34 LECTURE 11. 



long bones of the extremities of tlie colossal Iguaiiodons and Megalo- 

 saurs are as capacious as in any mammalian quadruped, and tlie white 

 crystallised spar with which these petrified bones are often filled, is 

 called, not unaptly, " fossil marrow" by the quarry-men. In the ordi- 

 nary marrow -bones of quadrupeds the walls of the cavity are thickest 

 and strongest at the middle, and become thin towards the ends, where 

 the peripheral concentric lamellae are separated by wider interspaces, 

 and are bi'oken up into a fine lattice or lace work. All the cavity and 

 the cells are lined by a delicate membrane, less vascular than the exter- 

 nal periosteum, which secretes and immediately contains the marrow ; 

 this fine oily fluid diminishes the brittleness of the bones. A special 

 artery called the " medullary," supplies the lining membrane of the 

 medullary cavity ; and the foramen and canal have the same relative 

 position and course in most Mammalia as in Man ; to wit, the canal 

 in the humerus and tibia inclines distad, in the femur and anti- 

 brachial bones proximad, as it approaches the medullary cavity : the 

 true Ruminants, however, present an exception as regards the femur, 

 in which the medullary artery, instead of penetrating the back part of 

 the shaft and running upwards, enters the fore part of the shaft at its 

 upper thii'd, and inclines downwards. 



The flat bones of Mammalia, e. g. those of the cranium, the sca- 

 pulae, and ilia, have a spongy texture, called diploe, included between 

 two compact plates ; the internal one in the cranial bones is called 

 the " vitreous table" from its density and brittleness. But the most 

 compact example of the osseous tissue is the bone containing the 

 organ of hearing, thence called " petrous," Avhich, with the tympanic 

 bone, reaches the maximum of density in the Cetacea, 



The bones of birds, especially those of flight, present the opposite ex- 

 treme of lightness ; not but that the osseous tissue itself is more com- 

 pact than in most Mammalia, but its quantity in any given bone is much 

 less, the most admirable economy being traceable throughout the skele- 

 ton of birds in the advantageous arrangement of the weighty material 

 for the ofiice it is destined to perform. Thus, in the long bones, the ca- 

 vities, analogous to the medullary in mammals, are more extensive, and 

 the solid walls of the bone much thinner ; a large aperture called the 

 " foramen pneumaticum," near one or both ends of the bone, com- 

 municates with its interior, and an air-cell or prolongation of the 

 lung is continued into and lines the cavity of the bone, which is thus 

 filled with rarefied air instead of marrow. The extremities of the 

 bone, instead of being occupied by a spongy diploe, present a light 

 open network, slender columns shooting across in diflerent di- 

 rections from wall to wall, and these columns are likewise hollow. 

 The vastly expanded beak, with its hornlike process, in the Hornbill 



