rection, a group met (Gregory, 1917) to systematize terms 

 for Permian tetrapods. An N.A.C. would depend to a great 

 extent upon agreement as to how similar structures must be 

 to be identified as homologs or how dissimilar they must be 

 to warrant use of different names. These are difficult 

 questions to answer as will be seen from the borderline 

 situations which will be described and discussed. 



Summarizing the above, the skeletal system, because it 

 is fossilized, is the only system that gives us a broad over- 

 view of the history of structural changes in the vertebrates. 

 The head skeleton is the most informative part of the 

 skeleton since its complexity offers many more opportunities 

 for divergent change. Because description of the head skele- 

 ton involves terms and since the basis of anatomical termi- 

 nology is the "Nomina Anatomica," the human should be 

 considered first. From man the terms can be extrapolated 

 to progressively more distant groups, i.e. reptiles, amphi- 

 bians, fishes. This procedure can be characterized as going 

 from the known to the unknown, or from the fixed to the 

 controversial. It also, in the final analysis, is going from the 

 simple to the more complex. In this process the limitations 

 of the concept of homology are encountered along with 

 problems of definition, observation, and interpretation. 



MAMMALIAN HEAD SKELETON 

 Introductory concepts 



Before beginnmg a description of the head skeleton, it 

 would be helpful to consider some general ideas concerning 

 it and its origin. These introductory remarks will be repeated 

 and enlarged as the study progresses. 



The skull was thought by Goethe, Oken, and Owen to be 

 made up of four to seven modified vertebrae. This view 

 was destroyed by Huxley in his Croonian Lecture of 1858. 

 The vertebra analogy is quite efTective in describing the skull 

 but is opposed by its embryological history. 



In most descriptions of the head skeleton, the bones are 

 separated into groups based on their origin. Some bones 

 arise in or around cartilaginous precursors and are called 

 chondral bones; other bones ossify directly in connective 

 tissue masses and are called dermal bones. Chondral bones 

 are of two types: those that arise at the surftice of the carti- 

 lage, inside of the perichondrium (the connective-tissue 

 envelope of the cartilage), and those that arise within the 

 cartilage. The former are perichondral, the latter endo- 

 chondral. Growth of a perichondral ossification may result 

 in replacement of the whole cartilage, or the bone may re- 

 main as an outer sheath. Endochondral bone may replace 

 the entire mass or retain some of the cartilage on its surface, 

 particularly in regions of contact or articulation with other 

 bones. A chondral bone may also have dermal extensions. 



The head skeleton is a functional structure which res- 

 ponds to the needs of the sense organs or the brain and 

 cranial nerves enclosed by it. Its bones also provide passage 

 for arteries and veins serving various parts of the head. The 



operation of the jaws modify this skeleton, depending on 

 the mode of articulation of the lower jaw, the attachments 

 of muscles, or the types of teeth and proportions. Bones are 

 usually thought of as forcing rigid conformity on other parts, 

 but this is not the case at all. Bone is very plastic, and this 

 plasticity is related in part to its late embryological appear- 

 ance. Bone or cartilage appear to be functional responses to 

 forces set up in the connective tissue system for support, for 

 mechanical lever action, or for resistance to compression 

 strain. As a continually responding tissue, bone can be laid 

 down or resorbed, and reformed to fit altered needs. 



Head skeleton of placental mammals 



Human head skeleton The human head skeleton is best 

 observed in the foetus (Figures 3-1, 3-4). It can be sub- 

 divided into the divisions and bones given in Table 3- 1 . The 

 reasons for some of these subdivisions have already been 

 stated or will be understood only after a study of the skulls 

 of other vertebrate groups. Bones found in other mammals 

 besides man are included in this list and set in italics. No 

 description will be given here, for the illustrations (Figures 

 3-1, 3-4) will help in locating each of the bones. The famili- 

 arity gained from locating the bones of the table in the 

 figures will serve in locating these same bones in a laborator)' 



Table 3- 1 THE BONBS Of THE HEAD SKELETON Of THE MAM- 

 MAL ARRANGED ACCORDING TO TYPE AND PLACE 

 OF ORIGIN 



MAMMALIAN HEAD SKELETON • 35 



