Aim and Method of Comparative Anatomy 



357 



breathing organ of a fish with the highly complex auditory organ of a 

 mammal. 



In the common dogfish, a small shark, there are six pairs of respira- 

 tory passages leading from the pharynx to the exterior (Fig. 287). On 

 the walls of all of these, except those of the first (most anterior) pair, 

 are well-developed branchial "filaments," thin projecting plates of soft 

 tissue densely filled with minute blood-vessels (Fig. 56). These are the 

 essential respiratory structures. The passages of the first pair, known 

 as spiracles, are short and broad or more nearly tubular as compared to 

 the thin, flat gill-chambers, and gill-filaments are only very weakly 

 developed in the spiracle, which is merely a passage for water. The 

 mammalian ear (Fig. 288) consists of three divisions. The more or less 

 funnel-like external ear leads into a broad passage (meatus) which 

 terminates against a taut tympanic membrane. The essential nervous 

 part of the ear is the complex "inner ear" embedded in the bone of the 

 skull. Between the tympanic membrane and the inner ear is the tym- 

 panic cavity ("middle ear"), which communicates with the nasal re- 

 gion of the pharynx by a long, narrow passage, the Eustachian tube. 

 Swung across the tympanic cavity are three very small bones movably 

 jointed together, and so forming a chain whose outer end is attached to 

 the tympanic membrane, while the inner end is movably inserted into a 



PETROSAL BONE 



OPENING INTO 

 PHARYNX 



Fig. 288. The human ear; diagrammatic section showing its three divisions. 

 External, consisting of the pinna and the auditory meatus. Middle, the tympanic 

 cavity. Internal, including the utriculus with its three semicircular canals and the 

 sacculus with its spiral cochlea. (After Howell and Czermak. Courtesy, Neal and 

 Rand: "Chordate Anatomy," Philadelphia, The Blakiston Company.) 



