NORMAL POSITION OF THE LIMBS. 37 



each side, and the more ce'^'hdXic— thorax — is enclosed by the ribs (costai) 

 and their cartilages {costicartilagines). 



The ventral and larger cavities of these three trunk regions are, strictly speaking, 

 cavum abdominis, cavum pelvis, and cavum thoracis; hut they are usually called by the 

 names of the regions themselves. 



The abdominal and pelvic cavities are continuous; but between the 

 abdomen and thorax, in the cat as in all other Mammals, there intervenes a 

 musculo-tendinous partition — the diaphragma. 



With all the lower Vertebrates, the diaphragm is absent or incomplete, so that the 

 three cavities are continuous, and constitute the acdum or trunk cavity. Sometimes it is 

 convenient to apply this general name with Mammals. 



A part of the alimentary canal is introdiiced for the sake of illustrating 

 a somewhat peculiar case in the designation of relative positions which will 

 be referred to in connection with the explanation of Fig. 7; § 91. 



§ 79. The Vertebrae.— As shown in Fig. 30, in the adult cat, the 

 Ycrtebrse form a continuous series from the caudal aspect of the cranium to 

 the tip of the tail. In this diagram, however, only the thoracic vertebrEe 

 are shown by a series of squares representing their centra (bodies). The 

 lines extended dextrad from the centra indicate the right costce (ribs). The 

 centra and costse are included in this diagram merely to illustrate the 

 method of notation of parts which form a series. The most cephalic is 

 numbered 1, etc. 



§ 80. Normal Position of the Limbs. — For accurate comparison of 

 the limbs with each other, or with those of other animals, it is desirable 

 that they should be placed, or regarded as placed, in some uniform normal 

 position. 



One of the many possible natural attitudes of the cat's limbs is shown in 

 Fig. 30, and there is great variety among other Mammals. 



Huxley has proposed (J) that all limbs should be compared as if in the 

 position shown in Figures 2 and 6. In this condition they stand out at right 

 angles with the body (soma), the extensor surfaces (with the convexities of 

 the elbow and knee) being placed dorsally, and the flexor surfaces ventrally, 

 with both pair of limbs. This is the position which they have in the 

 embryo Mammal, according to Kolliker, and it continues throughout life in 

 some Amphibia and Eeptiles. 



There is some difference of opinion as to whether the ventral aspects of the limbs 

 should not be regarded as facing more or less directly mesad ; but in other respects 

 Huxley's view has been adopted by several anatomists, including Mivart, Pageustecher, 

 EoUeston, Flower, Coues, and the senior author, in whose paper (10, 9-17) will be found 

 fuller references. See also Balfour, A, II, 508. 



Owing to changes which occur after the first formation of the limbs, it 



