Internal Organs (THIEME Atlas of Anatomy), Latin Nomenclature (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 intestinum tenue and endocrine organs are shown.

C Evolution of body cavities

While in fish (a) all internal organs are situated in a single common body cavity, in mamma ls (b), the diaphragma separates the cavitas thoracica from the cavitas abdominis. 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 cavitas abdominis from the cavitas pelvis. 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 cavitates abdominis and pelvis is of clinical significance as there are no anatomical barriers to restrict the spread of inflammation or tumors between these two compartments. The diaphragma acts as a barrier to stop tumors or inflammation from spreading from the cavitas abdominis to the cavitas thoracica and vice versa.

A Differentiation of the germ layers (after Christ and Wachtler)

After the formation of the trilaminar discus embryonicus at the end of the third week (see B) the primordia (precursor cells destined to become a specific tissue or organ) of the different tissues and organs are arranged according to the body plan. In the subsequent embryonic period (weeks 4 to 8), the three germ layers (ectoderma, mesoderma, and endoderma) give rise to all major external and internal organs (organogenesis). At the same time, the trilaminar discus embryonicus begins to fold, resulting in major changes in body form and internal structure. By the end of the embryonic stage, the major features of the body are recognizable and the organs have moved into their eventual position within and outside of the body cavities.

B Neurulation and Somitus Formation (after Sadler)

a, c, and e Dorsal views of the discus embryonicus 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 tubus neuralis from the lamina neuralis), the neurectoderma differentiates from the surface ectoderma, due to inductive influences from the notochorda, and the tubus neuralis and crista neuralis cells move inside the embryo.

a and b Discus embryonicus at 19 days: The sulcus neuralis is developing in the area of the lamina neuralis.

c and d Discus embryonicus at 20 days: In the mesoderma paraxiale, flanking both sides of the sulcus neuralis and notochorda, the first somiti have formed (they contain cellular material assigned to form the columna vertebralis, muscles, and subcutaneous tissue). Immediately lateral to the mesoderma paraxiale is the mesenchyma intermedium, and lateral to that is the mesoderma laminae lateralis. The sulcus neuralis is beginning to close to form the tubus neuralis and the embryo begins to fold.

e and f Discus embryonicus at 22 days: Eight pairs of somiti are seen flanking the partially closed tubus neuralis which is sinking below the ectoderma. In the mesoderma laminae lateralis, the coeloma intraembryonicum, or future body cavity, arises. It will later develop both a parietal and a visceral layer (somatopleure and splanchnopleure). On the side facing the coeloma, a mesothelial lining develops from the somato- and splanchnopleure. It later forms the tunicae serosae lining the cavitates pericardiaca, pleurali, and peritonealis. The tubus neuralis migrates deeper into the mesoderma, and the somiti differentiate into sclerotomus, myotomus, and dermatome.

C Formation of the coeloma intraembryonicum (after Waldeyer) a View into the chorionic cavity (coeloma extraembryonicum); b Cut through the cavitas amniotica, discus embryonicus and saccus vitellinus (the cavitas chorionica has been removed); c View of the discus embryonicus (the intraembryonic coelomic canal has been highlighted in red). The eventual definitive serous cavities (cavitates pericardiaca, pleuralis, and peritonealis) arise from the coeloma intraembryonicum which begins to form in week 4 when intercellular clefts (not shown) appear in the mesoderma laminae lateralis (see B). The coeloma intraembryonicum divides the mesoderma laminae lateralis into parietal and visceral layers (mesenchymata somatopleurale and splanchnopleurale). At the edges of the discus embryonicus, the mesenchyma somatopleurale adjacent to the surface ectoderm is continuous with the extraembryonic mesoderma of the amnion. The mesenchyma splanchnopleurale adjacent to the endoderma is continuous with the extraembryonic mesoderma of the saccus vitellinus. Thus, the coeloma intraembryonicum surrounds the opening of the saccus vitellinus like a ring (the coelomic ring). In the cranial part of the embryo, the coelomic ring closes off from the coeloma extraembryonicum (cavitas chorionica) and forms a horseshoe shaped intraembryonic coelomic canal, which is visible when viewed from above. The caudal coelomata intra and extraembryonicum (see D) continue to communicate with one another through the coelomic portals. Later, as a result of embryonic folding, the caudal coelomata intra- and extraembryonicum become...