I3O GLOSSARY. 



Si'nus (L. a hollow, a depression). A depression or cavity. 

 Osseous sinuses are cavities in bones which contain air. 

 The venous sinuses in the dura mater of the brain are 

 hollows in that membrane which contain blood, and thus 

 serve the purpose of veins. Similar venous sinuses play 

 the part of veins in some of the Invertebrata. 



Si'nus pocula'ris (L. cup-like sinus). A depression in the male 

 urethra which leads into the prostatic vesicle. 



Si'nusrhomboida'lis(L.rhomb-shapedsinus). A lozenge-shaped 

 depression left by the medullary folds of the embryo at 

 the hinder extremity of the medullary canal. Also a small 

 sinus found in the lumbar region of the spinal cord in 

 birds, which is the persistent sinus rhomboidalis of the 

 foetus. 



Si'nus terminalis (L. terminal sinus). The bounding line en- 

 circling the extreme margin of the vascular area in the 

 embryo. 



Si'nus urogenitinalis (L. uro-gcnital sinus). The ventral portion 

 of the cloacal chamber in the embryo, when shut off from 

 the intestinal portion by the development of a partition. 



Si'nus veno'sus (L. venous sinus). A term applied to the main 

 portion of the auricles of the heart to distinguish them 

 from the auricular appendages. In the lower Vertebrata 

 the sinus venosus is a distinct portion of the heart formed 

 by the junction of the large venous trunks, and is rhyth- 

 mically contractile. 



Si'phon (Or. a tube). One of the breathing tubes in the 

 Mollusca. Applied also to any similar tube. 



Sipho'nium (Gr. sip/ion, a tube). A bony tube which in some 

 birds conducts the air from the tympanum to the air cavity 

 in the articular piece of the mandible. 



Siphono'phora (Gr. siphon , a tube; phoreo, I bear). A group 

 of the Hydrozoa, comprising the marine forms. 



STphun'cle (! slpkunculus, a small pipe). The tube which 

 connects the chambers of the shells in the Tctrabranchiate 

 Cephalopoda. 



Sire'nia. A group of the Mammalia which comprises the 

 .Manatees and Dugongs. 



Smeg'ma (I* soap). The white soapy substance frequently 

 found adherent to the skins of new-born infants. 



Solenogly'phia (Gr. stifc/t, a channel ; glupho, I sculpture, 

 en-rave). A group of the Ophidia in which the maxillary 

 teeth are channelled. 



