84 VERTEBRATE ZOOLOGY 



The distal portion is composed of the ankle and the foot. The 

 ankle is made up of six small cartilages called the tarsals, two of 

 which form a transverse, proximal row and three a distal row, while 

 one is in the center. The typical number of bones or cartilages 

 in the vertebrate tarsus is ten, three being in the proximal and 

 five in the distal row, while one or two are in the center. In 

 the foot of Necturus four digits are present, each of which 

 is composed of a proximal metatarsal bone and two or three 

 phalanges, or toe joints. In the typical vertebrate foot five digits 

 are present. The digit which is lacking in the foot of Necturus 

 is the first. 



Exercise 21. Draw an outline of the skeleton of the leg on a scale of 

 2 or 3, showing accurately the bones and cartilages. 



The Axial Skeleton. The skull is made up of two regions, the 

 cranium and the visceral skeleton. The former incloses and pro- 

 tects the brain and the organs of special sense ; the latter surrounds 

 the mouth and pharynx, forming the framework of the jaws and 

 the hyobranchial apparatus. 



Both these regions are composed entirely of cartilage in an 

 early period of the animal's life, and certain portions remain 

 cartilaginous, this primitive cartilaginous skull being called the 

 chondrocranium. As the young animal develops, however, cer- 

 tain parts of the cartilage are replaced by bone. Besides these 

 cartilage bones, as they are called, other bones also make their ap- 

 pearance, which develop not in the cartilage but in the skin and 

 in connective-tissue membranes which cover the skull ; these are 

 called membrane bones. Thus, so far as the material of which it 

 is made up is concerned, the skull is composed of cartilage, carti- 

 lage bones, and membrane bones. 



Open the body cavity and remove the viscera. Strip off all the 

 muscles from the skeleton. It is well to begin the process with the 

 head and to lay bare and study first the hyoid and gill arches,— 

 which together form the hyobranchial apparatus,— and then the 

 lower jaw. These parts constitute the ventral portion of the skull 

 and belong to the visceral skeleton. The skull must be kept in 

 preserving fluid and not allowed to get dry. 



