Introditctioii to Aiiiiual ]\Iorpho!ogy. 19 



secondary areolae. In this case the bone columns are usually 

 arranged so as to give the greatest strength consistent with 

 the amount of solid material.* 



Within growing bone the unossified nucleated protoplasm, 

 formed by the proliferation of cartilage cells, becomes the 

 marrow, of which there are three kinds : yellow, gelatinous, 

 and red. The first consists of delicate connective tissue, ves- 

 sels and fat cells, and fills the shafts of long bones in most 

 Mammalia. The second form consists of an abundant trans- 

 lucent tissue (coagulable by acetic acid), and a few free nu- 

 cleated cells. The third form consists of a stroma of stellate 

 corpuscles entangling lymph cells, and surrounded by a close 

 capillary network of blood-vessels whose calibre is four times 

 greater than that of its feeding arteries. By the proliferation 

 of smaller cells, large colossal (myeloid) cells and polynuclear 

 masses (myeloplaxes) are formed. f In this tissue found in 

 the cancelli of young bone, Naumann, Bizzozero, and others 

 suppose the red blood corpuscles to be formed. (Red marrow- 

 is sparingly present in birds.) 



Bones may be long, flat, or irregular ; they constitute the 

 sclerome or skeleton. Those developed in the integument 

 are called exoskeletal, those in an internal cartilage basis 

 form the endoskeleton, which consists of — ist, an axial or 

 central column protecting the nervous and visceral systems, 

 and 2nd, an appendicular part, or the skeleton of the limbs. 

 Dermal or integumental bones are developed by ossification 

 in a fibrous matrix {parostosis), and have no granular stage. 

 Endoskeletal bones may be developed by endosiosis (ossifica- 

 tion beginning within the intercellular spaces of a cartilage 

 mass), or by ectostosis (ossification proceeding from the sur- 

 face inwards, beginning beneath the periosteum or fibrous 

 sheath which surrounds all bones). 



* ilf^_y^r distinguishes two sets of pillars in the cancelli, one set in the line 

 of greatest pressure (pressure lines, Driicklinien), and the other in the hner. 

 of traction (traction lines, Zuglinien). 



t Bredichin describes the colossal cells as formed from bone. 



C 2 



