ARSENIC. 



[ 62 ] 



ARTERIES. 



the succulent root-stocks or rhizomes of the 

 plants ; tlie arrow-root is extracted from the 

 grated root-stocks by washing, to separate 

 the cellular tissue and remove the often acrid 

 juices. See Starch. 



ARSENIC. — The common term for arse- 

 nious acid. Arsenious acid assumes two 

 crystalline forms and occm's also in an 

 amorphous state. 



The most common form is the octohedral 

 or tetrahedral. The second (right rhombic) 

 is less common, and is only obtained by sub- 

 limation. Attention to the form of the 

 crystals is important, because it is used as a 

 means of identifying arsenic in cases of 

 poisoning. It must, however, be borne in 

 mind that protoxide of antimony (Sb O3) 

 yields crystals by sublimation of exactly the 

 same form as those of arsenious acid (PI. 6. 



Solution of arsenious acid is sometimes 

 used as a preservative liquid for animal pre- 

 parations. 



BiBL. See Chemistry. 



ARTEMIA, Leach.— A genus of Ento- 

 mostraca, of the order Phyllopoda and family 

 Branchiopoda. 



Char. Abdomen prolonged in the form of 

 a tail, composed of nine segments or joints, 

 the end joint simply divided into two lobes ; 

 superior antennae slender and filiform in 

 both sexes; inferior antennse in the male 

 large, flat, ciu*ved downwards and two- 

 jointed, resembling horns ; in the female 

 short, pointed and slightly curved; basal 

 joint of male inferior, antennae provided with 

 a short conical process. 



A. salina. The Lymington shrimp or 

 brine-worm. Found in the salt-pans at 

 Lymington. Length about 1-2". 



Each segment of the thorax shortly bilobed 

 at the apex, and with a pair of branchial 

 feet ; each lobe of the end joint of abdomen 

 giving off several short setae. Agrees gene- 

 rally in structure with Branchijms. 



BiBL. Baud, Brit. Entom. ; Rackett, 

 Trmis. Liinncean Soc. xi. 



ARTERIES.— These are the tubes or 

 vessels which convey the blood from the 

 heart to the various parts of the body. The 

 structure of the arteries is very complicated 

 and difficult of investigation, and the coats 

 or tunics of which they consist are so inti- 

 mately connected as to be by no means 

 easily separable. 



In the larger arteries, three coats are 

 usually distinguishable, an outer or adven- 

 titious coat, a middle and an inner coat. 



Then' comjjosition and thickness varies in 

 arteries of different sizes. 



The middle coat is usually thick and 

 strong, consisting of several layers, and its 

 elements run transversely. In the largest 

 arteries it is yellow, very elastic and of great 

 strength; as the vessels become smaller, it di- 

 minishes in thickness, becoming redder and 

 more contractile ; and near the capillaries it is 

 very thin, finally disappearing. The inner coat 

 is always thin, yet thickest in the large 

 vessels ; whilst the outer coat is absolutely 

 thinner in these than in those of a moderate 

 size, in which it equals or even exceeds the 

 middle coat. 



In the smaller arteries the inner coat con- 

 sists of pale, flattened, fusiform cells with 

 longish, oval nuclei ; these possess no slight 

 resemblance, on the one hand, to the fusi- 

 form cells of pathologists (as also to the 

 formative cells of elastic and areolar tissue), 

 and on the other to contractile (smooth 

 muscular) fibre-cells ; yet they difi'er from 

 the former in the less acumination of their 

 ends and their paleness, and from the latter, 

 in their rigithty, the form of then' nuclei 

 and their chemical reactions. 



An elastic layer is expanded beneath the 

 epithelial layer in the living vessels, whilst 

 in these, when empty, it exhibits numerous 

 transverse or longitudinal folds. It forms 



Fig. 34. 



/I 



a. 



L.«a 



Magnified 350 diameters. 



Muscular fibre-cells from human arteries. 1, from the 

 popliteal : a. before, b, after the addition of acetic acid; 

 2, from a twig of the anterior tibial artery : a, nuclei. 



w hat is called a fenestrated membrane, gene- 

 rally exhibiting more or less distinct reticu- 

 lated fibres and usually small elongated 



