138 LOCOMOTION. [CHAP. vi. 



case there is rotation as perfect as in that just mentioned ; but the 

 head of the radius rolls in the ring, and the axis of motion is the 

 axis of the head and neck of the bone. Some anatomists consider 

 this joint a species of ginglymus, which they designate lateral. 



The terms Symphysis, Synchondrosis, Synneurosis, Syssarcosis, 

 Meningosis, have been employed by anatomists to designate certain 

 kinds of articulation, chiefly in reference to the nature of the con- 

 necting media. Symphysis, although originally employed with 

 great extent of meaning, seems to have been in later days applied 

 exclusively to denote the articulations of the pelvis, which we have 

 classed under Amphiarthrosis. We pass over the other terms, 

 because they ought to be discarded from use, as only tending to 

 encumber a vocabulary already too much crowded with difficult 

 and unnecessary terms. 



Mechanism of the Skeleton. We shall conclude this chapter with 

 some remarks upon the mechanical disposition of the various parts 

 of the skeleton, and their adaptation to the purposes they were 

 destined to fulfil. 



The skeleton consists of the head, trunk, and extremities. 



The head is composed of a cavity, surrounded by osseous walls 

 (cranium), destined to contain and protect the brain; and of an 

 expanded portion (the face), with which some of the organs of the 

 senses are connected, and upon which the features are formed. 

 The size of the cranium affords a good clue to determine the abso- 

 lute size of the brain ; and the proportion of the face to it, offers a 

 not inexact index of the relation which the intellectual faculties an( 

 the animal propensities bear to each other.* 



The spheroidal form of the cranium admirably adapts it for pi 

 tecting the organ which it contains against external violence. Th< 

 arched form is that which possesses most strength, and offers th< 

 greatest resistance. When, says Dr. Arnott, we reflect on th< 

 strength displayed by the arched film of an egg-shell, we need nol 

 wonder at the severity of the blows which the cranium can witl 

 stand. And he adds, in reference to the former, " what hard bloi 

 of the spoon or knife are often required to penetrate this wondei 

 defence of a dormant life ! " And this form, which gives so mu< 

 strength to the skull, favours the transmission of vibrations along it 

 walls, and thus saves the delicate viscus enclosed by them. Thi 

 blows inflicted upon the cranium become diffused; and sometime 



* See a good account of the comparative mensuration of the skull in Mr. 

 "Ward's Outlines of Human Osteology. 



