THE INTERNAL SKELETON. 199 
The tibia is generally the longest bone of the leg, and is some- 
what triangular in section at its upper part. At the anterior 
aspect of its upper end is an irregularly shaped prominence 
called the cnemial process. The lower, or distal, part of the tibia 
ends in two condyles or antero-posteriorly directed prominences 
with a median depression forming a pulley-like articular surface 
or trochlea, which is directed somewhat forwards as well as 
downwards. On the front of the lowest part of the tibia, just 
above the condyles, is a deep depression over which a band of 
bone (a bony bridge) generally passes. 
Tn front of the junction of the tibia with the femur there is 
usually a small bone, the patella, answering to our knee-pan. 
There may be two such bones, while sometimes there is no 
patella at all. 
The tibia answers to our shin-bone and more. It does so 
because the distal end of the bone, which is distinct in the 
young, answers to a portion of our ankle. 
The main, often the only, element of the skeleton of the next 
segment of the lower limb is termed the tarso-metatarsus. As 
already said *, it answers to the skeleton of the middle part of 
our foot which is called the “ metatarsus.” It consists of 
metatarsal bones more or less completely fused into one, and 
something more, since it also includes what answers to a portion 
of our ankle-bones. Now in human anatomy the skeleton 
of the whole of the ankle is called the “ tarsus,” and thus 
the skeleton of the “crus” of birds answers to our leg-bones 
together with the proximal part of our tarsus; while the skele- 
ton of the so-called “ tarsus ” of birds answers to the distal part 
of our tarsus together with the bones of the middle part of our 
foot. It is thus clear that the joint by which the so-called 
“tarsus” of birds articulates with their “crus” does not 
answer to our ankle-joint. It answers indeed to no conspi- 
cuously moveable joint which we possess. It answers to the 
interval between the proximal and the distal parts of our ankle 
or tarsus. The joint between the “crus” and “ tarsus ” of 
birds is thus only analogous to our ankle-joint and is not 
homologous therewith, 7. e. does not bear corresponding struc- 
tural relations with surrounding parts. 
The tarso-metatarsal bone is of complex nature over and above 
the fact that it contains elements of the ankle anchylosed with 
* See ante, p. 197. 
