The Muscular System of the Trunk and Head

This consists of two portions, which differ fundamentally in theii origin, and in their relations to the endoskeleton. The one takes its origin in the protovertebrae; each protovertebra becoming differentiated, as we have seen, into three parts; a spinal ganglion and a segment of the vertebral endoskeleton, in the same plane, and a more superficial sheet of muscular fibres. These muscular fibres are consequently situated above the endoskeleton, or arc episkeletal. Other muscular fibres are developed below the endoskeleton, and may be termed hyposkeletal muscles. The hyposkeletal muscles are separated from the episkeletal, not only by the endoskeleton of the trunk (or the vertebrae and their prolongations, the ribs), but by the ventral branches of the spinal nerves.

As the episkeletal muscles are developed out of the protovertebrae, they necessarily, at first, present as many segments as there are vertebrae, the interspaces between them appearing as intermuscular septa. The development of the hyposkeletal muscles has not been worked out, but it appears to take place much later than that of the episkeletal set.

In the lowest Vertebrata - as, for example, in ordinary fishes - the chief muscular system of the trunk consists of the episkeletal muscles, which form thick lateral masses of longitudinal fibres, divided by transverse intermuscular septa into segments (or Myotomes) corresponding with the vertebrae. The lateral muscles meet in the middle line below, and divide, in front, into a dorso-lateral mass connected with the skull, and a ventro-lateral attached, in part, to the pectoral arch, and, in part, continued forward to the skull, to the hyoidean apparatus, and to the mandible. Posteriorly, the lateral muscles are continued to the extremity of the tail. The hyposkeletal muscular system appears to be undeveloped.

In the higher Vertebrata, both the episkeletal and hyposkeletal muscular systems are represented by considerable numbers of more or less distinct muscles. The dorso-lateral division of the lateral muscle of the fish is represented by the superior caudal muscles, and by the erector spinae; which, as it splits up, anteriorly, and becomes attached to the vertebrae, and to the ribs, and to the skull, acquires the names of spinalis, semispinalis, longissimus dorsi, sacrohimbalis, intertransversalis, levatores costarum, complexus, spleiiius, recti postici, and recti laterales.

The ventro-lateral division of the fish's lateral muscle is represented, in the middle line of the trunk and head, by a series of longitudinal muscles; and, at the sides, by obliquely directed muscles. The former are the recti abdominis, extending from the pelvis to the sternum - the sterno-hyoidei, between the sternum and the hyoidean apparatus - the geniohyoidei, which pass from the hyoid to the symphysis of the mandible. The latter are the obliqui externi of the abdomen - the external intercostales of the thorax - the subclavius stretching from the first rib to the clavicle; the scaleni from the anterior dorsal ribs to the cervical ribs and transverse processes, and the sterno - and cleido-mastoidei from the sternum and clavicle to the skull.

The fibres of all these oblique muscles take a direction, from parts which are dorsal and anterior, to others which are ventral and posterior.

The trunk muscles of the lower Amphibia exhibit arrangements which are transitional between those observed in Fishes and that which has been described in Man, and which substantially obtains in all abranchiate Vertebrata.

The muscles of the jaws and of the hyoidean apparatus appear to be, in part, episkeletal, and, in part, hyposkeletal. The mandible is depressed by a muscle, the diagistric, arising from the skull, and supplied by a branch of the seventh nerve: it is raised by a muscular mass, which is separable into masseter, temporal, and pterygoid muscles, according to its connection with the maxillo-jugal bones, the sides of the skull, or the palato-pterygoid bones, and is supplied by the fifth nerve.

The proper facial muscles belong to the system of cutaneous muscles, and receive branches from the seventh nerve.

The hyposkeletal system is formed, partly, of longitudinal muscles which underlie the vertebral column; and partly, of more or less oblique, or even transverse fibres, which form the innermost muscular walls of the thorax and of the abdomen.

The former are the subcaudal intrinsic flexors of the tail; the pyriformis, psoas, and other muscles proceeding from the inferior faces of the vertebrae to the hind-limb; the longus colli, or intrinsic flexor of the anterior part of the vertebral column; and the recti capitis antici, or flexors of the head upon the vertebral column. The latter are the obliquus internus of the abdomen, the fibres of which take a direction crossing that of the external oblique muscle; and the transversalis, which lies innermost of the abdominal muscles, and has its fibres transverse. In the thorax, the intercostales interni continue the direction of the internal oblique, and the triangularis sterni that of the transversalis. The diaphragm and the levator ani must also be enumerated among the hyposkeletal muscles. The hyposkeletal muscles of the posterior moiety of the body attain a great development in those Vertebrata which have no hind-limbs, such as Ophidia and Cetacea.