INTRODUCTION 



part of the centrum. The articular surface of the anterior zygapophysis looks 

 upward, while that of the postzygapophysis looks downward. The first verte- 

 bra, which supports a rib united to the sternum, is commonly called the first 

 dorsal. As a rule there are thirteen 

 dorsals, less frequently from ten to twenty. 

 The lumbar vertebrae (Fig. 4, B) have no 

 ribs, the transverse processes are greatly 

 elongated and usually attenuated, the broad 

 spinous processes are directed obliquely 

 forward, and the centra are markedly 

 longer than those of the dorsal series. 



The sacrum supports the pelvis, and 

 is undifferentiated only in Cetacea and 

 Sirenia, which have lost their hind limbs. 

 Usually three to four vertebrae are fused 

 to form a sacrum, but occasionally only 

 two are involved, and more rarely as many 

 as eight or nine may be included. The 



centra and neural arches, and often the aspect). B, Second lumbar vertebra of the dog 



Qm'nnnc: nrnnpqtsP";; akn arp fncspd tnP'Pthpr O^teral aspect), c, Centrum; sp, neural spine; 



spinous processes aiSO, aie mseu LUgetuei ^^ diapopliysis ; s, anterior; z', posterior zyga- 



tO make an immovable sacral segment, pophysis ; m, metapophysls ; a, anapophysis ; t, 



. • e 1 facet for the tuberculum ; ca, facets for the 



which is broadest in front, and tapers capituium. 

 toward the tail. 



The caudal vertebrae are the most variable of all in number and form. As 

 a rule the most anterior ones have well-developed neural arches, spinous pro- 

 cesses, transverse processes, zygapophyses, and occasionally V-shaped haema- 

 pophyses (chevron bones). Toward the rear modifications gradually decrease, 

 the centra being more elongated, the arches and processes becoming vestigial or 

 disappearing entirely, the last centra taking on a more or less cylindrical form. 



The ribs are developed only in connection with the dorsal vertebrae, the 

 more anterior or true ribs being attached distally by means of cartilaginous or 

 imperfectly ossified costal cartilage to the sternum, while some of the posterior 

 or floating ribs end free in the muscular wall of the chest. Each rib has two 

 articulations, the tuberculum, which articulates with the transverse process, and 

 the capituium, which unites with the front of the centrum, or occasionally with 

 the lower part of the neural arch. It often happens, also, that this articulation 

 falls between two centra, so the capituium is united to the centrum in front, as 

 well as to the succeeding one. The sternum or breast bone is not a single 

 element, but consists of a median row of flat, separate, bony pieces suturally 

 united with each other. 



The skull (Figs. 5, 6) of mammals is distinguished from that of fishes, 

 amphibians, reptiles and birds, chiefly in the smaller number of constituent 

 bones, in the fusion of the upper jaws to the cranium, in the suppression of the 

 lower jaw supports, and direct connection of the dentary with the squamosal 

 bone. The absence of a distinct quadrate and the presence of two occipital 

 condyles are further characteristic of the mammalia. The form of the skull 

 depends on the size of the brain, the development of the jaws, the presence of 

 crests, bony protuberances, horns, etc., and shows tremendous variation in the 

 different orders and families. 



In the skull the bones bounding the brain cavity are called the cranial, 



