THE ANATOMY OF BIRDS. — OSTEOLOGY. 147 



— all of them anterior to the true sacrum of a bird. The sacrum proper (tig. 57, s) consists 

 of those few vertebrte — three, four, or five — from foramina between which issue the spinal 

 nerves that form the net-work called the sacral plexus. These true sacral vertebrae are ribless, 

 and may be recognized, in a general way, by the absence of anything like the cross-bars above 

 described, issuing from the vertebral centra ; though their neural arches send off some small 

 bars or plates to fuse with the ilia. These sacrals proper are at or near the middle of the 

 whole sacral mass. After these come a large number — from five to ten or more — of verte- 

 bra which, from their following the true sacrals, though consolidated therewith and with one 

 another, are considered to belong to what would be the caudal region of other animals, and 

 are hence called " tail-sacrals," uro-sacrals (Gr. ovpa^ tail, fig. 57, c.) These continue to send 

 oft' a series of little plate-like processes from their neural arches, just as the true sacrals do ; 

 hut, in addition to these, processes are given off from the bodies of the uro-sacrals, corre- 

 sponding in position and relation to those which proceed from the bodies of the lumbars, and 

 being apparently of the same morphological character (pleurapophysial). These "riblets" 

 are, however, quite slender, and also oblique in two directions ; for instead of being trans- 

 verse and nearly horizontal, they trend very obliquely backward and upward ; they also 

 shorten consecutively from before backward. The cross-bars of the latter uro-sacrals, however, 

 are stouter and altogether more like those of a lumbar vertebra. The appearances described 

 are those seen fi-om below, or on the ventral aspect. Above, on the back of the pelvis, the 

 line of confluent spinous processes of the dorso-lumbars is commonly distinct, separated a little 

 from the flaring lips of the ilia. Such distinct formation may continue throughout the sacral 

 and uro-sacral regions ; oftener, however, the line of spinous process sinks, flattens, and 

 widens into a horizontal plate which becomes perfectly confluent with the ilia along the pos- 

 terior portion of their extent ; such smooth, somewhat lozenge-shaped surface being quite 

 continuous with the superfcies of the pelvis, but perforated with more or fewer pairs of inter- 

 vertebral foramina. — Such is the general character of a bird's complex sacrum; the description 

 is taken chiefly fi-oni a raven {Corvus corax) ; the figure from the common fowl, after Parker. 

 The kidneys are moulded into the recesses between the sacral and uro-sacral vertebrae and in 

 the concavity of the ilia. The general shape of a " sacrum," viewed from below, is fusiform, 

 broadest across the sacral bodies proper or just in front of them, tapering toward either end; 

 tlie face of the sacrum is also flattest about the middle, more or less ridged before and behind 

 from compression of the vertebral bodies. It has little if any lengthwise curvature, and that 

 chiefly in the uro-sacral region, where the concavity is downward. The total number of bones 

 may be less than twelve, or more than twenty. The extensive anchyloses in this region of 

 the spine are in evident adaptation to bipedal locomotion, which requires fixity hereabouts, 

 that the trunk may not bend upon the fulcrum represented by a line drawn through the hip- 

 joints, which are situated about opposite the middle of the sacral mass, as shown by the arrow, 

 ac, in fig. 60. (The word " sacrum," a " sacred thing," curious in this application, is very 

 ancient in human anatomy, commemorating some superstitious or ritualistic notion, respecting 

 this part of the body.) 



The Coccyseal, or Caudal Vertebrae (fig. 56, dv) proper, terminate the spinal column. 

 They are called " coccygeal," from the fancied resemblance of the human tail-bones collectively 

 to the beak of a cuckoo (Gr. k6kkv$, kokkux). The caudals are all the free bones situated 

 behind the anchylosed uro-sacrals. The series commonly begins opposite the point where the 

 pelvic bones end; it consists of a variable number of bones, from the twenty long slender ones 

 which the Arch(Eopteryx possessed, down to seven or fewer separate ones. The usual ninnber 

 is eight witliout the jiygostyle. They are stunted, degraded vertebrae, whose chief office is to 

 support tlie tail-featlicrs ; for the Icasli of nerves wliicli emerge from the sjnnal canal to form 

 the sacral plexus by so much diuiinish the spinal cord that a mere thread is left to pene- 



