86 



OSTEOLOGY 



cranium, and partly from that which lies outside the entoderm of the fore-- 

 gut. They comprise the upper part of the occipital squama (interparietal), the 

 squamse and tympanic parts of the temporals, the parietals, the frontal, the vomer, 

 the medial pterygoid plates, and the bones of the face. Some of them remain 

 distinct throughout life, e. g., parietal and frontal, while others join with the bonen 

 of the chondrocranium, e. g., interparietal, squamae of temporals, and medial 

 pterygoid plates. 



Recent observations have shown that, in mammals, the basi-cranial cartilage, 

 both in the chordal and prechordal regions of the base of the skull, is developed 

 as a single plate which extends from behind forward. In man, however, its posterior 

 part shows an indication of being developed from two chondrifying centers which 

 fuse rapidly in front and below. The anterior and posterior thirds of the cartilage 

 surround the notochord, but its middle third lies on the dorsal aspect of the noto- 

 chord, which in this region is placed between the cartilage and the wall of the 

 pharynx. 



Optic foramen Small wing of sphenoid 



Great wing of sphenoid 



Nasal 

 capsule. 



Vomer 



Palatine 

 bone 



Mandible. 



Cricoid cart. 



Thyroid cart. 



\Styloid process 

 Fen. cochleae 



Canal for hypoglossal 



nerve 



Fio. 71. The same model as shown in Fig. 70 from the left side. Certain of the membrane bones of the right side 



are represented in yellow. (Hertwig.) 



BONE 



Structure and Physical Properties. Bone is one of the hardest structures of 

 the animal body; it possesses also a certain degree of toughness and elasticity. 

 Its color, in a fresh state, is pinkish-white externally, and deep red within. On 

 examining a section of any bone, it is seen to be composed of two kinds of tissue, 

 one of which is dense in texture, like ivory, and is termed compact tissue; the other 

 consists of slender fibers and lamellae, which join to form a reticular structure; 

 this, from its resemblance to lattice-work, is called cancellous tissue. The compact 

 tissue is always placed on the exterior of the bone, the cancellous in the interior. 

 The relative quantity of these two kinds of tissue varies in different bones, and 

 in different parts of the same bone, according as strength or lightness is requisite. 

 Close examination of the compact tissue shows it to be extremely porous, so that 

 the difference in structure between it and the cancellous tissue depends merely 

 upon the different amount of solid matter, and the size and number of spaces in 

 each; the cavities are small in the compact tissue and the solid matter between 



