320 ZOOLOGY FOR MEDICAL STUDENTS chap. 



The limb-girdle is seen in its least modified form in the pectoral 

 girdle which in the adult Dogfish remains in a condition probably departing 

 very little from that of the ancestral vertebrate. It forms a hoop-hke 

 cartilage, the right and left halves of which are continuous across the 

 mesial plane ventrally but not dorsally. In each half a smooth joint- 

 surface — the glenoid surface, serving for the attachment of the fin 

 skeleton — serves as a landmark between the dorsal or scapular region 

 and the ventral or coracoid. 



In the pelvic girdle we distinguish similarly between a dorsal iliac 

 and a ventral ischio-pubic region but in the Dogfish the former is reduced 

 to a mere knob of cartilage — the girdle consisting simply of a transverse 

 bar, representing the continuous ischio-pubic portions of the two sides, 

 and carrying the limb skeleton close to its outer end. 



As will have been gathered the main skeleton of the Dogfish is in the 

 cartilaginous phase of evolution. In many parts it is given increased 

 rigidity by the deposition of salts of lime in its intercellular matrix but 

 this calcified cartilage never becomes replaced by true bone. Bony 

 tissue is entirely confined to the placoid structures in the skin. 



The Heart is in an early stage of its development simply an enlarged 

 portion of the ventral vessel which, owing to its increase in length, its 

 ends being fixed, has taken a somewhat S-shaped curvature. As develop- 

 ment goes on certain portions of the primitive tubular heart become 

 dilated and modified, with the result that in the completed heart four 

 distinct chambers may be distinguished. Of these the posterior (Fig. 

 134, s.v), which receives the blood returned from the tissues of the body, 

 is the sinus venosus. This opens forwards into a very large chamber 

 with a thin wall like itself — the atrium (a). This opens downwards into 

 a rounded chamber — -the ventricle — characterized by the much greater 

 thickness of its walls, composed of masses of muscle-fibres with inter- 

 vening chinks. The ventricle opens in front into the conus arteriosus (c.) 

 — a slightly tapering tube with thick muscular walls which is continued 

 forwards directly into the ventral aorta (v.a). 



In the primitive tubular heart the contractions of its muscular walls 

 by which the blood is propelled onwards are in the form of simple waves 

 of contraction which pass along its length from behind forwards. As 

 the tube becomes segmented into the four chambers the originally con- 

 tinuous wave becomes modified, and the portion of wall belonging to 

 each chamber tends to contract by itself, the contraction of the several 

 chambers still taking place in the original sequence, (i) sinus venosus, 

 (2) atrium, (3) ventricle, and (4) conus. 



