Overview and Clinical Importance
This study guide covers six critical gross anatomy topics that are frequently tested on the BCSE examination. Understanding the anatomical relationships of the urogenital system, nervous system, lymphatic system, integumentary system, endocrine glands, and body cavities is essential for clinical practice and examination success.
Section 1: Urogenital System
Kidneys
The kidneys are paired, bean-shaped organs located in the retroperitoneal space of the dorsal abdominal cavity. They are positioned against the sublumbar muscles beneath the transverse processes of the lumbar vertebrae.
Kidney Position by Species
Kidney Morphology by Species
MEMORY AID: "COWS are LOBULATED" - Remember that bovine kidneys are the only domestic species with visible external lobulation (fetal lobulation that persists).
Kidney Internal Structure
The kidney consists of an outer cortex (contains glomeruli and convoluted tubules) and an inner medulla (contains loops of Henle and collecting ducts). The medulla projects into the renal pelvis as the renal papilla (or renal crest in unilobar kidneys).
The renal hilus is the indented medial border where the renal artery, renal vein, ureter, lymphatics, and nerves enter or exit. The arrangement from dorsal to ventral is typically: Artery, Vein, Ureter (VAU from dorsal).
Renal Blood Supply
Arterial supply: The renal arteries arise directly from the abdominal aorta. Multiple renal arteries occur in approximately 13% of dogs and 10% of cats (more common on the left side).
Venous drainage: The renal veins drain into the caudal vena cava. The right kidney is closely associated with the caudal vena cava medially.
Ureters
The ureters are muscular tubes that convey urine from the renal pelvis to the urinary bladder. They are divided into abdominal and pelvic portions. The ureters run retroperitoneally along the dorsal body wall before entering the bladder at the trigone (dorsolateral surface near the neck).
Clinical Pearl: The ureters enter the bladder obliquely through the bladder wall (intramural portion), which helps prevent urine reflux. In males, the ureters pass dorsal to the ductus deferens before entering the bladder.
Urinary Bladder
The urinary bladder is a hollow, musculomembranous organ that stores urine. It consists of three anatomical parts: the apex (vertex) which points cranially, the body, and the neck which continues into the urethra.
The trigone is a triangular area on the dorsal internal surface of the neck where the two ureters enter and the urethra exits. The bladder position varies with filling; when empty it lies within the pelvis, and when distended it extends cranially into the abdomen.
Species Note: Female dogs and cats have a shorter, wider urethra than males, and the bladder neck tapers more gradually. In cats, the urethra is relatively longer, so the bladder often appears more cranial in the abdomen.
Section 2: Nervous System
The nervous system is divided into the Central Nervous System (CNS) comprising the brain and spinal cord, and the Peripheral Nervous System (PNS) comprising cranial nerves, spinal nerves, and ganglia.
Brain Anatomy
The brain is protected by the cranial cavity and surrounded by three meninges: the dura mater (outermost), arachnoid mater (middle), and pia mater (innermost). Cerebrospinal fluid (CSF) circulates in the subarachnoid space.
Spinal Cord
The spinal cord extends from the foramen magnum to the lumbar region. In dogs and cats, it typically ends at L6-L7 as the conus medullaris. The spinal cord continues as the cauda equina (collection of nerve roots) and filum terminale caudally.
Two enlargements are present: the cervical intumescence (C6-T2, brachial plexus origin) and the lumbar intumescence (L4-S3, lumbosacral plexus origin).
Peripheral Nervous System
The PNS includes 12 pairs of cranial nerves (I-XII) and pairs of spinal nerves that emerge from the intervertebral foramina. Spinal nerves form important plexuses:
- Brachial plexus (C6-T2): Innervates thoracic limb
- Lumbosacral plexus (L4-S3): Innervates pelvic limb and pelvis
- Pelvic plexus: Autonomic innervation to pelvic viscera
Section 3: Lymphatic and Immune System
The lymphatic system consists of lymphoid organs (thymus, spleen, lymph nodes, tonsils, bone marrow) and lymphatic vessels that transport lymph fluid.
Primary and Secondary Lymphoid Organs
Clinically Palpable Lymph Nodes (Dog)
Knowing the location of palpable lymph nodes is essential for physical examination and disease staging:
MEMORY AID: "MAP SIP" = Mandibular, Axillary, Prescapular, Superficial inguinal, (and) Popliteal - the palpable peripheral lymph nodes in dogs.
Lymphatic Drainage
The thoracic duct is the largest lymphatic vessel. It originates from the cisterna chyli in the cranial abdomen and travels through the thorax to empty into the venous system at the junction of the left external jugular and subclavian veins. It drains lymph from the caudal body, left thorax, and left head/neck. Rupture of the thoracic duct causes chylothorax.
Section 4: Integumentary System
The integumentary system is the largest organ system, comprising 12-24% of body weight. It includes the skin and its appendages (hair, glands, claws/hooves, horns).
Layers of the Skin
Epidermal Layers (Superficial to Deep)
- Stratum corneum - Dead, keratinized cells; protective barrier
- Stratum lucidum - Only in thick skin (footpads); clear layer
- Stratum granulosum - Keratohyaline granules; waterproofing begins
- Stratum spinosum - Desmosomes connecting cells; Langerhans cells present
- Stratum basale - Single layer of dividing cells; melanocytes, Merkel cells
MEMORY AID: "Come, Let's Get Sun Burned" = Corneum, Lucidum, Granulosum, Spinosum, Basale (superficial to deep).
Skin Appendages
Hair follicles: May be simple (single hair shaft) or compound (multiple hairs per follicle opening). Dogs have compound follicles with a primary guard hair and multiple secondary hairs.
Sebaceous glands: Holocrine glands that produce sebum. Most numerous around paws, neck, rump, chin, and tail.
Sweat glands: Two types exist: Epitrichial (apocrine) glands open into hair follicles; atrichial (eccrine) glands are found only in footpads. Horses use epitrichial glands for thermoregulation; dogs rely primarily on panting.
Claws and Hooves
Claws (carnivores): Composed of keratinized epidermis (hard outer shell) covering the vascularized dermis (quick). The epidermis is firmly attached to the underlying distal phalanx (P3).
Hooves (ungulates): Similar structure but with epidermal and dermal laminae that interdigitate to attach the hoof wall to P3. Inflammation of the laminae causes laminitis in horses.
Section 5: Endocrine Glands
Endocrine glands are ductless glands that secrete hormones directly into the bloodstream. Knowledge of their anatomical location is essential for clinical examination, imaging interpretation, and surgery.
Section 6: Body Cavities and Regions
The body is divided into dorsal (cranial and vertebral cavities) and ventral (thoracic, abdominal, and pelvic cavities) compartments.
Serous Membranes
Body cavities are lined by serous membranes that secrete lubricating fluid:
- Pleura: Lines thoracic cavity and covers lungs (parietal and visceral pleura)
- Pericardium: Surrounds heart (fibrous and serous layers)
- Peritoneum: Lines abdominal and pelvic cavities and covers abdominal viscera
Retroperitoneal Structures
Organs that lie behind the peritoneum against the dorsal body wall are retroperitoneal. These include: kidneys, ureters, adrenal glands, abdominal aorta, caudal vena cava, and portions of the duodenum and pancreas.