LONGUS COLLI MUSCLE. 163 



in greatest quantity externally, so as to incase and unite them ; and it is 

 extended across the posterior part of the air tube. 



Dissection. On removing for about two inches the fibrous membrane 

 and the mucous glands from the interval between the cartilages at the back 

 of the trachea, the muscular fibres will appear. 



After the muscular fibres have been examined the membranous part of 

 the tube may be slit down, to see the elastic tissue and the mucous mem- 

 brane. 



Muscular fibres. Between the ends of the cartilages is a continuous 

 layer of transverse bundles of unstriated muscle, which is attached to the 

 truncated ends and the inner surface of the cartilages. By the one sur- 

 face the fleshy fibres are in contact with the membrane and glands, and 

 by the other with the elastic tissue. Some longitudinal fibres are super- 

 ficial to the transverse ; they are arranged in scattered bundles, and are 

 attached to the fibrous tissue. 



The elastic tissue forms a complete lining to the tracheal tube beneath 

 the mucous membrane ; and at the posterior part, where the cartilages are 

 deficient, it is gathered into strong longitudinal folds. This layer is closely 

 connected with the mucous membrane covering it. 



The mucous membrane of the trachea lines the tube, and resembles that 

 of the larynx in being furnished with a columnar ciliated epithelium. 



Connected with this membrane are numerous branched mucous glands 

 of variable size. The largest are found at the back of the trachea, in the 

 interval between the cartilages, where some are placed beneath the fibrous 

 membrane with the muscular fibres, and others outside that layer. 



Other smaller glands occupy the front and sides of the trachea, being 

 situate on and in the fibrous tissue connecting the cartilaginous rings. 



SECTION XVIII. 



PREVERTEBRAL MUSCLES AND VERTEBRAL VESSELS. 



Directions. On the part of the spinal column that was laid aside after 

 the separation of the pharynx from it, the student is to learn the deep 

 muscles on the front of the vertebras. 



Dissection. The prevertebral muscles will be prepared by removing 

 the fascia and areolar tissue. The muscles are three in number on each 

 side (fig. 45), and are easily distinguished. Nearest the middle line lies 

 the longus colli this is the longest ; the muscle external to it, which 

 reaches to the head, is the rectus capitis anticus major ; and the small 

 muscle close to the skull, which is external to the last and partly concealed 

 by it, is the rectus capitis anticus minor. The smaller rectus muscle is 

 often injured in cutting through the basilar process of the occipital bone. 



The LONGUS COLLI MUSCLE (fig. 45, A ) is situate on the bodies of the 

 cervical and upper dorsal vertebrae, and is pointed above, but larger below. 

 It consists of two parts internal and external, the former being vertical, 

 and the latter oblique in direction, as on the right side of the figure. 



The internal part arises by fleshy and tendinous processes from the 

 bodies of the two upper dorsal and two lower cervical vertebrae ; and the 

 external piece takes origin from the anterior transverse processes of four 



