ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



its inner edge (1<V i .:.} , 1 '>, v.a) and bearing externally the slender tapering 

 -ill-rays (,?) that support the thin part of the gill-septum. The originally 

 i-ontinuous hoop-like cartilage becomes segmented into a number of 

 as may be seen in Fig. 133 (3, 4; 5; 6 ; ?) While in the case of the 

 l.ranchial arches the details of this differentiation are of minor importance, 

 in the case of the mandibular and hyoid arches on the other hand some 

 of them have an important bearing upon the morphology of vertebrates 



neral and therefore must be alluded to. 

 The first visceral arch or mandibular arch lies between the mouth 



FIG. 133. 



Cartilaginous skeleton of head and visceral arches of Scyllium, as seen from the left side. (After 

 VV. K. Parker.) a, Artery; aud, auditory capsule; c.h, ceratohyal ; Cr, chondrocranium ; 

 h.m, hyomandibular ; I, labial cartilages ; M, Meckel's cartilage ; olf, olfactory capsule ; oph, ophthal- 

 mic ; pp, palatopterygoid cartilage ; ps.c, pre-spiracular cartilage ; Vs.o, VIIs.o, superficial oph- 

 thalmic nerves ; v, vein. 



Roman numerals indicate foramina for cranial nerves ; Arabic numerals, skeleton of visceral 

 arches. 



and the spiracle. The upper portion of the cartilage, next the cranium, 

 is reduced to a functionless vestige the prespiracular cartilage lying 



: front of the spiracle. The ventral portion on the other hand 

 becomes much enlarged, supports the lower edge of the mouth with its 

 teeth, and forms the lower jaw or Meckel's cartilage (Fig. 133, M). This 

 Meckel's cartilage is of great interest as it is the primitive lower jaw of 

 the vertebrate and is always present during early stages of development 

 tebrates in which the lower jaw of the adult is composed of 



Mid different in its morphological nature. The primitive upper jaw 

 of the vertebrate which is also seen typically in the Dogfish (Fig. 133, pp) 



