Vertehrata. 329 



in the distal ; and a oentrale (rarely two) ; five metatarsals 

 and five toes, each composed of several phalanges. 



The bony or cartilaginous pieces are united simply by a 

 connective tissue sheath which lies between them, or more rarely by 

 cartilage, in which case there is but little power 

 of movement between the parts. If the move- 

 ment be greater, a joint is usually present ; 

 that is, the skeletal pieces are separated at their 

 adjacent ends by a slit-like space (the joint- 

 cavity), filled with a small amount of fluid, and 

 are only united by a capsule of connective m ^ — - ^^ly hn 

 tissue surrounding this cavity. The apposed 



surfaces of these skeletal pieces, the articular ^ . ^ 



surfaces, are smooth and accommodated 



to one another, but are very diverse in form ; Fig. 271. Longitu- 



they are almost always covered with a thin dmal section of a joint. 



■'..•'. . A and B the two bones 



layer of cartilage (articular cartilages), a of the joint, ft periosteum, 

 remnant of that of which the whole bone ^ capsule, to articular 



... • T mi ■ • cartilages. — AiterGregen- 



primitively consisted. The connective tissue m baur. 



the region of the joint is often in part modified 



into firmer cords or ligaments, which reach from one bone to the 



other. With the exception of the articular surfaces, bones are 



everywhere covered by a fibrous connective tissue, the periosteum 



cartilage is similarly covered by the perichondrium. 



Bones, unless they are unusually thin, do not consist simply of osseous tissue, 

 but have cavities within them containing connective tissue and vessels. The 

 outer portion usually consists of a fii-m mass of compact hone, which 

 is perforated only by small canals (Haversian canals); the inner portion, 

 on the contrary, usually consists of spongy or cancellous bone in 

 which canals and spaces (marrow spaces), preponderate, separated by fine 

 trabeculse and laminae. In the middle of long bones, there is frequently an 

 expanded cavity, filled usually with adipose tissue, the marrow cavity. In 

 cartilages, too, a smaller number of fine canals are usually present, containing 

 connective tissue and blood vessels. 



Since the skeleton of Yertebrata is an internal one, the muscu- 

 lature is principally external, in contrast to Arthropoda, where 

 it lies within the skeleton. The muscles may be classified as those 

 of the trunk and those of the limbs. In Amphioxus and Pisces the 

 body muscles consist chiefly of large continuous masses, disposed on 

 the sides of the trunk and tail, not closely connected with the 

 skeleton, and divided by thin sheets of connective tissue into a 

 number of segments ; besides these, there are smaller muscles for 

 the movement of the visceral skeleton, the fin rays, etc. The limb 

 muscles are, as a rule, feebly developed in the Pisces. Similar 

 arrangements occur, in part, in the Amphibia, whilst in the higher 

 Vertebrata the musculature both of trunk and limbs is separated 

 into numerous individual muscles, extending from one bone to 



