to expose the abdominalcavity. There are two body cavities separated bya thin sheet of muscle called the diaphragm. Theexposed cavity is the abdominal cavity. Anteriorto that cavity and separated by the diaphragm isthe thoracic cavity.A prominent organ in the anterior of theabdominal cavity is the liver. This reddishbrown organ is divided into right and left lobesby the falciform ligament. Each of these lobes isfurther divided into median and lateral lobes,for a total of four lobes of the liver. The liverproduces some amino acids, filters the blood,converts harmful substances into harmlessones, and helps to regulate blood sugar.Additionally, in the digestive system, the liverproduces bile, which emulsifies fat duringdigestion. The bile produced by the liver isstored in the gallbladder. Raise the right lobe ofthe liver and observe the gallbladder, a dark,greenish-yellow sac.LiverLarge intestineDiaphragmSmall intestineFigure 8. Abdominal cavity9

LiverGallbladderLiverFigure 9. Abdominal cavity showing the gallbladderEsophagusAs you continue to examine the abdominalcavity, trace the path of food through thedigestive system. Each swallow of food forms abolus, which travels from the mouth down theesophagus. The majority of the esophagus,located in the thoracic cavity, is best examinedduring observation of the circulatory system.Some small portion may be visible at the mostanterior portion of the stomach. After leaving theesophagus, the food reaches the stomach,located on the left side of the rabbit, beneath thediaphragm. Lift the liver to view the entirestomach. The stomach consists of four portions:the cardia, the fundus, the body, and the pylorus.The cardiac portion is the area where thestomach joins the esophagus. In this section, thecardiac sphincter separates the esophagus fromthe stomach. (Malfunction of this sphincter is acause of acid reflux in humans.) The fundus isthe upper curve of the stomach that lies tothe left side of the esophagus. The body isthe main portion of the stomach. Thepyloric section lies to the right of theesophagus and terminates in the pyloricsphincter, which separates the stomach from theduodenum, the first portion of the smallCardiac regionFundusGreater cuLesrvaserturecurvatureCardiacsphincterPyloric regionPyloricsphincterFigure 10. Digestive system diagram10

LiverStomachFigure 11. Abdominal cavity showing the stomachStomachSpleenFigure 12. Abdominal cavity showing the spleen11

sections of the small intestine, which aredifficult to distinguish. The caudal end of theileum is modified into a round muscularenlargement called the sacculus rotundus. Thesacculus rotundus is a structure found only inrabbits. It has a large amount of lymphoid tissueand is involved in the immune system. Carefullycut longitudinally along a portion of the smallintestine and examine the lining with a handlens. The small intestine is lined with thousandsof circular folds, the plicae circulares. There arealso small projections called villi. Each villus hasits own projections, the microscopic, hair-likemicrovilli. The folds and projections increasethe surface area of the small intestine andmaximize the absorption of nutrients. The smallintestine ends at the ileocecal valve. Within themesentery of the small intestine is the pancreas,a pinkish-brown, somewhat granular mass. Thepancreas opens to the small intestine throughthe pancreatic duct at the caudal end of theduodenum. It is part of both the digestive andendocrine systems and secretes both enzymesand hormones.intestine. In the stomach, chemical andmechanical digestion continue as muscularcontractions mix the bolus with digestivesecretions, forming a mixture called chyme.Open the stomach and remove any food from theinner surface. Examine the internal surface of thestomach. Note the deep folds, or rugae, thatallow the stomach to expand without pressure.(A hands lens may help you see greater detail.)Though not part of the digestive system, thespleen is best viewed at this point. The spleen islocated on the greater curvature of the stomach,the larger convex curve. It is a small, dark,reddish-brown organ. The spleen is part of thelymphatic system and is designed to filter blood.From the stomach, the chyme travels throughthe pyloric sphincter into the duodenum, thefirst section of the small intestine. It is here thatthe common bile duct from the liver andgallbladder and the pancreatic duct from thepancreas meet the small intestine to releaseadditional enzymes to continue the process ofdigestion. From the duodenum, the mixturetravels to the jejunum and ileum, the otherStomachPancreasDuodenumFigure 13. Abdominal cavity showing the pancreas12

digestive system. The rabbit extracts additionalnutrients from the food this time, due to thefermentative changes that occurred in the cecum.Rabbits’ digestion exemplifies one type ofhindgut fermentation. Cattle and other ruminantsare called “foregut fermenters” becausemicrobial fermentation of their food occursbefore the food reaches the intestine.Although the rabbit colon can be divided intothe ascending, transverse, and descending colon,it is usually discussed in terms of the proximaland distal colon. A short, muscular portion calledthe fusus coli marks the division between thetwo. This section helps regulate the complexevents that move materials either forward orbackward and that determine whethercecotropes or dry feces are being produced. Thebeginning section of the proximal colon has alumpy appearance because it includes manypouches called haustra between longitudinalbands called taenia. The distal colon is a long,twisted tube that terminates with the rectum.Both dry feces and cecotropes exit the anus.Materials exit the small intestine through thesacculus rotundus to the large intestine—first toa short compartment called the ampulla cecaliscoli (a connection between the cecum and thecolon) and then to the colon. In the first section ofthe colon, the proximal colon, specialcontractions separate the more-digestible fromthe less-digestible materials and force thenutritious components backward into the cecum.The rougher materials continue along the distalcolon. The cecum is a large, coiled blind sac thatextends from the sacculus rotundus. The cecummakes up a larger proportion of the digestivetract in rabbits than in any other group ofanimals, and it is critical to the digestion of theirfood. Within the cecum, bacteria break down thematerial through fermentation. At certain times,material from the cecum is released to the colonwhere it is coated in mucus and excreted throughthe anus in the form of soft balls calledcecotropes, also known as night feces or softfeces. Rabbits immediately eat their cecotropes,and the materials pass again through theDistal colonHaustraColonTaeniaProximal colonCecumSacculus rotundusSmall intestineFigure 14. Abdominal cavity showing intestinal tract13Appendix

Dorsal Superficial Pelvic MusclesVentral Superficial Pelvic iform ligamentLiver, right central lobeOmentumTaeniaColonCecumRectumLiver, left central lobe10.11.12.13.14.15.16.17.18.Liver, left lateral lobeStomachSmall intestineAppendixEsophagusCystic ductGallbladderBile ductLiver, caudate lobeFigure 15. Stomach Pyloric stomachCardiac stomachSpleenPancreasIleumSacculus rotundusMesentery

Unit 5The Respiratory SystemTo view the respiratory system, open thethoracic cavity. Cut along the midline, from thediaphragm to the base of the neck. As you makeyour incisions, you will be cutting through somemusculature as well as the ribs. Fold back themusculature and ribs on both sides to expose thecavity. Pin the musculature down to expose thethoracic cavity.You have already examined the nares, theexternal portion of the rabbit’s nose. Rabbits areobligate nasal breathers; a rabbit cannot breathethrough its mouth due to the position of theglottis and epiglottis. These structures serve asthe opening of the respiratory system from thepharynx, the area just posterior to the mouthand common to both the respiratory anddigestive systems.As you examine the features of the respiratorysystem, follow the path of an inhaled breath.From the nares, air travels to the pharynx,past the glottis and epiglottis, to the larynx, andthen the trachea. To view the larynx and trachea,extend the midline incision toward the chin,making a shallow cut to avoid damaging theunderlying structures. Then, using a bluntinstrument, tease away any surrounding tissue.EsophagusTracheasLungHeart enclosed inthe pericardiumLungsAortic archFigure 16. Thoracic cavity, with the heart15

The larynx, a cartilaginous structure, protectsthe entrance to the trachea and is important invocalization. Posterior to the larynx is thetrachea. The trachea is a long tube ringed withbands of cartilage that provide support andprevent collapse. The trachea continuesposteriorly until it divides into left and rightbronchi dorsal to the heart. These bronchi lead tothe left and right lungs. In the lungs, the bronchibranch further into smaller bronchioles, whichterminate in vestibules containing alveoli, wheremost gas exchange occurs. The lungs of therabbit are divided into three portions: theanterior, middle, and posterior lobes. Due to theposition of the heart on the left side of the cavity,the left anterior lobe is much smaller than itscounterpart on the right.TracheasLungBronchusLungsFigure 17. Thoracic cavity, without the heart16

Unit 6The Circulatory SystemThe circulatory system transports varioussubstances through the body, includingdissolved gases, nutrients, hormones, defensivecells, and cellular wastes. Like other mammals(and like birds), rabbits possess a highlybranched network of vessels and a fourchambered heart.There are two circulatory pathways: pulmonaryand systemic. In pulmonary circulation,deoxygenated blood is pumped from the rightside of the heart to the lungs, and oxygenatedblood flows back to the left side of the heart. Insystemic circulation, this oxygenated blood ispumped from the left side of the heartthroughout the body, and deoxygenated bloodtravels back to the right side of the heart. To fullyview the heart, carefully remove the thymusgland, a component of the endocrine system.The heart is covered by a membrane calledthe pericardium, or pericardial sac.Carefully lift it away from the heartand make an incision in it. In a livingrabbit, the pericardial sac is full offluid, which protects and cushionsRight anteriorthe heart. Once you have removedvena cavathe pericardial sac, examine thefour chambers of the heart. Twodark, thinner-walled atria areanterior to the two thicker-walledventricles. The organization of theRight atriumheart keeps oxygenated anddeoxygenated blood separated.To trace the flow of bloodthrough the heart, slice a ventralopening by making a smooth,continuous partial incision into theheart’s left side. Carefully fold thePosteriorventral portion away from thevena cavadorsal portion, as if opening abook. Deoxygenated blood fromRight ventriclethe body enters the right atrium viathe right anterior vena cava, the leftanterior vena cava, and theposterior vena cava. From the rightatrium, blood flows through thetricuspid valve into the right17ventricle. The blood is then pumped through thepulmonary valve to the lungs by way of thepulmonary artery. Arteries are vessels that carryblood away from the heart. Valves prevent theblood from flowing back into the chamber fromwhich it came. In the lungs, the blood becomesoxygenated through gas exchange in the alveoli.The blood then returns to the heart through thepulmonary veins. Veins are vessels that carryblood toward the heart. The blood enters theheart again in the left atrium and travels throughthe bicuspid valve to the left ventricle. From theleft ventricle, the blood is pumped through theaortic valve and out of the heart through theaorta, the largest artery in the body.AortaLeft anteriorvena cavaLeft atriumLeft ventricleFigure 18. Heart, ventral view

brachiocephalic artery supplies blood to the rightsubclavian artery (which leads into the rightshoulder) and the right common carotid artery(which leads to the head). Likewise, the leftbrachiocephalic artery supplies the left subclavianartery and the left common carotid artery.Veins have less-muscular walls than arteriesand tend to be more superficial (i.e., majorarteries tend to lie deeper in the body than themajor veins). The veins merge until they reachthe venae cavae, which return blood to the rightatrium of the heart. The venous system is insome ways the inverse of the branching arterialsystem. The paired common iliac veins flow intothe inferior vena cava. As you trace the venacava cranially, you can locate the pairediliolumbar veins, the paired gonadal veins, andthe paired renal veins. The hepatic portal veindrains the liver and merges with the inferiorvena cava near the diaphragm before flowinginto the right atrium. Blood drains from thehead via the transverse jugular vein and theexternal jugular veins, and from there into thesubclavian veins, which merge to form thesuperior venae cavae.Following the path of the aorta, you will seethat it immediately makes a posterior turn, theaortic arch. Continuing caudally from the arch isthe dorsal aorta, which supplies blood to theposterior portion of the body (and may be calledthe abdominal aorta below the diaphragm). Thefirst branch is the celiac artery (or celiac trunk). Itdiverges to form the hepatic artery, whichsupplies the liver, and the gastrosplenic artery,which further subdivides to serve the stomachand spleen. Continuing caudally, locate thesuperior mesenteric artery, which supplies bloodto the small intestine. Next, find the paired renalarteries that carry blood to the kidneys and thegonadal arteries, which deliver blood to thetestes in males or the ovaries in females. Fartheralong the dorsal aorta you will see the pairediliolumbar arteries that carry blood to themuscles of the back. The last major branch formsthe right common and the left common iliacarteries, which supply blood to the legs.Blood travels through the aortic arch and to theanterior regions of the body through severalimportant vessels. The coronary arteries supplyblood to the heart muscle itself. The rightAnterior vena cavaPosterior vena cavaFigure 19. Heart, dorsal view18

Thoracic cavity, superficialHeart, 5Thoracic cavity, deep17Heart, 25121.2.3.4.5.6.7.8.9.10.11.12.Thymus glandHeartMediastinumRight lung, superior lobeRight lung, middle lobeRight lung, medial inferior lobeRight lung, lateral inferior lobeLeft lung, middle lobeLeft lung, inferior lobeRibXiphoid processDiaphragm, , centrum tendineumPhrenic v.EsophagusTransverse jugular v.External jugular v.Vertebral v.Left ventricleRight ventricleLeft atriumRight atriumLeft anterior vena cavaFigure 20. Diagram of the thoracic cavity with heart1924.25.26.27.28.29.30.31.32.33.34.Right anterior vena cavaPosterior vena cavaPulmonary v.Pulmonary a.Ligamentum arteriosumAortic archBrachiocephalic a.Right subclavian a.Left subclavian a.Left common carotid a.Pericardium

Unit 7The Urogenital SystemThe urogenital system consists of the urinaryand the reproductive systems, which share somecomponents. The urinary system removesnitrogenous wastes from the body as urine, andit also regulates osmotic pressure. Thereproductive system produces the gametes (themale’s sperm and female’s ova) necessary for theformation of offspring. It also includes thestructures involved in the sexual transfer ofsperm to ova and in the protection, development,and birth of young.into and out of the kidney. Also notice the adrenalglands, part of the endocrine system, one ofwhich is medial to each kidney.The kidneys filter waste from the bloodstreamand produce urine. The urine moves from thekidneys through the ureter to the urinary bladder,where it accumulates. As the bladder fills, it isstimulated to contract, expelling the urine outthrough the urethra. In the female rabbit, theurethra empties into the urogenital sinus,whereas in the male it continues to the penis,joining the reproductive system.Urinary SystemTo view the urinary system, remove therabbit’s liver and push the visceral organsto the right side of the specimen. Partiallyembedded on the dorsal surface of theabdominal cavity are two bean-shapedorgans, the kidneys. The kidneys filterwaste from the bloodstream, regulate theamount of water in the rabbit, andmaintain appropriate concentrations ofcertain ions. Using a sharp scalpel, cutone of the kidneys in half longitudinally,along its flattened plane. As you examinethe interior of the kidney, note the outerportion, the renal cortex, and the innerportion, the renal medulla. As youexamine the kidneys, notice the renalartery and renal vein, which carry blood11011928763451.2.3.4.5.6.KidneyHilusAbdominal aortaInferior vena cavaUreterRenal pelvis7.8.9.10.11.Renal medullaRenel cortexRenal a. and v.Dorsolumbar a. and v.Adrenal glandFigure 21. Kidneys and associated structuresUninary bladderKidneyUreterFigure 22. Upper portion of urinary system20

Reproductive Systemare released from the epididymis to the ductusdeferens, which transfers sperm from a testis tothe urethra. At the base of the bladder lies theseminal vesicle, which secretes a fluid into theductus deferens. This fluid combines with thesperm and with fluid from the prostate toproduce semen. The prostate is seen as athickening of the dorsal wall of the seminalvesicle. The semen passes through the urethraand out through the tip of the penis. The free endof the penis, the glans penis, typically lies withina pouch of skin called the prepuce. Inside theglans penis is a small bone called the baculum,which stabilizes the penis during copulation.MaleIn males the hormone testosterone and thesperm (the male ga

organs located in the mouth, the taste buds. Rabbits have a good sense of taste. Like humans, rabbits can taste the four basic categories of flavors: sweet, sour, bitter, and salt. It is not yet known if they are able to taste the umami flavor. The sense of taste helps a rabbit determine potential toxicity and nutritional value of the