TEXT-BOOK OF ZOOLOGY 



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Two arches, which coalesce, forming a ring, extend dorsally (in some 

 cases on the ventral side also) from, the body of the vertebra, or centrum. 

 The rings of all the vertebrae form a canal which encloses the spinal 

 marrow. On the ventral side of the body are placed the organs of 

 respiration, circulation and digestion. This position of these organs 

 relatively to the nervous system is peculiar to the vertebrata. (Compare 

 this respect other animals, e.g., the articulata.) A process, the 



(upper) spinous process, generally rises 

 from the middle of the vertebral arch, 

 and serves as a point of attachment 

 for the muscles. Frequently lateral 

 projections, the transverse processes, 

 also occur. Long bones, the ribs, the 

 number of which is variable, are con- 

 nected with the vertebrae of the trunk. 

 These act as a supporting framework 

 for the walls of the cavities in which 

 are lodged the internal organs (heart, 

 lungs, intestines, etc.). Those ribs 

 which are united with the plate-like 

 breast-bone, or sternum, are termed 

 true ribs ; those which do not reach 

 the sternum being called false ribs. 



At its anterior termination the 

 spinal canal widens out into a large 

 cavity, the cranial chamber, which 

 encloses the brain. The bones of this 

 cranial case are called cranial bones. 

 The other bones of the head, which 

 form the face, are called facial bones. 

 Both sets together form the cranial 

 skeleton, or skull. 



The limbs also are supported by 

 solid skeletal elements. They are 

 connected by special bones with the 

 vertebral column — the fore-limbs by 

 means of the shoulder girdle, the hind- 

 limbs by means of the pelvis. 

 3. Muscles. — How are the bones put in motion? If the skin is 

 removed from a vertebrate animal, it will be seen that the' flesh which 

 covers the bones does not form a continuous mass, but consists of 

 numerous smaller or larger bands. These are the muscles. At their 



Human Arm. A, Forearm extended ; 

 B, Forearm flexed. 



0., Humerus, or upper arm ; Sp., radius ; 

 E., ulna; Hw., carpus; M., metacarpal 

 bones ; 1, 2, 3, phalanges ; Mu., biceps 

 muscle (the part shaded deep black 

 shows the extent of the muscle when the 

 lower arm is flexed) ; S., tendons of the 

 biceps ; Sch., shoulder-blade (scapula) ; 

 Schb., collar-bone (clavicle). 



