MORPHOLOGY OF THE TRUNK-MUSCLES. 347 



rami ; those of the fore part of the muscle are directed transversely across the sub- 

 pubic arch in front of the urethra ; while those of the hinder and larger part pass 

 inwards, some transversely, others obliquely, and blend with the wall of the vagina. 

 The muscular substance consists in great measure of unstriped fibres. 



(On the perineal muscles in the female, see Lesshaft, " Ueber d. Muskeln u. Fascien der 

 Dammgegend beim Weibe," Morph. Jahrb., viii, 1884 ; Tschaussow, " Zur Frage von den Venen- 

 geflechten und Muskeln im vorderen Abschnitt d. weibl. Dammes," Arch. f. Anat., 1885.) 



MORPHOLOGY OP THE FASCLffi AND MUSCLES OF THE TRUNK AND HEAD. 



Fasciae. There is a general correspondence in the relation of the deep fascia to the 

 skeleton and masses of the trunk-muscles throughout vertebrate animals. In its simplest 

 and lowest form the general investing fascia is prolonged from the surface towards the 

 skeleton in four places, viz., two median, forming what have been called respectively the 

 neural and licemal septa, and two lateral, one on each side, running towards the transverse 

 processes of the vertebrae. The layers of the haemal septum are in close contact in the caudal 

 region, but they are separated and somewhat complicated in the rest of the trunk by the 

 interposition of the visceral cavity between them. 



In man and the higher animals the dorsal part of the general investing fascia is 

 represented by the tendinous attachments of the trapezius, latissimus dorsi, rhomboidei, and 

 serrati postici muscles, and by the vertebral aponeurosis, while the deep fascia of the side 

 and front of the trunk, and neck, and the aponeurotic sheaths of the limbs correspond with 

 its ventral portion. The neural septum remains as the ligamentum nuchae and the supra- 

 spinous and interspinous ligaments. The haemal septum partly constitutes the linea alba, 

 and is elsewhere separated into two as an investment of the visceral cavity, forming the 

 transversalis, iliac, and recto-vesical fascia?. The lateral septum, which is strongly developed 

 in fishes and amphibia, is only seen at all clearly in the middle layer of the lumbar fascia of 

 man and the higher animals, being in them situated much nearer the dorsal than the ventral 

 aspect of the body. This difference of position is coincident with the greater development 

 of the ventro-lateral muscles and the limbs in the higher than in the lower vertebrates. 



Muscles. It has already been stated (p. 201) that the muscles of the trunk fall into two 

 primary sets, separated by the embryonic vertebral axis, and known as the epaxial and 

 hypaxial muscles ; and that the former are again subdivided into a dorsal and a ventral 

 group, which are partly separated from each other by the above-mentioned lateral septum, 

 and which correspond to the dorso-lateral and ventro-lateral divisions of the great lateral 

 muscle of fishes and tailed amphibia. 



The hypaxial or subvertebral muscles, in man but little developed, are placed on the 

 ventral aspect of the vertebral column. They include the rectus anticus major and longus 

 colli in the neck, and the vertebral portion of the diaphragm in the dorso-lumbar region ; 

 while the occasionally present sacro-coccygeus anticus represents the prolongation of these 

 muscles on the ventral surface of the caudal vertebrae of some of the lower animals. It is 

 proper to state, however, that the independence of the muscles here termed hypaxial is by no 

 means certain, and it is probable that they are merely separated portions of the ventro- 

 lateral muscle. 



The dorso-lateral muscle consists of fibres which, more than any others, retain their 

 original segmented character and longitudinal direction. It is represented in man by the 

 mass of muscles which lies in the vertebral groove of the back, and which, arising from the 

 lower vertebras and the ilium, passes upwards to be inserted into other vertebrae, the ribs, and 

 the skull. The mass is divided throughout by a longitudinal cleft into two chief parts, 

 external and internal, which are supplied respectively by the external and internal branches 

 of the posterior primary divisions of the spinal nerves. The external division includes the 

 erector spinae and splenius : its fibres are mostly longitudinal, but in the splenius, owing to a 

 shifting inwards of their origin, they take an oblique direction upwards and outwards. The 

 internal division comprises the transverse -spinales with the complexus, the proper inter- 

 transversales (p. 318), and the interspinales with the suboccipital muscles : the muscular 

 fasciculi are generally shorter than in the external division, and run upwards and inwards, 

 but they retain the longitudinal direction in the intertransversales and interspinales. The 

 Buboccipital muscles are formed by a special differentiation of the deeper portion of this 

 column at its upper end in relation to the movements of the head. The mass between the 

 skull and the first two vertebrae is first divided in reptiles into mesial (rectus) and lateral 

 (obliquus) portions, the place of division being marked by the entrance of the posterior 

 division of the suboccipital nerve, which represents an internal branch only : the second 

 nerve is always external to the obliquus. A farther division occurs in each of these, in the 



