a2 ANATOMY OF THE RABBIT. 
In a comparison of the adult skeleton with the more primitive 
embryonic skeleton, several differences in the arrangement of the 
elements are evident. Thus many bones, notwithstanding their 
possession of several centres of ossification, are to be looked upon as 
individual either in the cartilage or in the bone condition. In other 
cases, as in the basal portion of the skull, separate bone elements 
are produced in a mass of cartilage primarily continuous. These 
either remain distinct throughout life, or, as in the occipital region, 
(Fig. 12), become fused together to form compound or composite 
bones. In still other cases, as in the vertebrae, the apparently 
single elements of the adult condition are the products not only of 
originally distinct bones, but also of primarily separate cartilage 
masses. 

Fic. 13. Vertical sections of elbow and knee of four-day-old rabbit. A 
elbow; c, capsule; eb, endochondral bone in the distal epiphysis of the humerus; 
ea, extensor muscles of the forearm; em, extensors of the hand; fa, flexors of the 
forearm; fm, flexors of the hand; h, humerus; ol, olecranon; r, radius; sc, 
synovial cavity; u, ulna. B, knee; a, anterior cruciate ligament; c, capsule; 
f, femur; lp, patellar ligament; p, posterior cruciate ligament; pv, popliteal 
vessels; t, tibia; x,x, anterior and posterior ligaments of the lateral meniscus; 
x’,x’, anterior and posterior ligaments of the medial mehiscus. 
The bones of the skeleton are united or articulated with one 
another by connective tissue in the form of ligaments, by cartilage, 
or in some cases by both together, i.e., by fibrocartilage. Liga- 
mentous union, distinguished as syndesmosis, is the most general 
type of articulation. Cartilage union or synchondrosis occurs 
in certain situations, as in the basal region of the skull. Union by 
fibrocartilage or symphysis is characteristic of the articulation 
of the two sides of the pelvis (symphysis pubis). 
5 
