Internal Organs (THIEME Atlas of Anatomy) (eBook)

A Overview of the internal organs of the human body

Anterior view of the human body, displaying the internal organs. For clarity, the nervous system and most of the small intestine and endocrine organs are not shown.

C Evolution of body cavities

While in fish (a) all internal organs are situated in a single common body cavity, in mammals (b), the diaphragm separates the thoracic cavity from the abdominal cavity. Due to shared evolutionary history, the structures of these two body cavities are basically identical. The different anatomical terms used for similar structures (e.g., pleura – peritoneum) are functionally meaningless. In mammals, there is no physical structure that separates the abdominal cavity from the pelvic cavity. They form a continuous space that in terms of its topographical anatomy is divided only by the superior border of the bony pelvis. The anatomical unit of the abdominal and pelvic cavities is of clinical significance as there are no anatomical barriers to restrict the spread of inflammation or tumors between these two compartments. The diaphragm acts as a barrier to stop tumors or inflammation from spreading from the abdominal to the thoracic cavity and vice versa.

B Neurulation and Somite Formation (after Sadler)

a, c, and e Dorsal views of the embryonic disc after removal of the amnion;

b, d, and f Schematic cross-sections of the corresponding stages at the planes of section as marked in a, c, and e; Age is in postovulatory days.

During neurulation (formation of the neural tube from the neural plate), the neuroectoderm differentiates from the surface ectoderm, due to inductive influences from the notochord, and the neural tube and neural crest cells move inside the embryo.

a and b Embryonic disc at 19 days: The neural groove is developing in the area of the neural plate.

c and d Embryonic disc at 20 days: In the paraxial mesoderm, flanking both sides of the neural groove and notochord, the first somites have formed (they contain cellular material assigned to form the spinal column, muscles, and subcutaneous tissue). Immediately lateral to the paraxial mesoderm is the intermediate mesoderm, and lateral to that is the lateral plate mesoderm. The neural groove is beginning to close to form the neural tube and the embryo begins to fold.

e and f Embryonic disc at 22 days: Eight pairs of somites are seen flanking the partially closed neural tube which is sinking below the ectoderm. In the lateral plate mesoderm, the intraembryonic coelom, or future body cavity, arises. It will later develop both a parietal and a visceral layer (somatopleure and splanchnopleure). On the side facing the coelom, a mesothelial lining develops from the somato- and splanchnopleure. It later forms the serous membranes lining the pericardial, pleural, and peritoneal cavities. The neural tube migrates deeper into the mesoderm, and the somites differentiate into sclerotome, myotome, and dermatome.

C Formation of the intraembryonic coelom (after Waldeyer)

a View into the chorionic cavity (extraembryonic coelom); b Cut through the amnionic cavity, embryonic disc and yolk sac (the chorionic cavity has been removed); c View of the embryonic disc (the intraembryonic coelomic canal has been highlighted in red).

The eventual definitive serous cavities (pericardial, pleural, and peritoneal) arise from the intraembryonic coelom which begins to form in week 4 when intercellular clefts (not shown) appear in the lateral plate mesoderm (see B). The intraembryonic coelom divides the lateral plate mesoderm into parietal and visceral layers (somatopleure and splanchnopleure). At the edges of the embryonic disc, the somatopleure adjacent to the surface ectoderm is continuous with the extraembryonic mesoderm of the amnion. The splanchnopleure adjacent to the endoderm is continuous with the extraembryonic mesoderm of the yolk sac. Thus, the intraembryonic coelom surrounds the opening of the yolk sac like a ring (the coelomic ring). In the cranial part of the embryo, the coelomic ring closes off from the extraembryonic coelom (chorionic cavity) and forms a horseshoe shaped intraembryonic coelomic canal, which is visible when viewed from above. The caudal intra- and extraembryonic coeloms (see D) continue to communicate with one another through the...