Laboratory Exercise 4: Musculoskeletal System of the Shark

Introduction

 The skeletal system of all chondrichthyeans is composed of hyaline cartilage. The shark skeleton is divided into axial and appendicular components which provide points of attachment for the skeletal musculature that provides for locomotion. Study of the skeletal system will utilize prepared skeletal material preserved in either alcohol or formalin. Please do not remove specimens from their containers.
 The muscular system will be examined by dissection of individual specimens. Use care in exposing individual muscles since they are often very small and fragile. Blunt dissection offers the best approach for preserving anatomical details.

Axial Skeleton

 The shark's skull consists of a chondrocranium which encloses the brain and a splanchnocranium consisting of the jaws and gill arches. In addition to the skull, the axial skeleton also includes the vertebral column. Examine the dorsal surface of the chondrocranium and locate the rostrum which encloses the precerebral cavity. The rostral fenestrae are the paired openings at the base of the rostrum. Lateral to the rostral fenestrae identify the olfactory capsules and the orbits. Delimiting the orbits are the anteorbital process, supraorbital crest, and post- orbital process. The epiphyseal foramen lies just posterior to the precerebral cavity in the midline of the chondrocranium and contains the pineal body. Locate the otic capsules and the supraotic crest. Between the otic capsules lies a deep depression, the endolymphatic fossa.
 On the ventral surface of the chondrocranium locate the rostral carina and the anteorbital shelf which demarcates the anterior margin of each orbit. Also notice that between the orbits the floor of the chondrocranium extends back as the infraorbital shelf then widens to form the basal plate. The posteriolateral extensions of the basal plate form the postotic processes. Examine the posterior aspect of the chondrocranium for the foramen magnum and occipital condyles.

 The splanchnocranium represents the visceral skeleton of the shark and consists of the seven visceral arches. The mandibular arch is the most anterior visceral arch and also the largest. It consists of paired palatopterygoquadrate cartilages which unite in the midline to form the upper jaw. Each of these bears an orbital process and a quadrate process. The lower jaw consists of the two Meckel's cartilages, also fused in the midline. Note the labial cartilages which extend forward from Meckel's cartilage at the angle of the jaw. The second visceral arch, named the hyoid arch, actually consists of five cartilages (two are paired, one is single): the hyomandibular cartilages laterally, ceratohyal cartilage beneath the chondrocranium, and the basihyal cartilage which forms a short bar in the midline. The remaining visceral arches (three - seven) comprise the pharyngobrachial cartilage which support the gills. These arches consist of the following elements: epibranchial cartilages laterally; ceratobranchial and hypobranchials ventrally; basibranchial cartilage medially. Notice that there is variation within individual arches. Identify the gill rakers and gill rays present on some of these elements.
 Examine the trunk vertebrae and identify the centrum and neural arch. Above the neural arch is the neural spine and adjacent intercalary plates.

Muscular System

 Muscles of the trunk and tail are metemerically arranged and don't require a great deal of dissection. Remove a 4-6 cm segment of skin from the middorsal to the midventral region and examine the following muscle and connective tissue elements. Myomeres are segmentally arranged muscle bundles which are separated from each other by connective tissue, myosepta. Note the horizontal septum which extends laterally from both sides of the centrum. Epaxial muscles lie above the horizontal septum while the hypaxials lie below. The dorsal and ventral vertical septa extend above and below the centra to divide the body into right and left halves. The ventral vertical septum unites on the mid-ventral body surface at the linea alba.

Appendicular Muscles

 Carefully remove the skin from a pectoral fin. Expose the levator muscles on the dorsal surface of the fin and the depressor muscles located on the ventral surface.

Branchiomeric Muscles

 Remove the skin from one side of the head. Branchiomeric muscles are paired and lie anterior to the pectoral fin. They control movement of the gill arches and jaws. The constrictor series is composed of six pairs of superficial muscles located between the eye and pectoral fin which compress the gill pouches and close the mouth. Locate the 1st dorsal constrictor which is composed of the spiracular and mandibular adductor muscles. The 2nd dorsal constrictor lies just behind the spiricle and above the amndibular adductor. The 3rd-6th dorsal constrictors are arranged sequentially above the gill slits. Also expose and identify each of the six pairs of ventral constrictors. Muscles that elevate the mandibular, hyoid, and gill arches are respectively, the first, second, and 3rd-6th levators (also called the cucullaris muscle). These should be exposed and identified.

Hypobranchial Muscles

 These seem to be the most difficult to expose. On one side, cut across and reflect to 1st ventral constrictor (intermandibular) muscle. Locate the coracomandibular muscle (medial) and the coracohyoid muscle which is lateral and just dorsal to the coraco- mandibular. Transect the coracohyoid to see the coracobrachialis group beneath. Also examine the common coracoarcurals which are short, thick muscles inserting onto the pectoral girdle.

Summary

 Before finishing your study of the muscles of the shark be sure to review the summary (Table 3-1) in your Atlas. Become familiar with the actions of each of the muscles examined.