10 THE ANATOMY OF VEBTEBRATED ANIMALS. 



respiration. In other Vertebrata all the visceral clefts 

 become closed and, with the frequent exception of the first, 

 obliterated; and no branchiae are developed upon any of 

 the visceral arches. 



In aU vertebrated animals, a system of relatively, or abso- 

 lutely, hard parts aifords protection, or support, to the softer 

 tissues of the body. These, according as they are situated 

 upon the surface of the body, oi- are deeper-seated, are called 

 exosheleton, or endosheleton. 



The Vertebrate Endosheleton. — This consists of connective 

 tissue, to which cartilage and bone may be added in various 

 proportions ; together with the tissue of the notochord and 

 its sheath, which cannot be classed tmder either of these 

 heads. The endoskeleton is distinguishable into two inde- 

 pendent portions — the one axial, or belonging to the head 

 and trunk ; the other, appendicular, to the limbs. 



The axial endoskeleton usually consists of two systems of 

 skeletal parts, the spinal system, and the cranial system, the 

 distinction between which arises in the following way in the 

 higher Vertebrata. 



The primitive groove is, at first, a simple straight depres- 

 sion, of equal diameter throughout ; but, as its sides rise, and 

 the dorsal laminae gradually close over (this process com- 

 mencing in the anterior moiety of their length, in the 

 future cephalic region), the one part becomes wider than 

 the other, and indicates the cephalic region (Fig. 4, A). The 

 notochord, which underlies the groove, tenninates in a point, 

 at a little distance behind the anterior end of the cephalic 

 enlargement, and indeed under the median of three dilata- 

 tions which it presents. So much of the floor of the enlarge- 

 ment as lies in front of the end of the notochord, bends 

 down at right angles to the rest ; so that the anterior en- 

 largement, or anterior cerebral vesicle, as it is now called, lies 

 in front of the end of the notochord ; the median enlarge- 

 ment, or the middle cerebral vesicle, above its extremity; 

 and the hinder enlargement, or the posterior- cerebral 

 vesicle, behind that extremity (Fig. 4, D and E). The under- 



