4386 



COMPARATIVE ANATOMY 



pull and pressure of its location which determines its shape, size, rapidity 

 of growth, and even its joints. 



The reason this has not been understood heretofore is because for- 

 mer experimenters took only sections from the growing bone itself for 

 their study, whereas Professor Carey has taken the complete embryo- 

 logical structure, including all muscles and related portions, which might 

 throw light upon the pull and pressure which affects such bone during 

 its growing period. 



Observing the ossification centers in the skull will throw light on 

 this subject (Fig. 406). There are many such centers, and they are 

 always found at exactly those points where there is an especial stress or 

 pressure. At these points it may be that sharper bends in the blood 

 vessels cause a slowing of the blood stream, which slowing in turn causes 

 lime salts to be laid down at the angles to a much greater extent than 

 -where the blood stream can rush past more swiftly. Then, with each 



Pin* H f M 



*%* 



Fig. 406. 



A diagram of the skull bones of a mammal, the mem- 

 brane bones shaded. BO., Basioccipital ; EO., exoccipital ; C, 

 condyle ; SO., supraoccipital ; Par., parietal ; Fr., frontal ; Na, 

 nasal ; Pmx., premaxilla ; ME., mesethmpid ; L, lachrymal ; Tu., 

 turbinal ; PS., presphenoid ; OC., orbitosphenoid ; AS., alis- 

 phenoid ; BS., basisphenoid ; SQ., squamosal ; P., periotic ; T., 

 tympanic ; PI., palatine ; Pt., pterygoid ; MX., maxilla ; Ju., 

 jugal ; T.H., tympanohyal ; S.H., stylohyal ; E.H., epihyal ; C.H., 

 ceratohyal; B.H., basihyal ; Th.H., thyrohyal ; vomer ; MN., 

 mandible. (From Borradaile, modified from Flower and 

 Weber.) 



succeeding deposit of such a hardening substance, a still greater number 

 of blood capillaries is affected so that more lime is laid down, and so 

 on, until all of the capillaries have been more or less obliterated and the 

 'entire cartilage or membrane has become ossified. 



Beginning with the axial skeleton, the skull becomes our first object 

 of attention. The cranium is that part of it which ericloses the brain 

 as well as the bony p^rts forming the eye-socket, the ear and nose, while 

 the more caudal portion, which is directly connected with the cephalic 

 <end of the digestive tract, is called the visceral skeleton. 



That portion of the skull which is cartilaginous is known as the 



