i8 THE SURGICAL ANATOMY OF THE HORSE 



represented in Plate XVUI. Inside the pouch are seen elevations 

 caused by the styloid cornu of the hyoid bone, and also by the eustachian 

 tube. On the outer side of the pouch is placed the occipito-styloid 

 muscle, the inner surface of which is covered by the membranous 

 wall of the pouch. The thickness of the muscle is evident, as also is 

 its relation to the superposed parotid gland. 



The probe is passed through the counter opening made in Hyoverto- 

 brotomy for the purpose of draining the pouch, and it enters the pouch 

 at its posterior diverticulum, which, when the pouch is distended with 

 purulent or other inflammatory products, becomes pressed towards the 

 angle of the inferior maxillary bone. 



In Plate VII. (facing page 20) we have a representation of a vertical 

 transverse section of the neck taken through the posterior third of the 

 atlas. A reference to this plate is of utility in studying the seat of 

 " poll-evil." Immediately under the skin is a considerable quantity of 

 adipose tissue, in which is embedded the funicular division of the 

 ligamentum nuchx. 



Below the ligament is a considerable mass of muscular tissue made up 

 chiefly of the rectus capitis posticus major and minor muscles. The 

 depth of muscular tissue is here about two inches, whilst clothing the 

 lateral aspect of the atlas we have a layer of muscular tissue which is 

 about equal in thickness to that placed on the superior aspect of the bone, 

 and which is made up chiefly of the obliquus capitis inferioris muscle. 



It will thus be seen that the structures usually involved in a sinus of 

 the poll are fatty, ligamentous, muscular and possibly osseous tissue, and it 

 will also be noticed that in an average sized animal where the opening is 

 at the highest point of the neck, the depth to which the sinus may be 

 probed in the vertical direction for operative purposes without any 

 danger of piercing an intervertebral joint is at least two inches, and this 

 depth increases as we proceed in a posterior direction down the neck. 



The atlas is cut through near its posterior extremity, so that the 

 posterior foramen in the wing is represented by a notch, the posterior 



