Musculoskeletal and Nervous System. The Adult Salamander

The musculoskeletal system of the amphibian has received the most attention of any organ system probably because it is the most common organ system to survive in fossil form. All other organ systems are fairly similar in appearance within an order, but the musculoskeletal system may vary dramatically between members of the same order, and it is often the characteristic used to define relationships among modern amphibians.

The elucidation of the particulars of the musculoskeletal anatomy is beyond the scope of this text. Rather, the approach is to familiarize the clinician with basic elements of the amphibian's musculoskeletal system.

The Adult Caecilian. The majority of the cranial components of a caecilian are fused. Caecilians lack pectoral and pelvic girdles as well as a sacrum. Except for the atlas and terminal vertebrae, the ribs of caecilians are double-headed.

The Adult Salamander. The cranium of the salamander is intermediate between the solid structure of a caecilian and the markedly reduced bone structure of an anuran. The hyoid may be modified to allow for suction feeding or the ejection of the tongue to capture prey items, as in tropical salamanders, Bolitoglossa spp. The vertebral column is poorly differen- tiated into cervical, trunk, sacral, caudal sacral, and caudal regions. Cleavage planes may be present in the caudal region. Pelvic girdles are lacking in sirens, Siren spp., and the dwarf siren, Pseudobranchus spp. Digits may be lacking in some species.

The Adult Anuran. The musculoskeletal system of the anuran is highly modified from that of other amphibians. Most of the modifications allow for saltatory locomotion, while features for other lifestyles may be secondarily derived from these original modifications. The nomenclature of some of the bones of the anuran skeleton is different from many other vertebrates and should be used appropriately.

The cranium bears two large orbits with no bony separation between the ocular globe and the oropharynx. The cranial elements are reduced when compared to the other amphibian orders. The vertebrae are fused, and three regions of the vertebral column are described: the presacral, sacral, and postsacral. A sacrum is lacking, and the pelvic girdle is highly modified. The limbs are highly modified. The distal long bones consist of a fused radio ulna in the forelimbs and a fused tibiofibula in the hind limb.

The pectoral girdle varies in shape among different genera of anurans, as does the amount of cartilaginous and bony elements making up the pectoral girdle. The pelvic girdle is fused and intimate with the last presacral vertebra. The coccyx or urostyle is a single fused element in most anurans, and forms a point above the dorsocaudal aspect of the pelvic girdle. In many genera the hyoid bones are adapted for ejecting the tongue to capture prey items.

Nervous System. The anatomy of the brain of a caecilian (Kuhlenbeck, 1973), a salamander (Herrick, 1948), and an anuran (Haslam, 1971) have been studied in great detail and compared (Noble, 1931). There is some debate as to the number of cranial nerves. One textbook (Duellman & Trueb, 1986b) lists the cranial nerves as follows: CN I Olfactory (also innervates the tentacle of caecilians); CN II Optic (absent or reduced in many caecilians and cave salamanders); CN III Oculomotor (does not connect with CN V in caecilians); CN IV Trochlear (also innervates an eye muscle); CN V Trigeminal (also innervates the tentacular sheath of caecilians); CN VI Abducens (innervates the retractor of the caecilian tentacle); CN VII Facial; CN VIII Auditory; CN IX Glossopharyngeal; CN X Vagus; CN XI Accessory; CN XII Hypoglossal, which may be considered a spinal nerve and not a true cranial nerve; Lateral-line nerves are innervated by branches of the cranial nerves. As is expected for limbless animals, the brachial and inguinal neural plexi are absent in caecilians, and reduced in salamanders with vestigial limbs (e.g., amphiumas, Amphiuma spp., dwarf siren, Pseudobranchus striatus, and sirens, Siren spp).