THE SKELETAL SYSTEM. 57 
incus is attached to the skull, while the malleus is extended as the 
cartilage of Meckel almost the whole length of the inner surface 
of the mandible. The bulk of the skull is formed already by 
the surface elements distributed in the characteristic fashion, but 
as yet only loosely associated (cf. Fig. 29). 
This condition of the developing skull in a mammal finds its 
explanation far back in the history of the vertebrates and is only 
made clear by the study of some one of the lower fishes such as 
HEAD SKELETON OF shark or sturgeon. Ina shark (Fig. 30) 
LOWER VERTEBRATES. _ the entire internal skeleton is formed in 
cartilage which persists throughout life. 
The principal part of the head sk2leton is a massive cartilaginous 
box (chondrocranium), enclosing the brain, and including as a 
result of growth and fusion the nasal and auditory capsules. The 
eye capsules are free, and their accommodation at 
the side of the cranium establishes the orbital 
depression to be seen on the skull of all vertebrates. 
This type of structure is obviously the basis of the 
mammalian skull, except that in the latter the 
cartilage mass is at once more restricted to its basal 
portion, and is more specialized by its replacement 
by definite bone centres. 
It is, however, in respect of the visceral arches 
suspended from the chondrocranium that the structure of the shark 
skull is most illuminating. The first or mandibular arch forms 
the upper and lower jaw. Its composition reflects that of the 
external part of the auditory chain in the mammalian embryo, and 
establishes a fact, for a long time scarcely believed by zoologists, 
that in the history of mammals this arch has undergone a profound 
change of function. The second or hyoid arch, though developed 
to an extent out of keeping with the degree of development of the 
primitive tongue, is nevertheless obviously homologous with the 

Fic. 31. Shagreen 
denticles of the 
smooth _ dogfish, 
Mustelus; enlarged. 
hyoid arch of a mammal and in most sharks has an important 
accessory function in the support of the lower jaw. Following 
the hyoid arch are five ordinary or branchial arches supporting 
the filaments of the gills and serving as pillars of the gill apertures. 
These arches are then the parts of an extensive system out of which 
those portions of the laryngeal cartilage which are of visceral rather 
than tracheal origin have been specialized in mammals. 
