OOLOGY 



cancellous tissue is called the diploe, and this, in certain regions of the skull, 

 becomes absorbed so as to leave spaces filled with air (air-sinuses') between 

 the two tables. The flat bones are: the occipital, parietal, frontal, nasal, lacrimal, 

 vomer, scapula, os coxae (hip bone), sternum, ribs, and, according to some, th 

 patella. 



Irregular Bones. The irregular bones are such as, from their peculiar form, 

 cannot be grouped under the preceding heads. They consist of cancellous tissue 

 enclosed within a thin layer of compact bone. The irregular bones are: the 

 vertebrae, sacrum, coccyx, temporal, sphenoid, ethmoid, zygomatic, maxilla, mandible, 

 palatine, inferior nasal concha, and hyoid. 



Surfaces of Bones. If the surface of a bone be examined, certain eminences 

 and depressions are seen. These eminences and depressions are of two kinds: 

 articular and non-articular. Well-marked examples of articular eminences are 

 found in the heads of the humerus and femur; and of articular depressions in the 

 glenoid cavity of the scapula, and the acetabulum of the hip bone. Non-articular 

 eminences are designated according to their form. Thus, a broad, rough, uneven 

 elevation is called a tuberosity* protuberance, or process, a small, rough prominence, 

 a tubercle ; a sharp, slender pointed eminence, a spine ; a narrow, rough elevation, 

 running some way along the surface, a ridge, crest, or line. Non-articular depres- 

 sions are also of variable form, and are described as fossae, pits, depressions, grooves, 

 furrows, fissures, notches, etc. These non-articular eminences and depressions serve 

 to increase the extent of surface for the attachment of ligaments and muscles, and 

 are usually well-marked in proportion to the muscularity of the subject. A short 

 perforation is called a foramen, a longer passage a canal. 



DEVELOPMENT OF THE SKELETON. 



The Skeleton. The skeleton is of mesodermal origin, and may be divided into 

 (a) that of the trunk (axial skeleton), comprising the vertebral column, skull, ribs, 

 and sternum, and (6) that of the limbs (appendicular skeleton). 



The Vertebral Column. The notochord (Fig. 19) is a temporary structure and 

 forms a central axis, around which the segments of the vertebral column are devel- 

 oped. 1 It is derived from the entoderm, and consists of a rod of cells, which lies 

 on the ventral aspect of the neural tube and reaches from the anterior end of the 

 mid-brain to the extremity of the tail. On either side of it is a column of paraxial 

 mesoderm which becomes subdivided into a number of more or less cubical seg- 

 ments, the primitive segments (Figs. 19 and 20). These are separated from one 

 another by intersegmental septa and are arranged symmetrically on either side of 

 the neural tube and notochord: to every segment a spinal nerve is distributed. 

 At first each segment contains a central cavity, the myoccel, but this is soon filled 

 with a core of angular and spindle-shaped cells. The cells of the segment become 

 differentiated into three groups, which form respectively the cutis-plate or derma- 

 tome, the muscle-plate or myotome, and the sclerotome (Fig. 64) . The cutis-plate 

 is placed on the lateral and dorsal aspect of the myoccel, and from it the true skin 

 of the corresponding segment is derived ; the muscle-plate is situated on the medial 

 side of the cutis-plate and furnishes the muscles of the segment. The cells of the 

 sclerotome are largely derived from those forming the core of the myoccel, and lie 

 next the notochord. Fusion of the individual sclerotomes in an antero-posterior 

 direction soon takes place, and thus a continuous strand of cells, the sclerotogenous 

 layer, is formed along the ventro-lateral aspects of the neural tube. The cells of 

 this layer proliferate rapidly, and extending medialward surround the notochord; 

 at the same time they grow backward on the lateral aspects of the neural tube 

 and eventually surround it, and thus the notochord and neural tube are enveloped 



1 In the amphioxus the notochord persists and forms the only representative of a skeleton in that animal. 



