SEC. IV ANATOMY OF BIRDS 213 



the costal process, so prominent in Passeres (Fig. 58). The posterior 

 lateral piece is the metosteon (Gr. /iera, meta, after). lYom the 

 latter are derived the pair, or two pairs, of lateral processes which 

 the posterior border of the sternum has in many birds. In fine, the 

 extent of ossification of the lophosteon and metostea, and the mode 

 of their coossification, determines all those Various shapes of the 

 posterior border of the sternum which, being commonly character- 

 istic of genera and higher groups, are described for purposes of 

 classification. Thus, if the lophosteon and the metostea are com- 

 pletely ossified and to the same extent behind, the posterior border 

 of the sternum will be transverse, and perfectly bony. Such a 

 sternum is said to be entire. If the lophosteon is longer than 

 the lateral pieces, the sternum will have a median pointed or 

 rounded projection; such a formation is called the middle xiphoid 

 process (Gr. ^t'^os, xiphos, a sword : cfSos, eidos, form). The pro- 

 jection of the metostea, not infrequent, similarly gives a pair of 

 external lateral xiphoid processes. But such processes oftener 

 result merely from defects of coossification between the elements of 

 the sternum. Thus, there is often a deep notch in the posterior 

 border of the sternum between the lophosteon and the metosteon 

 of each side ; the sternum is then said to be single-notched or single- 

 ermrginate (one pair of notches, one on each side; Fig. 58). This 

 conformation prevails throughout the great group Passeres, possibly 

 without exception ; it is therefore highly characteristic of that 

 order, though a great many other birds also have it. In the 

 natural state, the notch is filled in with membrane. Such a notch 

 may also be converted into a " fontanelle " or fenestra (Lat. fenestra, 

 a window), which is simply a hole in the bone, the metostea having 

 grown to the lophosteon at their extremities, but left an opening 

 between. Such a sternum is called fenestrate, more exactly uni- 

 fenestrate (Lat. unus, one ; one window on each side). Now, the 

 parts remaining .as before, let each half of the lophosteon, or each 

 metosteon, be notched or fenestrate ; obviously, then, such a sternum 

 is double-notched or birfenestrate, having four notches, or holes, two on 

 each side — two notches, or two holes ; or notched and fenestrate, 

 having a notch and a hole on each side. The latter is very fre- 

 quent : when occurring, the hole is generally nearest the middle 

 line, the notch exterior. Irregularity of ossification, converting a 

 hole into a notch, and conversely, may in any case result in lack of 

 symmetry ; but this is a mere individual peculiarity. When there 

 are two notches on each side, as in Fig. 56, the sternum has evi- 

 dently a median and two lateral backward extensions, which are 

 then called respectively the middle, internal lateral, and external 

 lateral xiphoid processes. Notching of the lophosteon in the middle 

 line, at least to any extent, must be very rare, if indeed it ever 



