24 LECTURE II, 



The skeleton is not entirely external or dermal in tlie Invertebrata. 

 Independently of the true cartilaginous endo-skeleton of the Cepha- 

 lopods, and of the entapophyses, sometimes also cartilaginous, of the 

 annular segments of the exo-skeleton of Insects and Crustacea, there 

 ai'e parts, as the calcified framework supporting the gastric teeth, 

 and giving attachment to the muscles which work them, in the 

 lobster, and the calcareous gastric plates of the Bulla, which relate 

 more particularly to the functions of the internal organs, or contained 

 viscera ; and we find a corresponding group of parts of the general 

 skeleton in most Vertebrate animals. The cartilages or bone^ of the 

 larynx, trachea, and bronchia of the air-breathing Vertebrates, the 

 bones and cartilages supporting the branchiae in fishes and batra- 

 chians, the bones in the hearts of certain birds and mammals, are 

 examples of the visceral series of hard parts, or the " splanchno 

 skeleton," as it has been termed by Carus ; and very neai'ly and 

 naturally connected with this primary division of hard and dry parts 

 are those bones and gristles which form capsules, or support the 

 appendages of the special organs of the senses ; as, for example, the 

 sclerotic osseous cups or plates of the eye, the petrous capsule of the 

 labyrinth, the ossicles and cartilages of the tympanum and external 

 ear, the turbinate bones and gristles of the nose. But some of these 

 " sense capsules " are connected and intercalated with the true bones 

 of the endo-skeleton, and subservient to similar functions, besides 

 their own special uses, so that they are generally described as ordi- 

 nary bones of the skull. As in all arrangements of natural objects, 

 where nature is followed in selecting their characters, so in classi- 

 fying the parts of the general skeleton of the Vertebrata, the primary 

 groups blend into one another at their extremes, and make it difiicult 

 to draw a well-defined boundary line between them. Thus the 

 hyoid and branchial arches closely resemble each other in fishes. 

 Bones of the dermo-skeleton combine with those of the endo-skeleton 

 to form the opercular and the single median fins. But we must not 

 on that account abandon the advantage of arrangement and classi- 

 fication in acquiring an intelligible and tenable knowledge of a 

 complex system of organs, when typical characters clearly indicate 

 the general primary groups. Clearly appreciating the existence of 

 such characters in the very numei'ous and diversified parts of the 

 general skeleton of the Vertebrate animals, I, therefore, adopt the 

 primary division of those parts into endo-skeleton, exo-skeleton, and 

 splanchno-skeleton. 



The endo-skeleton may present itself to our observation under 

 three histological conditions, the fibrous, the cartilaginous, and the 



