**6.3 Digestive organs**

After a ventral midline incision through the abdominal wall has been made, the greater omentum (omentum majus) that covers the majority of abdominal organs can be observed (**Figure 45A**). It consists of the parietal and visceral sheets that enclose the virtual omental bursa. The parietal sheet is attached to the greater curvature of the stomach, while the visceral sheet is attached to the dorsal abdominal wall. The abdominal organs can only be observed after retraction or excision of the greater omentum (**Figure 45B**).

**Figure 44.** *Dorsal view of the tongue. 1: papillae fungiformes, 2: papillae circumvallatae, 3: papillae foliatae, 4: papillae filiformes.*

#### **Figure 45.**

*Ventral view of the abdominal cavity after a ventral midline incision was performed. A: The omentum majus is still present with 1: urinary bladder, 2: parietal sheeth of the omentum majus. B: The omentum majus is excised with 1: diaphragm, 2: lobus hepatis dexter lateralis, 3: lobus hepatis dexter medialis, 4: lobus hepatis sinister medialis, 5: ligamentum falciforme, 6: curvatura major of the stomach, 7: cecum, 8: colon transversum, 9: colon descendens, 10: mesocolon, 11: jejunum.*

The esophagus presents a cervical, thoracic and abdominal segment. The cervical segment lies at the left side of the trachea. It bends to the right side of the body when reaching the thorax and deviates to the left side again to perforate the diaphragm (hiatus oesophageus). Its muscular layer is composed of an outer layer of longitudinally orientated fibers and an inner layer of circular fibers that enable peristalsis. The abdominal segment contains smooth muscle cells, while the other two segments present striated muscle fibers. The esophagus finally enters the stomach a few centimeters caudal to the diaphragm. Here, the cardiac sphincter is located. Relaxation of this sphincter and antiperistalsis in the esophagus allow for vomiting.

The stomach (**Figure 46**) consists of the fundus, the body, the pyloric canal and the pyloric antrum. The fundus is large and extends cranially left to the esophagus. The corpus is continuous with the esophagus and cannot be delineated from the fundus by any anatomical landmark. The pyloric antrum is continuous with the corpus. It can be distinguished from the body by its smaller diameter. The narrow short tube that follows is the pyloric canal that ends at the pyloric sphincter.

The reddish spleen is tongue-shaped and lies at the left side of the abdomen. It is connected to the stomach by means of the gastro-splenic ligament (**Figure 46**). The spleen is, however, a lymphoid organ.

The isolated intestinal tract of the rhesus monkey is presented in **Figure 47**. The small intestine measures approximately 175 cm in length and is composed of the duodenum, the jejunum and the ileum. The duodenum presents a long descending part (duodenum pars descendens/duodenum descendens) that is located at the right side of the abdomen, a short transverse part (duodenum pars transversa/duodenum

*Anatomy of the Rhesus Monkey (*Macaca mulatta*): The Essentials for the Biomedical Researcher DOI: http://dx.doi.org/10.5772/intechopen.99067*

#### **Figure 46.**

*Dorsal view of the stomach with the spleen attached. 1: oesophagus, 2: pars cardiaca, 3: fundus ventriculi, 4: corpus ventriculi, 5: pars pylorica, 6: pars cranialis duodeni, 7: lien.*

#### **Figure 47.**

*Isolated intestinal tract. 1: oesophagus, 2: stomach, 3: duodenum descendens, 4: duodenum transversum, 5: duodenum ascendens, 6: jejunum, 7: ileum, 8: caecum, 9: colon ascendens, 10: colon transversum, 11: colon descendens, 12: rectum.*

transversum) in which the chyme travels from right to left in the caudal half of the abdominal cavity, and a short ascending part (duodenum pars ascendens/duodenum ascendens) at the left side of the abdomen. The basis of the mesentery lies in the middle of the J-shaped duodenum. The common bile duct (ductus choledochus) and the pancreatic ducts (i.e. ductus pancreaticus and ductus pancreaticus accessorius) enter the descending part of the duodenum at 1/3 of its length. The accessory pancreatic duct enters the duodenum separately on the minor duodenal papilla, whereas the common bile duct and the principal pancreatic duct join to terminate on the major duodenal papilla. Within the mesoduodenum descendens, the tail or lobus dexter of the pancreas is found. This organ measures approximately 12 cm by 2 cm. Its body (corpus pancreatis) and left lobe (lobus sinister pancreatis) lie within visceral sheet of the greater omentum against the stomach and in the mesocolon ascendens (**Figure 48**). The jejunum presents several loops and continues as the ileum that is anatomically defined as that segment of the small intestine that is attached to the cecum by means of the plica ileocecalis. The ileum finally enters the cecum (ostium ileocecale).

The large intestine measures approximately 63 cm in length ans consists of the cecum, colon and rectum. The cecum can be found at the junction between the ileum and colon at the right side of the abdomen (**Figure 45B**). The cecum (**Figure 49**) is relatively large, measures 7 cm in length and lacks an appendix. Ventral and dorsal teniae that consist of smooth muscle fibers are present. They give origin to the several sacculations called haustra. The U-shaped colon consists of the ascending part (colon pars ascendens/colon ascendens), the transverse part (colon pars transversa/colon transversum) and the descending part (colon pars descendens/colon descendens). Its total length is approximately 46 cm. It present two teniae that give origin to haustra. The descending colon travels along the left side of the abdomen and passes insensibly into the rectum that is approximately 10 cm long and is defined as that segment of the large intestine that is located within the pelvic cavity.

The liver lies most cranial in the abdomen (**Figure 45B**). It measures approximately 15 cm by 10 cm. Its diaphragmatic side is located against the diaphragm while its visceral side faces the viscera, in particular the stomach. The esophagus runs in a fissure between the left and the caudate lobes. The lobulation of the liver is presented in **Figure 50**. The falciform ligament runs from the umbilicus to the liver, in between the left and right liver lobes towards the diaphragmatic side. This side is attached to the diaphragm by means of the left and right triangular ligaments and the coronary ligament. At the visceral side, the gall bladder is lodged in between the quadrate lobe and the right medial lobe.

The common bile duct joins the principal pancreatic duct to enter the duodenum on the major duodenal papilla. The portal vein and hepatic artery enter the liver at the porta hepatis. Both vessels join at the level of the sinusoids. The blood within the

#### **Figure 48.**

*Pancreas of the rhesus monkey in situ (A) and ex corpore (B) with 1: corpus ventriculi, 2a: lobus pancreatis sinister, 2b: lobus pancreatic dexter, 3: omentum majus paries profundus, 4: hepar.*

*Anatomy of the Rhesus Monkey (*Macaca mulatta*): The Essentials for the Biomedical Researcher DOI: http://dx.doi.org/10.5772/intechopen.99067*

**Figure 49.** *Isolated cecum. 1: ileum, 2: plica ileocecalis, 3: apex ceci, 4: tenia, 5: corpus ceci, 6: colon ascendens.*

#### **Figure 50.**

*Liver. A: visceral side with 1: lobus hepatis dexter lateralis, 2: lobus hepatis dexter medialis, 3: lobus hepatis sinister lateralis, 4: lobus hepatis sinister medialis, 5: processus anonimus, 6: processus papillaris of lobus caudatus, 7: processus caudatus of lobus caudatus, 8: vesica biliaris, 9: lobus quadratus. B: diaphragmatic side with 1 – 4 idem as in A, 5: ligamentum falciforme, 6: ligamentum triangulare dextrum, 7: ligamentum coronarium.*

sinusoidal system flows towards the central veins in the center of the liver lobules. These finally join to form multiple hepatic veins that ultimately drain into the caudal vena cava that runs at the dorsal margin of the liver (**Figure 51**).

#### **Figure 51.**

*Vascular corrosion cast of the liver, visceral side. 1: a. hepatica, 2: v. portae, 3: ductus choledochus, 4: vesica biliaris, 5: liver sinusoids.*

#### **6.4 Urinary tract and adrenal glands**

The brownish, bean-shaped kidneys measure approximately 5 cm by 3 cm. The lateral margin is convex while the medial margin is concave (**Figure 52B**). The cranial pole of the left kidney lies against the left lobe of the pancreas and lies more caudal than the right kidney that makes contact with the caudate lobe of the liver. As a result, this liver lobe presents a renal impression. An adipose capsule surrounds the kidneys that are overlaid with a fibrous capsule (**Figure 52A**). At the hilus, the renal artery and renal vein enter the kidney, while the ureter leaves the kidney. After a longitudinal section of the kidney has been performed, the red cortex, brown medulla and the pale pelvis can be observed (**Figure 52C**).

The ureters lead the urine into the urinary bladder. Like the kidneys, they lie retroperitoneally. The abdominal part travels dorsal to the a. and v. ovarica or a. and v. testicularis. The pelvic part is located within the pelvic cavity and crosses the a. and v.

#### **Figure 52.**

*A: Kidney (1) encapsulated by the capsula adiposa (2) and capsula fibrosa (3). B: Left kidney and adrenal gland ex corpore with 1: margo lateralis, 2: margo medialis, 3: hilus renalis, 4: glandula adrenalis, 5: polus cranialis, 6: polus caudalis, 7: ureter. C: Longitudinally sectioned kidney of which the blood vessels are filled with white latex rubber showing the cortex (1), medulla (2) and pelvis renalis (3).*

*Anatomy of the Rhesus Monkey (*Macaca mulatta*): The Essentials for the Biomedical Researcher DOI: http://dx.doi.org/10.5772/intechopen.99067*

iliaca externa ventrally. The intramural part travels obliquely within the wall of the urinary bladder.

The urinary bladder measures approximately 10 cm in length and 7 cm in width when filled with urine. It is attached to the abdominal wall by means of the median ligament (ligamentum vesicae medianum) and the left and right lateral ligaments (ligamenta vesicae lateralia). When the urinary bladder is cut longitudinally from the cervix, over the corpus to the apex, the mucosa can be studied. In the cervix, a left and right ostium ureteris is present on the respective columna ureterica. These distally elongate to form the left and right plica ureterica that distally join at the crista urethralis. As such, the trigonum vesicae is delineated. At the ostium urethrae internum, the urethra finds its origin (**Figure 53**).

The female urethra is rather short as it opens ventrally into the vagina. This opening, the ostium urethrae externum, forms the border between the actual vagina and the vestibulum vaginae.

The adrenal glands are located within the adipose capsules of the kidneys, at their cranial poles (**Figure 57B**). They have a pink color, are lobulated and measure approximately 1 cm in length and a few mm in width (**Figure 52B**). The pink cortex produces mineralocorticosteroids, glucocorticosteroids and androgens. The brown medulla produces adrenalin and noradrenalin.

#### **6.5 Female reproductive organs**

The oval-shaped ovaries measure approximately 8 mm in length and 6 mm in width. At the margo mesovaricus, they are attached to the abdominal wall by means of the mesovarium. The margo liber is devoid of any ligaments. The ligamentum suspensorium ovarii connects the ovary with the lateral pelvic wall. The a. and v. ovarica lie within this ligament. The ligamentum ovarii proprium links the ovary to the uterus.

The coiled fallopian tubes or oviducts lie lateral to the ovaries. They are attached to the abdominal wall by means of the mesosalpinx. The tapered infundibulum that lies against

#### **Figure 53.**

*A: Urinary bladder in situ showing the apex vesicae (1), corpus vesicae (2), cervix vesicae (3), ligamentum vesicae laterale dextrum (4), ligamentum vesicae laterale sinistrum (5). B: Opened urinary bladder with indication of the ostia ureteria (1), columnae uretericae (2), plicae uretericae (3), crista urethralis (4).*

the ovary presents fimbriae to collect the ovulated ovum. Fertilization takes place within the wider ampulla. The isthmus is narrower and opens up into the uterus.

The uterus of the rhesus monkey is of the simplex type. The fundus uteri, corpus uteri and isthmus uteri measure approximately 5 mm, 10 mm and 5 mm in length, respectively. The isthmus is in continuation with the canalis cervicis uteri that is the central canal within the cervix. The uterus is connected with the abdominal wall by means of the mesometrium. Together with the mesosalpinx and the mesovarium, it forms the broad uterine ligament. The ligamentum teres uteri attaches to the uterine body and travels through the inguinal canal. Terminal fibers of this ligament disperse into the vulva lips (**Figure 54**).

The vagina begins distal to the cervix. The portio vaginalis cervicis is the protrusion of the cervix into the vagina. The fornix vaginae is surrounding this structure. The vaginal mucosa is slightly keratinized and presents irregular folds. The vestibulum vaginae lies more distal and can be reached through the ostium vaginae. The border between the vagina and the vestibulum is formed by the urethral opening. This perineal opening that is located ventral to the anal opening is enclosed by a pair of vulva lips.

#### **6.6 Male reproductive organs**

The primary genital glands of the male rhesus monkey are the testes. These are located in the scrotum that lies caudoventrally in the perineal region (**Figure 2**). The scrotal skin is thin and a has a limited number of hairs. This favors thermoregulation. The raphe scroti is visible in the midline and is continuous with the internal septum scroti that divides the scrotum in two separate cavities (cavum vaginale). These cavities can be reached by incision through the scrotal skin.

After transecting the wall of the vaginal cavity, i.e. the tunica vaginalis, the testis can be observed. This egg-shaped organ measures approximately 5 cm in length and 3 cm in width. It is encapsulated by the pale tunica albuginea that consist of dense connective tissue (**Figure 55**).

The a. testicularis is responsible for the testicular blood supply. It is surrounded by the venous plexus pampiniformis that cools the arterial blood. The ductus deferens is closely associated with the testicular blood vessels as they form the funiculus spermaticus that is enclosed by the tunica vaginalis. The ductus deferens is the continuation of the ductus epididymidis. This duct is extremely coiled, forming the epididymis, a solid structure adjacent to the testis. It can be divided into the caput, corpus and cauda epididymidis. The corpus lies against the medial side of the testis and is connected with this structure through the ligamentum testis proprium. The cauda epididymidis is attached to the tunica vaginalis by means of the ligamentum caudae epididymidis (**Figure 56**).

The ductus deferens leaves the vaginal cavity through the inguinal canal. After it has entered the abdominal cavity, it presents a caudal flexion dorsal to the ureter to flow into the pelvic part of the urethra. Along this urethral segment, three accessory glands are present. The ellipsoid, lobulated vesicular glands are large, 5–6 cm in length. They are positioned against the neck of the urinary bladder. Their caudal parts have contact with the prostate. This gland is spherical with a diameter of approximately 1 cm. Its body is positioned in between the caudal parts of the vesicular glands, at the dorsal side of the urethra. Some glandular tissue, however, surrounds the urethra. The bulbourethral glands are very small. They can be found caudolateral to the prostate gland (**Figure 57**).

*Anatomy of the Rhesus Monkey (*Macaca mulatta*): The Essentials for the Biomedical Researcher DOI: http://dx.doi.org/10.5772/intechopen.99067*

#### **Figure 54.**

*A: The female reproductive tract in situ. 1: rectum, 2: ovarium, 3: tuba uterina, 4: isthmus tubae uterinae, 5: fundus uteri, 6: corpus uteri, 7: isthmus uteri, 8: ligamentum teres uteri, 9: vesical urinaria, 10: ligamentum latum uteri, 11: anulus inguinalis profundus. B: Isolated female reproductive tract. 1: margo liber ovaricae, 2: margo mesovaricus, 3: ovarium, 4: mesosalpinx, 5: tuba uterina, 6: ligamentum ovarii proprium, 7: ligamentum latum uteri, 8: ligamentum teres uteri, 9: fundus uteri, 10: corpus uteri, 11: isthmus uteri, 12: cervix, 13: vagina. C: Isolated female reproductive tract with opened uterus. 1: fundus uteri, 2: corpus uteri, 3: isthmus uteri, 4: canalis cervicis uteri, 5: fornix vaginae, 6: portio vaginalis cervicis, 7: vagina, 8: myometrium, 9: endometrium.*

#### **Figure 55.**

*A: Penis and scrotum of the male rhesus monkey. 1: scrotum, 2: raphe scroti, 3: radix penis, 4: corpus penis, 5: glans penis, 6: preputium. B: Incision through the scrotal skin showing the testes. 1: scrotal skin, 2: septum scroti, 3: tunica vaginalis (partially incised), 4: tunica albuginea, 5: cauda epididymidis, 6: raphe scroti.*

**Figure 56.**

*Left testis and epididymis of the rhesus monkey. 1: testis, 2: extremitas dorsalis, 3: extremitas ventralis, 4: caput epididymidis, 5: corpus epididymidis, 6: cauda epididymidis, 7: ductus deferens, 8: plexus pampiniformis.*

The penis of the rhesus monkey is of the cavernous type. When the penis is transected, the paired corpora cavernosa can be recognized by their pale brown color. Both are divided by the penile septum. The free part of the penis is approximately 5 cm long. It consists of the corpus penis and the glans penis. The urethral opening is

*Anatomy of the Rhesus Monkey (*Macaca mulatta*): The Essentials for the Biomedical Researcher DOI: http://dx.doi.org/10.5772/intechopen.99067*

#### **Figure 57.**

*A: Ventral view of the isolated male urogenital tract. 1: ren, 2: glandula adrenalis, 3: ureter, 4: vesical urinaria, 5: glandula vesicularis, 6: ductus deferens, 7: plexus pampiniformis, 8: testis, 9: epididymis, 10: tunica vaginalis, 11: penis, 12: prostata, 13: glandula bulbourethralis. B: Larger magnification of the urinary bladder (1) and the accessory genital glands comprising the glandula vesicularis (2) and the prostata (3).*

located at the ventral side of the glans. The glans is covered by the preputium (**Figure 58**).

#### **6.7 Lungs**

The major intrathoracic organs are the lungs. The lungs can be examined by auscultation are medical imaging in the region from the 2nd to the 8th intercostal space. They consist of the left and right lungs that are separated by the mediastinum. Both are divided into lung lobes by fissures. The left lung consists of a cranial and caudal lung lobe that are separated by the interlobar fissure. The left cranial lung lobe is additionally divided into a cranial part and a caudal part by the cardiac scissure. The right lung has four lobes. The presence of the cranial and caudal interlobar fissures allows for the determination of the cranial, middle and caudal lung lobes. In addition, an accessory lung lobe is present in the right lung (**Figure 59**).

Each lung lobe is ventilated by a principal bronchus (bronchus principalis sinister et dexter). These are the terminal bifurcation of the trachea. This structure counts approximately 27 cartilaginous rings and measures approximately 10 cm in length and 1 cm in diameter. Intrathoracically, the trachea lies ventral to the esophagus and is crossed by the aortic arch at its left side. From the left and right principal bronchi, two and three specific bronchi (bronchi lobares) for the several lung lobes branch off, respectively. The bronchus for the left cranial lung lobe further splits into a bronchus for the cranial part and one for the caudal part. The bronchus for the accessory lobe of the right lung is a branch from the caudal lobar bronchus (**Figure 60**).

#### **Figure 59.**

*Lungs. A: Left lateral view of the left lung with 1: trachea, 2a: lobus cranialis, pars cranialis, 2b: lobus cranialis, pars caudalis 3: lobus caudalis, 4: incisura cardiaca, 5: fissura interlobaris. B: Right lateral view of the right lung with 1: trachea, 2: lobus cranialis, 3: lobus medius, 4: lobus caudalis, 5: lobus accessorius, 6: fissura interlobaris cranialis, 7: fissura interlobaris caudalis.*

*Anatomy of the Rhesus Monkey (*Macaca mulatta*): The Essentials for the Biomedical Researcher DOI: http://dx.doi.org/10.5772/intechopen.99067*

#### **Figure 60.**

*Polyurethane cast of the lungs, ventral view. 1: trachea, 2: bifurcatio tracheae, 3a: bronchus principalis sinister, 3b: bronchus principalis dexter, 3a1: bronchus lobaris for the left cranial lung lobe, 3a2: bronchus lobaris for the left caudal lung lobe, 3b1: bronchus lobaris for the right cranial lung lobe, 3b2: bronchus lobaris for the right middle lung lobe, 3b3: bronchus lobaris for the right caudal lung lobe, 4a: pars cranialis lobi cranialis pulmonis sinistri, 4b: pars caudalis lobi cranialis pulmonis sinistri, 5: lobus caudalis pulmonis sinistri, 6: lobus cranialis pulmonis dextri, 7: lobus medius pulmonis dextri, 8: lobus caudalis pulmonis dextri, 9: lobus accessorius pulmonis dextri, 10: incisura cardiaca, 11: fissura interlobaris, 12: fissura interlobaris cranialis, 13: fissura interlobaris caudalis.*

#### **6.8 Heart**

The heart lies in the thoracic cavity in the region from the 2nd to the 4th intercostal space. It is located between the lungs, in de middle mediastinum, and is enclosed by the pericardium. This fibrous structure dorsally attaches to the basis of the heart and ventrally to the sternum by means of the sternopericardiac ligament. After removal of the left thoracic wall, the blunt apex of the heart, which is formed by the left ventricle, can be observed in between the left cranial and caudal lung lobes as the longitudinal axis of the heart presents a deviation of approximately 45° towards the left. As a result, both the left and right auricle can be observed from the left. The left lateral aspect of the heart is therefore called the auricular side (facies auricularis). Both atria are visible from the right side. This side of the heart is the atrial side (facies atrialis). After removal of the right thoracic wall, it can be observed that the right heart including the right auricle and ventricle rests on the sternum as a result of the counterclockwise quarter rotation of the longitudinal cardiac axis (**Figure 61**).

#### **Figure 61.**

*Right lateral view of the thoracic cavity with 1: lobus cranialis pulmonis dextri, 2: lobus medius pulmonis dextri, 3: lobus caudalis pulmonis dextri, 4: lobus accessorius pulmonis dextri, 5: heart within the pericardium, 6: diaphragma, 7: n. phrenicus.*

The left atrium, that is enlarged by the presence of the left auricle, receives oxygenated blood from the lungs via the four pulmonary veins. The left atrium is smaller in volume than the right atrium and has a smoother inner surface. From here, the blood flows to the left ventricle. The bicuspid left atrio-ventricular valve, i.e. the mitral valve, separates the left auricle from the left ventricle. It is attached to the papillary muscles in the wall of the left ventricle by means of the chordae tendineae. The latter presents a well trabeculated wall (trabeculae carneae) of approximately 6–9 mm in width. A septomarginal trabecula can be observed. Subsequently, blood flows to the ascending aorta. The aortic valve contains three valvulae.

The right atrium, with its right auricle, receives the systemic venous blood through the cranial and caudal vena cava that join at the level of the tuberculum intervenosum. Its wall is characterized by the mm. pectinati. The right auriculo-ventricular valve presents three cusps and is therefore known as the tricuspid valve. The wall of the right ventricle is also trabeculated and measures 1–2 mm in width. The right lumen contains a septomarginal trabecula. The pulmonary valve has the typical arrangement with three valvulae (**Figures 62** and **63**).

The left and right coronary arteries (a. coronaria sinistra et dextra) branch off the short ascending aorta, which runs craniodorsally, just above the aortic valve. These can initially be seen in the coronary sulcus. The a. coronaria dextra gives the a. interventricularis subsinuosus that runs in the sulcus interventricularis subsinuosus. It ultimately joins the ramus circumflexus of the a. coronaria sinistra. This coronary artery runs initially in the coronary sulcus, gives the ramus interventricularis paraconalis that runs in the sulcus interventricularis paraconalis, and continues as the ramus circumflexus that joins the right coronary artery. The left coronary artery is more pronounced than the right (**Figure 63**).

*Anatomy of the Rhesus Monkey (*Macaca mulatta*): The Essentials for the Biomedical Researcher DOI: http://dx.doi.org/10.5772/intechopen.99067*

#### **Figure 62.**

*External anatomical landmarks of the heart. A: Left view, facies auricularis with 1: basis cordis, 2: apex cordis, 3: margo ventricularis dexter, 4: margo ventricularis sinister, 5: sulcus coronarius, 6: sulcus interventricularis paraconalis, 7: auricula sinistra, 8: auricula dextra, 9: ventriculus dexter, 10: ventriculus sinister, 11: aorta descendens, 12: truncus pulmonalis, 13: v. cava cranialis, 14: vv. pulmonales. B: Right view, facies atrialis with 1: basis cordis, 2: apex cordis, 3: margo ventricularis sinister, 4: margo ventricularis dexter, 5: sulcus coronarius, 6: sulcus interventricularis subsinuosus, 7: atrium dextrum, 8: sulcus terminalis, 9: ventriculus dexter, 10: atrium sinistrum, 11: ventriculus sinister, 12: aorta descendens, 13: vv. pulmonales, 14: v. cava cranialis, 15: v. cava caudalis, 16: sinus venarum cavarum.*

#### **Figure 63.**

*Heart and larger vessels. A: Latex injected specimen, B: Vascular corrosion cast. 1: a. coronaria sinistra ramus interventricularis paraconalis, 2: ventriculus sinister, 3: ventriculus dexter, 4: atrium sinistrum, 5: arcus aortae, 6: vv. pulmonales, 7: v. cava caudalis, 8: aorta thoracica.*

The aortic arch presents a branching pattern that is dissimilar to that in humans. Only two branches can be seen, the brachiocephalic trunk and the left subclavian artery. From the short initial segment of the brachiocephalic trunk, also known as the truncus communis, branches the left common carotid artery after a few mm to 1 cm. In humans, the left common carotid artery is a direct branch of the aortic arch. The right common carotid artery branches off subsequently. The continuation of the brachiocephalic trunk is the right subclavian artery (**Figure 64**).

#### **Figure 64.**

*Branching vessels from the aortic arch. A: Native specimen, B: Vascular corrosion cast. 1: aorta ascendens, 2: arcus aortae, 3: aorta descendens, 4a: truncus communis, 4b: truncus brachiocephalicus, 5: a. subclavia dextra, 6a: a. carotis communis sinistra, 6b: a. carotis communis dextra, 7: a. subclavia sinistra.*
