| arch of the aorta | The curved portion between the ascending and descending parts of the aorta; it begins as a continuation of the ascending aorta posterior to the sternal angle, runs posteriorly and slightly to the left as it passes over the root of the left lung, and becomes the descending aorta as it reaches and begins to course along the vertebral column; it gives rise to the brachiocephalic trunk, the left common carotid and left subclavian arteries. Any member of the several pairs of arterial channels encircling the embryonic pharynx in the mesenchyme of the brachial arches; there are potentially six pairs, but in mammals the fifth pair is poorly developed or absent. The first and second pairs are functional only in very young embryos; the third pair is involved in the formation of the carotids; the fourth arch on the left is incorporated in the arch of the aorta; the sixth pair forms the proximal part of the pulmonary arteries. A series of arterial channels encircling the embryonic pharynx in the mesenchyme of the branchial arch's. There are potentially six pairs, but in mammals the fifth pair is poorly developed or absent. The first and second pairs are functional only in very young embryos; the third pair is involved in the formation of the carotids; the fourth arch on the left is incorporated in the arch of the aorta; the sixth pair forms the proximal part of the pulmonary arteries. Synonym: arcus aortae. (05 Mar 2000) |
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| arch of the palate | The vaulted roof of the mouth. (05 Mar 2000) |
| arch of thoracic duct | See: thoracic duct. (05 Mar 2000) |
| arch-loop-whorl system | See: Galton's system of classification of fingerprints. (05 Mar 2000) |
| archae | <microbiology> This is a classification of odd bacteria that are neither prokaryotes nor eukaryotes, some scientists believe they represent a separate kingdom. The primary species is Archaebacteria, whose members fall in three categories: microbes that can live in extremely salty environments (halophiles), microbes that produce methane (methanogens), and microbes that can live in extremely hot environments (thermophiles). All are of interest to biotechnologists because they have unique biochemical features (for example, the enzymes of the theromophiles are extremely stable at high temperatures). (10 Jan 1998) |
| archaea | A unique group of microorganisms. They are called bacteria (Archaeobacteria) but they are genetically and metabolically different from all other known bacteria. They appear to be living fossils, the survivors of an ancient group of organisms that bridged the gap in evolution between bacteria and the eukaryotes (multicellular organisms). The name Archaea comes from the Greek archaios meaning ancient. (12 Dec 1998) |
| archaeal proteins | Proteins found in any species of archaeon. (12 Dec 1998) |
| archaean | <geology> The earliest period in geological period, extending up to the Lower Silurian. It includes an Azoic age, previous to the appearance of life, and an Eozoic age, including the earliest forms of life. This is equivalent to the formerly accepted term Azoic, and to the Eozoic of Dawson. Source: Websters Dictionary (01 Mar 1998) |
| Archaebacteria | <microbiology> One of two major subdivisions of the prokaryotes. There are three main Orders, extreme halophiles, methanogens and sulphur dependant extreme thermophiles. Archaebacteria differ from Eubacteria in ribosomal structure, the possession (in some cases) of introns and in a number of other features including membrane composition. (18 Nov 1997) |
| archaeocerebellum | Synonym: archicerebellum. Origin: G. Archaios, ancient, + cerebellum (05 Mar 2000) |
| archaeocyte | <organism> An amoeboid cell type of sponges (Porifera). (18 Nov 1997) |
| archaeoglobales | An order of extremely thermophilic, sulfate-reducing archaea, in the kingdom euryarchaeota. The single family archaeoglobaceae contains one genus archaeoglobus. (12 Dec 1998) |
| archaeoglobus | A genus of extremely thermophilic, sulfate-reducing archaea, in the family archaeoglobaceae. (12 Dec 1998) |
| archaeoglobus fulgidus | A species of extremely thermophilic, sulfur-reducing archaea. It grows at a maximum temperature of 95 degrees c. In marine or deep-sea geothermal areas. (12 Dec 1998) |
| archaeography | A description of, or a treatise on, antiquity or antiquities. Origin: Gr. Ancient. Source: Websters Dictionary (01 Mar 1998) |