326 METAZOAN PHYLA 



Bone tissue consists of an intercellular matrix of salts of lime which 

 masks the cellular elements. Under the microscope, however, the cells, 

 called hone corpuscles, appear, lodged in spaces called lacunae. Small 

 canals called canaliculi run from one lacuna to another and contain the 

 branching processes of the cells. In the living animal the bones are sur- 

 rounded, except on articular surfaces, by a fibrous covering, or periosteum, 

 firmly anchored by fibers which penetrate the bone itself ; the articular sur- 

 faces are covered with cartilage. Blood vessels and nerves enter the 

 mass and are distributed through it, reaching, in many bones, a central 

 cavity containing fatty, yellow marrow. The marrow, of two kinds, 

 red and yellow, is a form of connective tissue. The red marrow is com- 

 posed largely of cells, from which are produced the new red blood cor- 

 puscles needed continually to replace those worn out by use. 



The skull may be divided into two parts, a cranial part, or cranium, 

 made up of the bones which surround the brain, and a visceral part, 

 which includes the bones developed about the mouth and nasal chambers. 

 To the latter are added other bones which lie in lower forms about the 

 gill sUts and which in the higher forms are more or less intimately associ- 

 ated with the skull. 



The vertebral column consists of a series of individual bones known as 

 vertebrae. It extends through the neck, trunk, and tail and is divided 

 into four regions. The cervical vertebrae are in the neck; the thoracic 

 vertebrae in the anterior part of the trunk; the sacral vertebrae in the 

 posterior part of the trunk where, with the pelvic girdle, they form the 

 pelvis; and the caudal vertebrae in the tail. Ribs may be developed in 

 connection with all the regions of the vertebral column, though they are 

 most constant and reach their greatest development in the thoracic 

 region, where in the highest forms they, together with the sternum, form 

 a thoracic basket inclosing the lungs. Several ribless vertebrae lying 

 between the thoracic and sacral regions, corresponding to what is known 

 in the human body as the small of the back, are in the mammals recog- 

 nized as a fifth region and called lumbar vertebrae. 



The girdle to which is attached the anterior pair of appendages is 

 known as the pectoral girdle. The girdle of the posterior appendages is 

 known as the pelvic girdle, and with the sacral vertebrae forms the pelvis. 

 . The bones forming the skeletons of the two limbs correspond, and this 

 correspondence may be brought out by reference to diagrams (Fig. 227). 

 The homology between the limbs also includes blood vessels, muscles, 

 and nerves. 



347. Muscular System. — The voluntary muscles of vertebrates are 

 usually attached to some part of the skeleton and are known collectively 

 as the flesh of the animal. Their attachment is frequently by means of a 

 tendon, which is a dense mass of parallel connective tissue fibers continu- 

 ous on the one hand with the muscle sheaths and on the other intermin- 



