Page:Laboratory Manual of the Anatomy of the Rat (Hunt 1924).djvu/23

Rh the occipital, interparietal, parietals, frontals, nasals, pre-maxillaries, maxillaries, zygomatics, squamosals, and lacrimals. The position and structure of each of these bones will be described in detail later.

Exercise I. Draw and label the dorsal view of the skull.

Lateral aspect of the cranium. The nuchal plate bears ventrally the occipital condyle, by which the skull articulates with the backbone. The jugular process is a long, pointed, triangular structure lateral to and slightly in front of the occipital condyle. Between the two is a semi-circular depression, the jugular fossa. The mastoid portion of the petrosal bone is immediately anterior to the jugular process, and is located in a pronounced depression of the lateral part of the occipital bone. The tympanic bulla is a hollow spheroidal capsule anterior to the jugular process, opening to the outside through the external acoustic meatus. This opening is closed in life by the tympanic membrane, which transmits sound waves from the outer ear to the three small bones of the middle ear.

Where the mastoid portion connects anteriorly with the squamosal bone there is a ventral extension of the latter, the post-tympanic hook, which helps to hold the tympanic bulla in place.

The orbito-temporal fossa and zygomatic arch are seen from the lateral side also. Within the fossa are foramina through which the nerves emerge from the brain. The foramen ovale is located directly ventral to the posterior end of the zygomatic arch, and anterior to the tympanic bulla. A branch of the fifth cranial nerve emerges here. The foramen communicates by a perforation of the external pterygoid process with the ventral surface of the skull. Another small foramen is immediately in front of the foramen ovale. A few millimeters anterior to the foramen