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These needs include (1) formation of a vascular association • It helps eliminate metabolic wastes purchase evista 60mg online. Developmental © The McGraw−Hill Anatomy purchase evista 60mg mastercard, Sixth Edition Development Anatomy, Postnatal Companies, 2001 Growth, and Inheritance Chapter 22 Developmental Anatomy, Postnatal Growth, and Inheritance 763 Amniotic fluid Embryonic disc Amnion Allantois Implanted embryo Yolk sac Body of uterus Chorion (a) Amnion (b) Connecting (a) stalk Villi of chorion Umbilical cord Placenta frondosum Yolk sac Chorion Allantois frondosum (c) Creek Chorion Amnion FIGURE 22. Amniotic sac containing amniotic fluid Amniotic fluid is formed initially as an isotonic fluid ab- Yolk sac sorbed from the maternal blood in the endometrium surrounding the developing embryo. Later, the volume is increased and the concentration changed by urine excreted from the fetus into the amniotic sac. Amniotic fluid also contains cells that are sloughed off from the fetus, placenta, and amniotic sac. Because all of these cells are derived from the same fertilized egg, all have the same (b) Umbilical blood vessels genetic composition. Many genetic abnormalities can be detected by aspirating a sample of this fluid and examining the cells ob- FIGURE 22. Genetic diseases, such as Down syndrome, or trisomy 21 (in which there are three instead of two centa. Prior to delivery, the amnion is naturally or surgically rup- number–21 chromosomes), can be detected by examining chromo- tured, and the amniotic fluid (bag of waters) is released. Diseases such as Tay-Sachs disease, in which there is a de- As the fetus grows, the amount of amniotic fluid increases. For the near-term baby, almost 8 liters of fluid are completely replaced each day. Amniotic fluid is normally swallowed by the fetus and ab- sorbed in the GI tract. The fluid enters the fetal blood, and the Yolk Sac waste products it contains enter the maternal blood in the pla- The yolk sac is established during the end of the second week as cells from the trophoblast form a thin exocoelomic (ek'so-se˘- Down syndrome: from John L. Developmental © The McGraw−Hill Anatomy, Sixth Edition Development Anatomy, Postnatal Companies, 2001 Growth, and Inheritance 764 Unit 7 Reproduction and Development Decidua basalis Chorion frondosum Maternal vein Maternal Umbilical artery Chorion cord Amnion Amniotic sac containing amniotic fluid Placenta FIGURE 22. Blood from the embryo is carried to and from the chorion frondosum by the umbilical arteries and vein. The maternal tissue between the chorionic villi is known as the de- cidua basalis; this tissue, together with the villi, form the functioning placenta.

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Few dissections of cadavers were performed other than in attempts to determine the cause of death in criminal cases order evista 60 mg amex. Medicine was not preventive but was limited purchase 60 mg evista overnight delivery, almost without exception, to the treatment of soldiers injured in battle. Later in Roman history, laws were established that attested to the influence of the Church on medical practice. According to Roman law, for exam- ple, no deceased pregnant woman could be buried without prior removal of the fetus from the womb so that it could be baptized. The scientific documents that have been preserved from the Roman Empire are mostly compilations of information ob- FIGURE 1. New anatomical information was scant, and for the most part was derived from dissections of animals other than human. Through vivisections and dissections of human cadavers, he pro- Most of what is currently known about the Alexandrian school vided excellent descriptions of the skull, eye, various visceral or- of medicine is based on the writings of the Roman encyclopedist gans and organ relationships, and the functional relationship of Cornelius Celsus (30 B. Two monumental works by tion into an eight-volume workcalled De re medicina. He re- only limited influence in his own time, however, probably be- garded the brain as the seat of intelligence and described many of cause of his use of Latin rather than Greek. It was not until the its structures, such as the meninges, cerebrum, cerebellum, and Renaissance that the enormous value of his contribution was fourth ventricle. A Greek living under Roman domination, he tions than structure and is frequently referred to as the father of was certainly the most influential writer of all times on medical physiology. For nearly 1,500 years, the writings of Galen repre- servations on the heart, vessels, brain, and cranial nerves. Erasis- sented the ultimate authority on anatomy and medical treat- tratus noted the toxic effects of snake venom on various visceral ment.

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The remaining non- important signal to the kidneys to direct the excretion or collagen portion (5%) of organic matter is referred to as retention of calcium and phosphate buy evista 60 mg with amex. Ground substance consists of a mixture of various proteoglycans discount 60 mg evista with visa, high-molecular-weight com- Renal Handling of Calcium. As discussed in Chapter 24, pounds consisting of different types of polysaccharides filterable calcium comprises about 60% of the total calcium linked to a polypeptide backbone. This eralized bone, they lose much of their bone-forming ability orderly association of hydroxyapatite crystals with the colla- and become quiescent. At this point they are called osteo- gen fibers is responsible for the strength and hardness char- cytes. A loss of either bone mineral or organic teoblast stage are maintained into the osteocyte stage. These connections become visible channels or canaliculi Complete demineralization of bone leaves a flexible collagen that provide direct contact for osteocytes deep in bone framework, and the complete removal of organic matrix with other osteocytes and with the bone surface. It is gen- leaves a bone with its original shape, but extremely brittle. The three principal cell types involved in bone Osteoclasts are cells responsible for bone resorption. Osteoclasts promote bone resorption by secreting Osteoblasts are located on the bone surface and are re- acid and proteolytic enzymes into the space adjacent to sponsible for osteoid synthesis. Surfaces of osteoclasts facing bone are tively synthesize proteins for export, osteoblasts have an ruffled to increase their surface area and promote bone abundant rough ER and Golgi apparatus. First, gaged in osteoid synthesis are cuboidal, while those less ac- osteoclasts create a local acidic environment that in- tive are more flattened. Numerous cytoplasmic processes creases the solubility of surface bone mineral.