1 42 MORPHOLOGICAL TYPES 



in having lost the endocuneiform (ist tarsal), otherwise its 

 tarsus is easily comparable to that of Triton. The foot has 

 four toes, the ist or hallux having disappeared. There are 

 consequently four metatarsals, and the digital formula for the 

 number of phalanges is o, 3, 3, 3, 3. 



It is common to find small irregular bones on the under or 

 palmar side of the joints of several of the fingers and toes, and 

 covering certain joints of the arm and leg. These are the 

 sesamoid bones. They arise in connexion with the insertion 

 of tendons on to bones. Examples are the patella, and the 

 pisiform which underlies the joint between the ulna and the 

 cuneiform bone of the wrist. Sesamoids are important 

 functionally, but they have not much significance in com- 

 parative anatomy, since they are not constant from group to 

 group. 



Sternum and Ribs. — The breast-bone or sternum is sub- 

 divided into six sections, called sternebrae, and a posterior piece 

 called the xiphisternum. The most anterior of these (manu- 

 brium) is attached to the clavicles. The first seven ribs 

 articulate ventrally with the sternum. The dorsal part of each 

 of these ribs is bony, the ventral part cartilaginous. The next 

 two ribs are attached ventrally not to the sternum but to the 

 seventh rib, and the remainder end freely and are not attached 

 to any skeleton. 



Ccelom. — An important characteristic of mammals is the 

 fact that the perivisceral coelomic cavity is completely divided 

 into two by a transverse partition, anteriorly convex, the 

 diaphragm. The lungs are in front of this diaphragm, enclosed 

 in the pleural cavities. The pericardium is also in front of the 

 diaphragm, but its cavity is separated from that of the pleural 

 cavities. The remaining viscera lie in the general peritoneal 

 cavity behind the diaphragm (see Fig. 127). 



The diaphragm, which is of course pierced by the alimentary 

 canal, the aorta, and the inferior vena cava, divides the trunk 

 effectively into thoracic and abdominal regions. It is muscular, 

 and plays an important part in the process of respiration, 

 assisting the ribs in increasing the capacity of the thoracic box, 

 and causing air to rush into the lungs. It is developed in 



