170 THE ARTICULATIONS. 



The number of cranial vertebrae is invariable, as each is destined to lodge 

 the organs of one of the four senses. The occipito-hyoideal receives the principal 

 organs of hearing ; the parieto-maxillary osteodesm protects the sense of taste ; 

 finally, the organs of vision are sustained by the fronto-mandibular vertebra, 

 while the naso-turbinal contains the sense of smell. 



Several anatomists, at the head of whom are Huxley and Gegenbauer, do not 

 entirely share these views. They certainly admit the existence of the occipital 

 vertebra, but it appears to them to be impossible to recognize the others. They 

 remark that all the bones which constitute the spine are found in the primary 

 cartilaginous skeleton ; so that, in order to establish the vertebral constitution 

 of the head, it would be necessary to allot to the same cranial vertebra : 1. The 

 pieces that are found in the cartilaginous cranium. 2. The other pieces which are 

 developed in the fibrous tissue — those skeletal tegumentary pieces (temporals, 

 parietals, etc.). It must also be observed that the division into vertebral bodies 

 of the parts which form the base of the cranium, far from being easy in the lowest 

 animals, is, on the contrary, only possible, with some trouble, in the highest classes. 

 So that if the vertebral constitution of the head were a fact, it must be admitted 

 that the differentiation of the bones at the base of the cranium is less advanced 

 in Mammals than in the lower vertebrates. 



The vertebral type is not, therefore, universally accepted by all anatomists. 



SECOND SECTION. 

 The Articulations. 



THE ARTICULATIONS IN GENERAL. 



The different pieces constituting the solid framework of the animal body are, as 

 has been said, united in such a manner that they can move one upon the other. 

 From this union results the articulations, or articular Joints, the construction 

 of which will now be referred to in a general manner, before commencing a 

 particular description of each. 



Arthrology, or Syndesmology, is the name given to that division of anatomy 

 which treats of the articulations. To form articulations, the bones correspond 

 with each other by certain determined points of their periphery, which are named 

 articular surfaces. Every articulation is, therefore, essentially constituted by two 

 opposite osseous surfaces, simple or complex, which are moulded to each other. 

 These are either contiguous, independent, and very movable — continuous with 

 each other by means of a cartilaginous substance which confines them, if not to 

 total immobility, at least to very limited movements ; or united by a fibro- 

 cartilage, the elasticity of which permits a certain degree of displacement between 

 the bones in contact. 



In the first case, the articulations are classed as diarthroses^ or movable 

 articulations. 



In the second, they are designated synarthroses, sutures, or immovable 

 articulations. 



