228 VOCABULARY. 



ALVE'OLUS (pi. alveoli). The alveoli are the sockets of the 

 teeth, into which they are, as it were, driven. Their size 

 and shape are determined by the teeth which they receive, 

 and they are pierced at the apex by small holes which give 

 passage to the dental vessels and nerves. 



ANASTOMO'SIS (' a mouth '). Communication between two ves- 

 sels. By considering the nerves to be channels, in which a 

 nervous fluid circulates, their communication likewise has 

 been called anastomosis. By means of anastomoses, if the 

 course of a fluid be arrested in one vessel, it can proceed 

 along others. 



ANISODAC'TYLE. Hoofed quadrupeds with toes (on the hind- 

 feet at least) in uneven numbers, as one, three, or five, the 

 latter being manifested by the Proboscidians. All these have 

 a simple stomach and an enormous caecum. Examples : 

 Horse, tapir, rhinoceros, elephant. R. Owen. 



ANTE'RIOR (ante, ' before '). Great confusion has prevailed with 

 anatomists in the use of the terms before, behind, &c. 



(A practical definition of anterior appears to be (1) parts 

 in front, supposing the body to be equally divided longi- 

 tudinally from right to left ; (2) parts nearest the operator, 

 parts beyond being posterior.) 



ANTRUM. A cavern. A name given to certain cavities in 

 bones, the entrance to which is smaller than th'e bottom. 



ANTRTJM OF HIGHMORE. A deep cavity in the substance of the 

 superior maxillary bone, communicating with the middle 

 meatus of the nose. It is lined by a prolongation of the 

 Sclmeiderian membrane. 



ARACHNOID MEMBRANE. A name given to several mem- 

 branes, which, by their extreme thinness, resemble spider- 

 webs. The moderns use it for one of the membranes of the 

 brain, situate between the dura mater and pia mater. It is a 

 serous membrane, composed of two layers, the external being 

 confounded, in the greater part of its extent, with the dura 

 mater, and, like it, lining the interior of the cranium and spi- 

 nal canal ; the other is extended over the brain, from which 

 it is separated by the pia mater, without passing into the 

 sinuosities between the convolutions, and penetrating into 

 the interior of the brain by an opening at its posterior part, 

 under the ' corpus callosum.' It forms a part of the investing 



