. The chordates. Chordata. 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 spiracl
. The chordates. Chordata. 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.). Please note that these images are extracted from scanned page
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