OF THE MOTORY APPARATUS IN MAN. 129 



disposed as to favour rapidity of motion at the expense of 

 force. Thus, in lowering the extended arm, if the rapidity 

 with which the muscles contract be such that their point of 

 insertion be displaced three inches in a second, the extremity of 

 the limb will pass from its original position with a rapidity 

 of nearly three feet per second. 



Description of the Motory Apparatus in Man. 



268. The motory apparatus as we have already 

 mentioned is composed in man and all vertebrata of the 

 skeleton, the muscles, and the articular apparatus. 



The skeleton is divided into the head, trunk, arid extremities. 



269. The Head. The 

 skeleton of the head is composed * 

 of two portions the cranium 

 and face. The cranium, lodg- 

 ing the brain, cerebellum, and n 

 pous, with their membranes, the na 

 vessels proceeding to and from c 

 them, and the roots of the cranial * 

 nerves, is composed of eight 

 bones : the frontal or coronal 

 (Fig.76/); the two parietal, p; Fig. 76.* 



the two temporal, t; the occi- 

 pital, o, behind; and the sphenoid, *; and the ethmoid 

 below. These bones are generally broad and flat, of a com- 

 pact tissue externally and internally, and are immovably united 

 by sutures dovetailed into each other. This adds greatly to 

 the strength of the cranium and of the arch it forms : the 

 interlocking of the sphenoid also with all the other bones 

 of the cranium contributes greatly to its general strength. 



At the base of the skull will be seen a great number of 

 apertures for the egress of the cranial nerves, and of the 

 bloodvessels entering or leaving the cranium. In the occipital 

 bone, close to its condyles, is the foramen magnum, by which 

 the medulla spinalis passes towards the brain ; the vertebral 

 arteries, also intended to supply the brain, enter by this large 

 aperture. The cranium is articulated by means of these two 

 condyles of the occipital bone with the atlas or first cervical 



* f, frontal or coronal ; p, parietal; t, temporal; o, occipital; *, the sphe- 

 noid; 11, the nasal; ms, superior maxillary ;./, jugal or cheek bone; -rot, 

 inferior maxillary or lower jaw bone; na, anterior opening of the nostrils ; 

 fa, auditory canal or foramen ; az, zygomatic arch ; aA, cd, lines forming the 

 facial angle. 



K 



